ELEMENTS 

OF 

PHYSIOLOGICAL  PSYCHOLOGY 


ELEMENTS 


OP 


PHYSIOLOGICAL  PSYCHOLOGY 


A  TREATISE  OF  THE  ACTIVITIES  AND  NATURE 
OF  THE  MIND 

FROM  THE  PHYSICAL  AND  EXPERIMENTAL  POINTS  OF  VIEW 

(THOROUGHLY  REVISED  AND  RE-WRITTEN) 


BY 

GEORGE  TRUMBULL  LADD,  LL.D. 

IERITUS  PROFESSOR  OF  MORAL  PHILOSOPHY  AND  METAPHYSICS  IN  YALE  UNIVERSITY 

AND 

ROBERT  SESSIONS  WOODWORTH,  PH.D. 

PROFESSOR  OF  PSYCHOLOGY  IN  COLUMBIA  UNIVERSITY 


ILLUSTRATED 


NEW   YORK 

CHARLES  SCRIBNER'S  SONS 
1911 


L2- 


BIOLOGY 

LIBRARY 

G 


COPYRIGHT  1887,  1911,  BY 
CHARLES  SCRIBNER'S  SONS 

Published  April,  1887 

Reprinted  July  1888 ;  July  1889 ;  Jan.  1890;  Jan.,  Sept.  1892; 
Nov.  1894;  Nov.  1896;  Dec.   1897;  Mar.  1900;  Feb.  1907 


New  Edition  Published  May,  1911 


PREFACE 

IT  is  now  nearly  a  quarter  of  a  century  since  the  appearance  of 
the  first  edition  of  this  treatise,  which  attempted  to  gather  within  the 
limits  of  a  single  volume  the  main  results  of  the  then  modern  study 
of  psychology  from  the  physiological  and  experimental  points  of 
view.  It  was  at  that  time  (1887)  the  only  work  of  its  kind  in 
English;  and  so  far  as  the  author  was  aware,  it  was  the  only  work 
in  any  language,  with  the  sole  exception  of  Professor  Wundt's 
Grundzuge  der  physiologischen  Psychologic,  which  was  then  in  its 
second  edition.  The  task  involved  in  any  such  attempt  was  even 
so  long  ago  exceedingly  large  and  difficult.  But  the  prompt  favora- 
ble response  given  to  its  accomplishment,  not  only  in  this  country, 
but  also  from  abroad,  was  on  this  account  all  the  more  unexpectedly 
gratifying. 

During  the  interval  since  the  first  edition,  many  trained  and 
skilled  workmen  have  assiduously  devoted  themselves  to  the  solu- 
tion of  the  various  subordinate  problems  involved  in  the  study  of 
mental  life  and  mental  development  with  the  carefully  guarded 
methods  prescribed  by  modern  science.  Much  light  has  been 
thrown  on  these  problems,  their  nature  and  their  probable  solu- 
tion; while  many  new  ones  or  new  aspects  of  old  problems  have 
been  uncovered  and  are  still  awaiting  future  investigation.  And 
there  is  no  prospect  that  this  process  will  come  to  an  end.  But  it  is 
just  this  fact  which  evinces  in  an  unmistakable  way  the  vitality 
of  the  science  of  psychology,  as  it  does  the  vitality  of  all  the  other 
forms  of  human  knowledge.  On  the  other  hand,  it  is  entirely 
safe  to  say  that  neither  the  extravagant  hopes  nor  the  extravagant 
fears  of  twenty-five  years  ago,  with  reference  to  the  results  of  the 
so-called  "  new  psychology,"  have  been  verified.  The  fundamental 
problems  with  regard  to  the  nature  of  man's  mind  and  its  relations 
to  the  organism,  its  place  in  the  scale  of  development  and  its  destiny, 
remain  essentially  unchanged. 


281804 


vi  PREFACE 

Those  acquainted  with  the  first  edition  will  notice  several  impor- 
tant changes  on  comparing  with  it  the  present  revised  edition.  Of 
these  changes  perhaps  the  most  important  may  be  summed  up  in 
the  fact  that,  of  the  two  editions,  this  is  much  the  most  distinctly 
physiological.  The  reason  for  this  change  is  chiefly  the  following: 
While  many  excellent  volumes  and  articles  have  meantime  appeared 
to  give  the  results  of  experimental  psychology,  more  strictly  so-called, 
comparatively  little  has  been  published,  of  a  worthy  character, 
which  could  be  brought  under  the  head  of  physiological  psychology, 
with  any  strict  application  of  this  latter  term.  With  the  same  end  in 
view,  two  entire  chapters — one  on  THE  PLACE  OF  THE  NERVOUS 
MECHANISM  IN  THE  ANIMAL  KINGDOM,  and  the  other  on  THE 
DEVELOPMENT  OF  THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL — have 
been  added  to  Part  I,  and  all  the  other  chapters  of  this  part  have 
been  carefully  rewritten  and  in  most  cases  considerably  expanded. 
The  new  chapters  on  THE  LOCALIZATION  OF  CEREBRAL  FUNCTIONS 
have  been  transferred  to  it.  In  this  way  the  aim  has  been  to  give 
a  compact  and  complete  summary  of  the  nervous  mechanism  in  its 
relations  to  mental  life.  The  hope  has  also  been  indulged  that 
the  book  might  thus  be  made  more  useful  to  students  of  physiology 
and  medicine,  who,  in  the  opinion  of  the  authors,  ought  to  know 
far  more  than  they  now  do  about  the  science  of  psychology  as 
approached  from  the  physiological  point  of  view. 

The  subjects  more  definitely  falling  under  the  head  of  experi- 
mental psychology  have,  however,  by  no  means  been  neglected. 
Indeed,  so  far  as  these  are  connected  with  the  functions  of  the 
organs  of  sense,  and  with  the  measurement  of  all  sorts  of  reactions 
to  all  kinds  of  stimuli,  they  are  themselves  distinctly  physiological 
in  their  character  and  in  their  conclusions.  And  even  when  the 
students  of  experimental  psychology  attempt  the  more  difficult 
problems  connected  with  the  mental  functions  involved  in  memory, 
association,  thinking,  and  learning,  they  can  not  possibly  quite  sep- 
arate themselves  from  the  closely  allied  physiological  problems. 
On  the  other  hand,  the  problems  which  are  attacked  and  more  or 
less  successfully  handled  by  the  most  purely  experimental  methods 
are  available  and  important  data  for  the  student  of  the  same  phe- 
nomena from  the  physiological  point  of  view.  While,  then,  Part  II 


PREFACE  vii 

has  not  been  correspondingly  extended,  it,  too,  has  been  revised  with 
equal  thoroughness,  with  the  intention  of  making  it  a  fairly  complete 
but  succinct  treatment  of  similar  data  as  regarded  from  a  somewhat 
changed  point  of  view.  In  all  this,  however,  the  controlling  pur- 
pose of  the  book  has  never  been  lost  out  of  sight,  which  is  to  sum- 
marize what  modern  science  knows,  or  reasonably  conjectures,  about 
the  correlations  existing  between  the  nervous  mechanism  and  the 
mental  life  of  man. 

The  other  principal  change  which  will  be  noted  is  the  great  re- 
duction made  in  the  Third  Part  of  the  book.  This  has  been  partly 
on  account  of  economy  of  space.  But  it  has  been  chiefly  due  to 
the  fact  that  the  author  of  the  first  edition  has,  since  its  appear- 
ance, expressed  his  views  on  the  more  distinctly  speculative  ques- 
tions involved,  in  a  series  of  monographs.1  The  two  chapters  to 
which  this  part  has  been  reduced — to  summarize  them  in  a  single 
word — simply  maintain  the  contention  that  the  study  of  ultimate 
psychological  problems,  from  the  physiological  and  experimental 
points  of  view,  finally  leaves  them  in  essentially  the  same  condition 
as  that  in  which  it  finds  them;  and  this  position  is  that  of  a  naive 
and  so-called  "  common-sense "  dualism,  as  distinguished  from  the 
metaphysical  theories  of  parallelism,  materialism,  or  subjective 
idealism.  To  this  dualism,  neither  psychology  nor  physiology 
and  physics  have  anything  to  oppose  on  scientific  grounds.  The 
settlement  of  these  questions,  then,  if  they  are  to  be  settled  at  all, 
must  be  relegated  to  philosophy. 

While,  as  has  already  been  stated,  the  entire  subject  has  been 
studied  afresh  and  the  book  rewritten  throughout,  it  has  been  all 
the  more  gratifying  to  find  that  as  candid  and  objective  a  reconsider- 
ation of  his  own  views  as  it  is  easily  practicable  for  any  student  to 
give,  did  not  seem  to  require  essential  modifications  in  any  of  the 
psychological  theories  advanced  by  the  first  edition  on  alleged 
scientific  grounds.  But  if  the  same  candidly  critical  reception 
is  accorded  to  this  edition  which  was  so  happily  experienced  by  its 
predecessor,  it  will  be  all  that  can  be  justly  asked — perhaps  more 
than,  under  existing  circumstances,  can  be  reasonably  expected. 

The  authors  of  this  edition  wish  to  make  general  acknowledg- 

>  Especially  "  Philosophy  of  Mind"  (1895),  and  "A  Theory  of  Reality"  (1899). 


viii  PREFACE 

ment  of  obligations  to  the  many  authorities  whom  they  have  con- 
sulted and  without  whose  assistance,  of  course,  the  book  could 
never  have  been  written.  So  far  as  was  practicable,  they  have 
endeavored  to  mention  them  by  name,  either  in  the  text  or  in  the 
foot-notes.  But  they  desire  to  make  special  mention  in  this  place 
of  their  obligations  to  the  writers  and  publishers  of  the  neurological 
journals  and  treatises  from  which  so  many  of  the  figures  illustrat- 
ing the  anatomy  of  the  nervous  system  have  been  borrowed. 

During  the  progress  in  preparation  of  this  edition,  the  author 
of  the  first  edition  found  that  his  obligations  to  Professor  R.  S. 
Woodworth  could  be  justly  discharged  only  by  placing  his  name 
upon  the  title-page  as  its  joint  author.  And  this  act  of  justice 
was  cheerfully  rendered. 

GEORGE  TRUMBULL  LADD. 

NEW  HAVEN,  May,  1911. 


PREFACE  TO  THE  FIRST  EDITION 

THERE  can  be  no  doubt  that  an  important  movement  in  psychol- 
ogy has  ar^en  in  recent  times  through  the  effort  to  approach  the 
phenomena  of  mind  from  the  experimental  and  physiological  point 
of  view.  Different  students  of  psychological  science  will  estimate 
differently  both  the  net  result  already  reached  by  this  effort  and 
the  promise  of  further  additions  to  the  sum  of  our  knowledge  from 
continued  investigation  of  the  same  kind.  Some  writers  have  cer- 
tainly indulged  in  extravagant  claims  as  to  the  past  triumphs  of  so- 
called  Physiological  Psychology,  and  in  equally  extravagant  expec- 
tations as  to  its  future  discoveries.  On  the  other  hand,  a  larger 
number,  perhaps,  have  been  inclined  either  to  fear  or  to  depreciate 
every  attempt  to  mingle  the  methods,  laws,  and  speculations  of  the 
physical  sciences  with  the  study  of  the  human  soul.  These  latter 
apparently  anticipate  that  some  discovery  in  the  localization  of 
cerebral  function,  or  in  psychometry,  may  jeopard  the  birthright 
of  man  as  a  spiritual  and  rational  being.  Or  possibly  they  wish 
to  regard  the  soul  as  separated,  by  nature  and  with  respect  to  its 
modes  of  action,  from  the  material  body  in  such  a  way  as  to  render 
it  impossible  to  understand  more  of  the  one  by  learning  more  about 
the  other. 

As  a  result  of  some  years  of  study  of  the  general  subject,  I 
express  with  considerable  confidence  the  opinion  that  there  is  no 
ground  for  extravagant  claims  or  expectations,  and  still  less  ground 
for  any  fear  of  consequences.  In  all  cases  of  new  and  somewhat 
rankly  growing  scientific  enterprises,  it  is  much  the  better  way  to 
waive  the  discussion  of  ac  ual  or  possible  achievements,  as  well  as 
of  welcomed  or  dreaded  revelations  of  new  truth,  and  proceed  at 
once  to  the  business  on  hand.  It  is  proposed  in  this  book  to  follow 
this  better  way.  It  will  be  the  task  of  the  book  itself  to  set  forth 
the  assured  or  alleged  results  of  Physiological  Psychology;  and 
this  will  be  done  at  every  step  with  such  degree  of  assurance  as 

ix 


x  PREFACE  TO  THE  FIRST  EDITION 

belongs  to  the  evidence  hitherto  attainable  upon  the  particular 
subject  discussed.  With  declamation,  either  in  attack  or  defence 
of  the  "old  psychology,"  of  the  "introspective  method,"  etc.,  one 
may  dispense  without  serious  loss. 

The  study  of  the  phenomena  of  consciousness  by  the  method  here 
proposed  necessarily  requires  some  acquaintance  with  a  considera- 
ble circuit  of  sciences  which  are  not  usually  all  alike  closely  allied. 
The  number  of  scholars  who  can  form  opinions  with  equal  freedom 
and  confidence  in  all  of  these  sciences  is  very  small.  Moreover, 
since  all  psycho-physical  laws  are  supposed — as  the  very  term  indi- 
cates— to  govern  the  correlations  of  phenomena  of  consciousness 
with  phenomena  of  the  nervous  system,  a  peculiar  mystery  belongs 
to  much  of  the  domain  within  which  psycho-physical  science  is 
compelled  to  move.  These  facts  may  fitly,  on  the  one  hand,  excite 
caution  in  the  writer;  and,  on  the  other  hand,  excuse  him  for 
many  inevitable  failures  to  set  forth  with  perfect  defmiteness  and 
confidence  the  conclusions  he  has  to  propose.  Much  will  be  said 
that  must  be  accepted  as  provisional,  as  only  probably  true.  Much 
room  must  also  be  made  for  conjecture  and  speculation.  What  is 
most  important,  however,  is  that  conjecture  should  not  be  put 
forth  as  ascertained  fact,  or  speculation  as  unquestioned  law. 

It  would  have  been  a  great  assistance  to  me  if  I  had  had  more 
predecessors  in  the  path  which  I  am  to  take.  But  with  the  excep- 
tion of  Wundt's  masterly  work  (Grundzuge  der  physiologischen 
Psychologic,  second  edition  in  1880),  no  one  book  has  attempted  to 
cover,  even  in  a  summary  way,  the  entire  ground.  The  number 
of  monographs,  however,  which  have  dealt  with  individual  ques- 
tions subordinate  to,  or  part  of,  the  main  inquiry  is  very  great. 
These  two  facts  also  render  the  attempt  at  a  general  survey  of 
Physiological  Psychology  for  readers  of  English  both  peculiarly 
attractive  and  peculiarly  difficult.  I  can  only  indulge  the  hope 
that  I  have  done  something  toward  breaking  this  path  and  render- 
ing it  easier  and  more  secure,  both  for  myself  and  for  others,  in  the 
future. 

The  investigators  and  authors  to  whom  I  am  under  obligations 
for  material  upon  the  various  questions  discussed,  or  statements 
made,  in  this  book  are  by  no  means  all  mentioned  by  name.  Of 


PREFACE  TO  THE  FIRST  EDITION  xi 

course,  much  of  what  is  said  on  the  structure  of  the  nervous  system, 
and  on  the  phenomena  of  sensation  and  perception,  has  already 
become  part  of  that  general  fund  of  facts  and  laws  which  belongs 
alike  to  all  students  of  the  subject.  But  by  quoting  certain  author- 
ities in  the  text,  and  by  a  few  (in  comparison  with  the  number 
which  might  have  been  cited)  references  in  foot-notes,  I  have  con- 
nected some  of  the  discoveries  and  views  of  modern  psycho-physical 
science  with  their  authors.  These  may  serve  somewhat  as  guide 
to  those  persons  who  wish  to  pursue  such  studies  still  further. 

I  am  under  particular  obligations  to  Dr.  James  K.  Thacher, 
Professor  of  Physiology  in  the  Yale  Medical  School,  for  valuable 
assistance  in  that  description  of  the  Nervous  Mechanism,  its  struct- 
ure and  functions,  which  the  First  Part  of  the  book  contains.  If  I 
have  escaped  the  mistake  of  assuming  to  teach  more  than  is  really 
known  upon  this  subject,  it  has  been  in  large  measure  due  to  his 
friendly  and  skilful  guidance.  Valuable  assistance  has  also  been 
received  from  Russell  H.  Chittenden,  Professor  of  Physiological 
Chemistry,  and  Charles  S.  Hastings,  Professor  of  Physics — both  of 
the  Sheffield  Scientific  School. 

The  method  and  arrangement  of  the  book  have  been  chosen  so 
as  to  fit  it  for  use,  both  as  a  text-book  by  special  students  of  the 
subjects  of  which  it  treats,  and  also  by  the  general  reader  who  is 
interested  in  knowing  what  results  have  been  reached  by  the  more 
modern — and  even  the  latest  — psycho-physical  researches. 

GEORGE  T.  LADD. 

YALE  UNIVERSITY,  NEW  HAVEN,  February,  1887. 


TABLE  OF  CONTENTS 


PAGE 

INTRODUCTION  . .  1-10 


PART  FIRST 
THE  NERVOUS  MECHANISM 

CHAPTER  I 

THE  PLACE  OP  THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 13-35 

§  1,  Necessity  of  the  Comparative  Method. — §  2,  Primary  Division  of 
Animals. — §§  3-4,  Description  of  the  Amoeba. — §  5,  Specialization  of  Re- 
ceptors and  Effectors. — §§  6-8,  Nerve-Net  Type  of  Nervous  System. — 
§§  10-11,  Segmented  Type  of  Nervous  System.— §  12,  Centralized  Type 
of  Nervous  System.— §§  13-15,  Nerve-Centres  of  Vertebrates. — §  16, 
Structure  of  the  Cerebellum.— §  17,  The  Mid-Brain.— §  18,  Inter-Brain 
and  End-Brain.— §  19,  Development  of  the  Cortex.— §§  20-21,  Relation  of 
Size  to  Intelligence. — §  22,  Essential  Function  of  Nervous  Tissue. 

CHAPTER  II 

THE  DEVELOPMENT  OP  THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 36-62 

§§  1-2,  Reproduction  in  Unicellular  Organisms. — §  3,  Formation  of 
the  Neural  Groove.— §  4,  Constitution  of  the  Neural  Tube.— §§  5-6, 
Growth  of  Nerve  Axons  and  Fibres. — §  7,  Formation  of  the  Spinal  Cord. 
— §§  8-9,  Growth  of  the  Five  Brain  Vesicles.— §§  10-11,  Development  of 
the  Cortex.— §§  13-14,  The  Twelve  Pairs  of  Cranial  Nerves.— §§  15-16, 
Factors  in  Growth  of  the  Nervous  System. — §  17,  Function  of  the  Myelin 
Sheath. — §§  18-20,  Variations  in  Weight  of  Human  Brain. 

CHAPTER  III 

GROSS  STRUCTURE  OP  THE  NERVOUS  SYSTEM 63-96 

§  1,  Importance  of  Relation  between  Nervous  Elements. — §§  2-3,  Dif- 
ferentiation of  Nervous  Function. — §  4,  Distinction  of  Sympathetic  and 
Cerebro-Spinal. — §  5,  Membranes  of  Brain  and  Spinal  Cord. — §§  6-7,  Col- 
umns and  Commissures  of  the  Cord. — §§  8-9,  White  and  Gray  Matter  of  the 
Cord.— §§  10-11,  Nerve-Fibres  and  Nerve-Cells  in  the  Cord. — §  12,  Different 
Aspects  of  the  Encephalon.— §  13,  Twelve  Pairs  of  Cranial  Nerves. — §§  14- 

xiii 


xiv  TABLE  OF  CONTENTS 

15,  Internal  Structure  of  Brain-Stem. — §§  16-18,  Continuations  of  the 
Brain-Stem.— §§  19-21,  Structure  and  Relations  of  Inter-Brain. — §  22,  The 
Cerebellum. — §§  23-24,  Tracing  and  Naming  of  Nerve-Tracts. — §§  25-27, 
Nerve-Tracts  and  Their  Connections. — §  28,  Principal  Motor  Pathway. — 
§§  29-35,  Systems  of  Nerve-Centres  and  Connections. 

CHAPTER  IV 

PAGE 

ELEMENTS  OF  THE  NERVOUS  STRUCTURE 97-116 

§§  1-2,  Development  of  the  Nervous  Elements. — §  3,  Structure  of  the 
Nerve-Fibre. — §  4,  Size  of  Different  Nerve-Fibres. — §  5,  Constitution  of  the 
Gray  Matter. — §  6,  Size  and  Shape  of  Nerve-Cells.— §§  7-8,  Internal  Struct- 
ure of  Nerve-Cells. — §  9,  Arrangement  of  Nerve-Cells. — §  10,  Structure 
and  Office  of  the  Dendrites.— §  11,  Nerve-Cells  of  the  Cerebellum.—!  12, 
Structure  and  Function  of  the  Axon. — §  13,  The  Neurone  Theory. — §14, 
The  Neuroglia  Cells. 

CHAPTER  V 

CHEMISTRY  OF  THE  NERVOUS  SYSTEM 117-126 

§§  1-2,  General  Chemistry  of  Brain  and  Nerves. — §  3,  Chief  Chemical 
Elements  of  Brain. — §§  4-5,  The  Lipoids  of  the  Nervous  Substance. — §  6, 
Non-Fatty  Substances  in  Brain. — §  7,  Cholesterin. — §§  8-11,  The  Phosphor- 
ized  Lipoids  in  the  Nervous  Substance. — §  12,  Specific  Chemistry  of  Nervous 
Elements. 

CHAPTER  VI 

THE  NERVES  AS  CONDUCTORS 127-144 

§§  1-2,  Most  General  Function  of  the  Nerves. — §  3,  Methods  of  Experi- 
mentation.— §  4,  Excitability  of  the  Nerve. — §  5,  The  Laws  of  Du  Bois- 
Reymond. — §  6,  The  Refractory  Period. — §  7,  Speed  of  the  Nervous 
Current. — §  8,  Alterations  in  Conductivity  of  Nerves. — §  9,  Double  Con- 
duction of  the  Nerves. — §  10,  Nature  of  the  Process  Conducted.—!  11, 
v  Phenomena  of  Fatigue. — §  12,  The  Current  of  Action. — §§  13-14,  Electro- 
tonic  Changes  in  Nerves. — §  15,  The  Core  Model  or  Core  Conductor. — 
§§  16-17,  The  Current  of  Rest.— §  18,  Theories  of  Nervous  Function. 

CHAPTER  VII 

REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 145-174 

§§  1-2,  Nature  of  Reflex  Arc. — §  3,  Instinctive  and  Simple  Reflexes.— 
§§  4-5,  Automatic  Action  of  Nervous  Centres.— §§  6-7,  The  Ganglionic 
Reflexes. — §  8,  Reflexes  of  the  Skeletal  Musculature. — §§  9-12,  Local  Re- 
flexes of  the  Spinal  Cord. — §§  13-14,  Reflexes  of  the  Cerebellum. — §  15, 
Reflexes  of  the  Medulla. — §  16,  Influence  of  Cerebrum  on  Lower  Centres. 
— §§  17-18,  Nervous  Mechanism  of  Reflexes.— §§  19-20,  Phenomena  of 
Inhibition. — §§  21-22,  The  Refractory  Period. — §  23,  Diphasic  Impulses. 
— §  24,  Latent  Time  of  Nervous  Reflexes.— §§  25-26,  Intensity  and  Ex- 


TABLE  OF  CONTENTS  xv 

tent  of  Reflexes. — §§  27-28,   Facilitation  of  Reflexes.— §§  29-30,   Inter- 
ference of  Reflexes. — §  31,  General  Characteristics  of  Reflex  Action. 


CHAPTER  VIII 

PAGE 

END-ORGANS,  OR  RECEPTORS,  OF  THE  NERVOUS  SYSTEM 175-212 

§§  1-3,  Function  of  End-Organs  in  the  Nervous  System. — §§  4-5,  End- 
Organs  of  Smell. — §§  6-7,  End-Organs  of  Taste. — §§  8-9,  End-Organs  of 
Touch.—§  10,  The  Pacinian  Corpuscles.— §§  11-12,  Structure  of  the  Eye- 
Ball. — §  13,  Refracting  Media  of  the  Eye. — §  14,  Muscles  of  the  Eye-Ball. — 
§  15,  The  Problem  Before  the  Organ  of  Vision. — §  16,  Indices  of  Refraction. — 
§  17.  Mechanism  of  Accommodation. — §  18,  The  Stimulus  of  Vision. — 
§§  19-20,  Nervous  Elements  of  the  Retina. — §  21.  The  Yellow  Spot  and  the 
Blind-Spot. — §  22,  Office  of  the  Rods  and  Cones. — §  23,  Chemical  Changes 
in  the  Retina. — §  24,  General  Description  of  the  Eye. — §  25,  Three  Princi- 
pal Parts  of  the  Ear.— §  26,  Structure  of  the  Middle  Ear.— §  27,  Office 
of  the  Tympanum.—§  28,  The  Eustachian  Tube.— §  29,  Structure  of  the 
Internal  Ear.— §  30,  Distribution  of  the  Auditory  Nerve. — §  31-32,  The 
Organ  of  Corti. — §  33,  Theory  of  Sympathetic  Vibration. — §§  34-36,  Func- 
tion of  the  Semicircular  Canals. — §  37,  End-Organs  of  Motion. 

CHAPTER  IX 

THE  CEREBRAL  HEMISPHERES  AND  THEIR  FUNCTIONS 213-234 

§§  1-4,  The  Problem  of  Cerebral  Functions. — §  5,  Evidence  from  Com- 
parative Anatomy. — §§  6-7,  Evidence  from  Removal  or  Injury. — §  8,  The 
Problem  Stated. — §  9,  General  Description  of  Cerebral  Hemispheres. — §  10, 
Principal  Divisions  of  the  Cortex. — §  11,  Projection  and  Association  Fibres. 
— §§  12-13,  Nervous  Elements  of  the  Cortex. — §  14,  History  of  the  Investiga- 
tion.—§  15,  The  Three  Lines  of  Evidence. — §  16,  The  Method  of  Extirpa- 
tion.— §  17,  Evidence  of  Human  Pathology. — §  18,  Evidence  from  His- 
tology.— §  19,  Summary  of  the  Situation. 

CHAPTER  X 

THE  CEREBRAL  HEMISPHERES  AND  THEIR  FUNCTIONS  [Continued] ....  235-274 

§  1,  The  Work  of  Fritsch  and  Hitzig. — §§  2-3,  Localization  in  the  Brains 
of  Monkeys  and  Anthropoid  Apes. — §  4,  Paralyses  of  the  Motor  Area. — 
§§  5-7,  Definition  and  Function  of  the  Motor  Area. — §  8,  Restitution  of 
Motor  Functions.— §§  9-11,  The  Somesthetic  Area. — §  12,  The  Visual  Area. — 
§§13-14,  Phenomena  of  Psychical  Blindness. — §  15,  The  Auditory  Centre. — 
§§16-17,  Centres  of  Smell  and  Taste. — §  18,  The  So-called  "Silent  Areas." 
— §  20,  Results  of  Injuries  to  the  Occipital  Lobe. — §  21,  To  the  Temporal 
Lobe.— §§  22-23,  To  the  Parietal  Lobe.— §§  24-25,  Disturbances  of  Speech 
Functions. — §§  26-29,  Discussion  of  Broca's  Speech  Centre. — §  30,  Localiza- 
tion of  a  Writing  Centre. — §§  31-33,  Functions  of  the  Frontal  Lobe. — §  34, 
Summary  of  the  Results.— §§  35-38,  Cell-Layers  in  the  Cortex.— §§  39-40, 
Histological  Mapping  of  the  Cortex. — §§  41-43,  Grouping  of  Functions  in 
the  Cortex. 


xvi  TABLE  OF  CONTENTS 

CHAPTER  XI 

PAGE 

MECHANICAL  THEORY  OF  THE  NERVOUS  SYSTEM 275-293 

§§  1-3,  Machine-like  Nature  of  the  Organism. — §§  4-5,  Significance  of 
Chemical  Constitution. — §§  6-7,  Arrangement  of  the  Nervous  Elements. 
§  8,  Equilibrating  of  Different  Parts. — §§  9-10,  Imperfect  Character  of 
Present  Theory.— §§  11-12,  Theory  of  Specific  Energies.— §§  13-15,  Mech- 
anism of  the  Nerve-Centres. — §§  16-17,  Function  of  the  Nerve-Cells. — 
§§  18-19,  Significance  of  the  Synapse. — §  20,  Rival  Metabolic  Theories. — 
§§  21-23,  Review  of  the  Evidence. 


PART  SECOND 

CORRELATIONS  OF  THE  NERVOUS  MECHANISM 
AND  MENTAL  PHENOMENA 

CHAPTER  I 
THE  QUALITY  OF  SENSATIONS 297-323 

§§  1-3,  Changes  in  Points  of  View. — §§  4-5,  Classification  of  the  Sensa- 
tions.— §§  6-7,  Need  of  Further  Analysis. — §  8,  The  So-called  Simple 
Sensation  a  Fiction. — §  9,  Questions  Requiring  an  Answer. — §  10,  Bear- 
ing of  the  Specific  Energy  of  the  Nerves. — §§  11-12,  Stimuli  of  Olfactory 
Sensations. — §§  13-15,  Analysis  of  Olfactory  Sensations. — §  16,  Stimuli 
of  Gustatory  Sensations. — §§  17-18  Analysis  of  Gustatory  Sensations. — §  19, 
Chemical  Stimulus  of  Sensations  of  Taste. — §§  20-21,  General  Nature  of 
Auditory  Sensations. — §  22,  Characteristics  of  Musical  Sounds. — §  23,  The 
Pitch  of  Musical  Sounds. — §  24,  Sensitiveness  of  the  Ear  to  Differences  of 
Pitch. — §  25,  And  Purity  of  Interval. — §  26,  Means  of  Judging  Musical 
Sounds. — §  27,  Nature  of  the  "Clang." — §  28,  Sounds  Used  in  Music. — 
§§  29-30,  Theory  of  Consonance  and  Dissonance. — §  31,  The  Difference- 
Tone. 

CHAPTER  II 

THE  QUALITY  OF  SENSATIONS  [Continued] 324-352 

§  1,  Intricacy  of  Visual  Sensations. — §  2,  Stimulus  of  Visual  Sensations. 
— §  3,  Local  Values  of  the  Retina.— §  4,  The  General  Problem  of  Visual 
Sensations. — §  5,  The  Spectral  Color-Tones. — §  6,  Relative  Brightness  of 
Color-Tones. — §  7,  Composite  Nature  of  Ordinary  Colors. — §  8,  Number 
of  Colors  Distinguishable. — §§  9-12,  Theory  of  Complementary  Colors. — 
§§  13-14,  Phenomena  of  Color-Blindness. — §§  15-16,  Negative  and  Positive 
After-images. — §  17,  Phenomena  of  Contrast. — §§  18-21,  Theories  of 
Color  Vision. — §  22,  Symbolic  Representation  of  Visual  Sensations. — 
§§  23-24,  Sensations  of  the  Skin. — §  25,  Nature  of  Temperature  Stimuli. — 
§§  26-27,  Sensations  of  Pain.— §  28,  The  So-called  Muscular  Sense.— §  29, 
Labyrinthic  Sensations. — §  30,  Visceral  or  Organic  Sensations. — §  31,  Sum- 
mary of  Evidence  for  the  Specific  Energy  of  the  Nerves. 


TABLE  OF  CONTENTS  xvii 

CHAPTER  III 

PAGE 

THE  QUANTITY  OF  SENSATIONS 353-379  * 

§§  1-2,  Distinction  of  Quantity  from  Quality. — §§  3-4,  Unscientific 
Form  of  Ordinary  Usage. — §§  5-6,  Character  of  the  Quantitative  Problems. 
— §  7,  Method  of  Determining  the  Limits. — §  8,  Methods  of  Determining 
the  Least  Perceptible  Difference. — §§  9-11,  Weber's  Law  and  Fechner's 
Formulas. — §§  12-13,  The  Perception  of  Weight. — §  14,  Sensitiveness  to 
Light  Pressure. — §  15,  Discriminations  of  Temperature. — §§  16-17,  Sensi- 
tiveness of  Acoustic  Perception. — §  18,  Least  Perceptible  Difference  in  In- 
tensity of  Tones. — §§  19-20,  Quantitative  Discriminations  of  Sight. — §  21, 
Weber's  Law  Applied  to  Visual  Sensations. — §  22,  Perceptible  Minimum  of 
Sensations  of  Light. — §  23,  Extensive  Sensations  of  Light. — §§  24-25,  In- 
tensity of  Gustatory  Sensations. — §  26,  Intensity  of  Sensations  of  Smell, — 
§  27,  Review  of  Weber's  Law. — §  §  28-29,  Explanations  of  Weber's  Law, — 
§  30,  Fechner's  Intepretation  of  the  Phenomena. — §  31,  Summary  of  Re- 
sults. 

CHAPTER  IV 

PRESENTATIONS  OF  SENSE,  OR  SENSE-PERCEPTIONS 380-412  */ 

§  1,  Artificial  Character  of  Simple  Sensations. — §  2,  Necessity  of  Analy- 
sis.— §§  3-4,  Nature  and  Stages  of  Sense-Perception. — §  5,  Necessity  of 
Mental  Synthesis  or  Fusion. — §§  6-7,  Nativistic  and  Empiristic  Schools. — 
§  8,  Nature  of  a  Spatial  Series. — §  9,  Nature  of  the  Local  Signs. — §  10,  Stages 
of  Sense-Perception. — §  11,  Activity  of  Higher  Faculties. — §  12,  Perceptions 
of  Smell. — §  13,  Perceptions  of  Taste. — §  14,  Perceptions  of  Hearing. — 
§§  15-18,  Localization  of  Sounds. — §§  19,  Construction  of  the  Field  of 
Touch. — §§  20-22,  Explanation  of  Weber's  "Sensation-Circles." — §§  23-25, 
The  "Two-Point  Threshold." — §  26,  Sensuous  Basis  of  Discrimination. — 
§  27,  Mixed  and  Tangled  Skin-Sensations. — §  §  28-29,  Localization  of  Tem- 
perature.— §  30,  Nature  of  the  Muscular  Sense. — §§  31-32,  Judgments  of 
Bodily  Movements. — §  33,  Co-operation  of  Eye  with  Hand. — §  34,  Feelings 
of  Double  Contact. 

CHAPTER  V 

PRESENTATIONS  OF  SENSE,  OR  SENSE-PERCEPTIONS  [Continued] 413-469  *^ 

§  1,  Special  Difficulties  of  Visual  Perceptions.— §§  2-3,  Data  of  Visual 
Perceptions. — §§  4-5,  Formation  of  the  Retinal  Field. — §  6,  Statement  of 
the  Problem. — §  7,  Values  of  Different  Retinal  Sensations. — §  8,  Muscles 
of  the  Eye-Ball  and  Its  Movements. — §  9,  The  Law  of  Listing. — §  10,  Effects 
of  Accommodation. — §§  11-13,  Conditions  of  Binocular  Vision. — §  14,  Vis- 
ual Perception  of  Depth. — §§  15-16,  Visual  Perception  of  Distance  and 
Size. — §  17,  Visual  Perceptions  of  Motion. — §§  18-19,  Judgment  in  Errors 
of  Sense. — §  20,  Geometrical  Optical  Illusions. — §  21,  Central  Factor  in 
Illusions. — §  22,  Illusions  of  Angles. — §  23,  Illusions  of  Areas. — §  24, 
Theories  of  Visual  Illusions. — §  25,  The  Central  Theories. — §  26,  The 
Dynamic  Theory. — §  27,  The  Confusion  Theory. — §  28,  Binocular  Mixing 
and  Contrast  of  Colors. — §§  29-30,  Upright  and  Inverted  Vision. — §  31, 
Inferences  from  Errors  of  Sense. — §  32,  General  Conclusions  as  to  Theory 
of  Vision. — §  33,  Influence  of  Eye-Movements  on  Space  Perception. — 
§  34,  Graphic  Records  of  Eye-Movements. — §§  35-36,  Speed  of  Eye-Move- 


xviii  TABLE  OF  CONTENTS 

ments. — §  37,  Value  of  Objective  Measurements. — §§  38-39,  Development 
of  Visual  Perception. — §  40,  Relations  of  Visual  and  Muscular  Perception. — 
§  41,  Perception  as  an  Achievement  of  Mind. 

CHAPTER  VI 

PAGE 

TIME-RELATIONS  OF  MENTAL  PHENOMENA 470-499 

§§  1-2,  Experiments  in  Reaction-Time. — §  3,  Simple  Reaction-Time. 
— §  4,  Inertia  of  the  End-Organs. — §  5,  Measurement  of  Smallest  Intervals. 
— §  6,  The  Point  of  Starting. — §§  7-8,  Variations  in  Reaction-Time.— §  9, 
Effect  of  Increasing  Intensity. — §  10,  Character  of  Reacting  Movement. — 
§  11,  Influence  of  Central  Conditions.—!  12,  The  Three  Periods.—!  13, 
Wundt's  Analysis. — §  14,  So-called  Sensorial  and  Muscular  Reactions. — 
§  15,  Complex  Reactions. — §§  16-18,  Reaction  with  Discrimination. — §  19, 
Discernment  of  Intensities. — §§  20-22,  Associative  Reaction-Time. — §§  23- 
24,  Amount  of  Conscious  Process  Involved. — §§  25-26,  General  Character  of 
Reaction-Time. 

CHAPTER  VII 
FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 500-541 

§§  1-2,  Variable  Conceptions  of  the  Nature  of  Feeling. — §§  3-4,  Difficulties 
of  Analysis. — §  5,  Methods  of  Investigation. — §  6,  Physiological  Theories 
of  Feeling. — §  7,  Kinds  of  Feeling. — §  8,  Pleasantness  and  Unpleasantness. 
— §  9,  Induced  Electrical  Changes. — §§  10-11,  Other  Theories  of  Feeling. 
— §  12,  General  Classification  of  the  Feelings. — §  13,  Changeable  Charac- 
teristics of  all  Feeling. — §§  14-15,  Pleasureable  and  Painful  Tone  of  Feeling. 
— §  16,  Forms  of  Common  Feeling. — §§  17-18,  Feeling-Tone  of  Sensations. — 
§  19,  Mixtures  of  Feeling.— §§  20-22,  Nature  of  the  Emotions.—!  23, 
Peripheral  Theory  of  the  Emotions. — !  24,  Teleological  Value  of  the  Emo- 
tions.— !  25,  The  Intellectual  Feelings. — §§  26-27,  The  ^Esthetic  Feelings. 
— §  28,  Nature  of  the  Sentiments. — §§  29-30,  Dynamogenic  Effects  of 
the  Emotions. — !  31,  Automatic  and  Ideo-Motor  Movements. — !!  32-33, 
Phenomena  of  Fatigue.—!!  34-35,  Causes  of  Fatigue.— §  36,  The  Kinds 
of  Fatigue.—§  37,  The  Refractory  Period. 

CHAPTER    VIII 
MEMORY  AND  THE  PROCESS  OF  LEARNING 542-592 

§  1,  Memory  as  Recognitive,  Inexplicable. — §§  2-3,  Processes  Involved 
in  Memory. — !  4,  Learning  Among  Invertebrates. — §  5,  Learning  Among 
Vertebrates. — !  6,  Process  of  Selection  in  Learning. — !  7,  Adjustment  of 
Psycho-physical  Mechanism. — §  8,  Learning  by  Trial  and  Error. — §!  9-10, 
Learning  by  Ideas. — !  11,  Solution  of  Mechanical  Puzzles. — !  12,  Acquire- 
ment of  Skill.— §§  13-14,  Analysis  of  Feats  of  Skill.—!  15,  The  Rate  of 
Improvement  by  Practice. — !  16,  Relation  of  Consciousness  to  Learning. 
— !!  17-19,  Transference  of  Learning — !!  20-21,  Processes  of  Memorizing. 
— !  22,  Methods  of  Studying  Memory. — §  23,  The  Curve  of  Forgetting. — 
§!  24-25,  Forming  of  Association. — !  26,  The  Psycho-physical  Mechanism. — 
§§  27-28,  The  Culture  of  Memory. — !!  29-31,  Phenomena  of  Reproduction. 
— !  32,  Inhibition  of  Reproductive  Processes. — !  33,  Phenomena  of  Per- 
severation. — §§  34-35,  Varieties  of  Association. — §§  36-37,  Recall  with 
Recognition. — §  38,  The  Guarantee  of  Memory. 


TABLE  OF  CONTENTS  xlx 

CHAPTER  IX 

PAGE 

THE  MECHANISM  OF  THOUGHT 593-625 

§§  1-2,  Logical  Terms  Inadequate.—!  3,  Perception  as  a  Form  of  Re- 
action.— §  4,  The  Process  of  Abstraction. — §  5,  The  Span  of  Attention. — 
§§  6-7,  Shifting  and  Fluctuation  of  Attention. — §  8,  The  Direction  of  Atten- 
tion.— §  9,  Development  of  Abstract  Concepts. — §  10,  The  Nature  of 
Comparison. — §  11,  The  Psychology  of  Reasoning. — §  12,  Reasoning  as 
Search  for  Premises. — §  13,  Immediate  and  Mediate  Reasoning. — §§  14-15, 
Correlations  of  Mechanism  and  Mental  Life. — §§  16-18,  Cerebral  Conditions 
of  Consciousness. — §  19,  The  Mechanism  of  Attention. — §  20,  The  Power 
of  Varied  Reaction.— §§  21-22,  Physiology  of  Local  Signs.— §§  23-25, 
Cerebral  Action  in  Memory. — §§  26-28,  The  Mechanism  of  Associations. — 
§§  29-30,  Physiology  of  the  Higher  Units.— §§  31-33,  Collecting  and  Dis- 
tributing Mechanisms. — §  34,  Limitations  of  all  Explanation. 


PART  THIRD 
THE  NATURE  OF  THE  MIND 

CHAPTER  I 
GENERAL  RELATIONS  OF  BODY  AND  MIND 629-667 

§§  1-2,  The  Metaphysical  Problem  Proposed. — §  3,  Nature  of  Uncriti- 
cal Dualism.—§  4,  Reality  of  Correlations.— §§  5-7,  The  Brain  as  "Seat" 
of  the  Mind.— §§  8-10,  The  Brain  as  "Organ"  of  the  Mind.— §  11,  The 
Conception  of  a  Bond  Between  Brain  and  Mind. — §  12,  The  Body  as  the 
Tenement  of  Mind. — §§  13-14,  The  Conception  of  Product  as  Applied  to 
the  Case. — §§  15-16,  Reciprocal  Influence  of  Body  and  Mind. — §§  17-18, 
General  Objections  to  Causal  Theory. — §§  19-20,  Analysis  of  Conceptions. 
— §§  21-23,  Conservation  of  Cerebral  Energy. — §  24,  Some  Kind  of  Causal 
Relation  Necessary. — §  25,  Necessity  of  a  Further  Hypothesis. — §  26,  The 
Three  Processes  Involved. — §§  27-29,  The  Conception  of  Development 
Necessary. — §  30,  Mental  Condition  of  the  Embryo. — §  31,  Earliest  Develop- 
ment of  the  Child. — §  32,  Development  of  the  Adult. — §  33,  The  Changes  of 
Old  Age. — §  34,  Argument  from  Phenomena  of  Decay. — §  35,  Peculiarly 
Mental  Elements. — §  36,  Peculiarly  Mental  Operations. — §  37,  The  Con- 
clusion Drawn. 

CHAPTER  II 
REALITY  AND  UNITY  OF  THE  MIND 668-687 

§§  1-2,  Nature  of  the  Claims  Discussed. — §  3,  Conceptions  of  Reality 
and  Unity  Expounded. — §  4,  Unity  as  Involving  Plan. — §§  5-6,  Reality 
and  Unity  of  Mind,  not  Merely  Phenomenal. — §§  7-8,  The  Activity  of 
Apperception. — §§  9-11,  Mental  Unity  not  Atomic. — §§  12-13,  The  Dis- 
tinction of  Ego  and  Non-Ego. — §§  14-15,  Mind  as  Unifying  Actus. — §  16, 
Mind  not  a  Static  Unity.— §§  17-18,  The  Spirituality  of  Mind.— §§  19-20, 
Mind  as  a  Unity  of  Growth. — §§  21-22,  Limits  of  Physiological  Psychology. 

INDEX  OF  AUTHORS 689 

INDEX  OF  SUBJECTS  . .  .   694 


PHYSIOLOGICAL   PSYCHOLOGY 


INTRODUCTION 

§  1.  A  clear  conception  of  Physiological  Psychology  requires  some 
knowledge  of  the  nature  and  methods  of  those  two  sciences,  the 
results  of  whose  investigations  it  endeavors  to  combine.  These 
sciences  are  Psychology  and  Physiology — the  latter  being  under- 
stood so  as  to  include  also  various  applications  of  the  general 
theory  of  physics  to  the  functions  of  the  animal  organism.  But  as 
the  form  taken  by  this  compound  term  would  itself  seem  to  indicate, 
the  two  do  not  stand  upon  precisely  the  same  level  in  effecting  this 
combination,  whether  we  consider  the  end  that  science  desires  to 
reach,  or  the  means  that  it  employs  to  reach  the  end.  For  the  noun 
("  psychology")  in  the  compound  term  may  be  said  more  particularly 
to  define  this  end;  while  the  adjective  ("physiological")  defines  the 
character  of  the  means  which  it  is  proposed  especially  to  employ. 
Hence  "Physiological  Psychology"  can  scarcely  claim  to  be  an  in- 
dependent science;  it  is  rather  to  be  regarded  simply  as  psychology 
approached  and  studied  from  a  certain — the  so-called  "physiologi- 
cal " — side  or  point  of  view.  It  is  necessary,  then,  in  the  first  place, 
to  define  what  we  understand  by  the  science  of  psychology. 

§  2.  Perhaps  the  most  common  definition  of  psychology,  until 
recent  times,  has  regarded  it  as  "  the  science  of  the  human  soul." 
If  this  definition  had  always  been  taken  only  in  a  provisional  way, 
and  with  the  implied  confession  that  it  is  the  business  of  psychol- 
ogy itself  to  demonstrate  the  existence  of  "  the  soul, "  and  to  show 
how  such  an  entity  is  needed  to  explain  the  phenomena  of  con- 
sciousness, then  little  valid  objection  could  have  been  made  to  it. 
But  such  has  by  no  means  been  the  case.  Objections  have,  there- 
fore, been  more  or  less  fitly  and  forcefully  urged  against  this  defi- 
nition as  ordinarily  employed.  It  has  been  said  that  clearly  we 
have  no  right  to  assume  any  such  entity  as  the  soul ;  and  it  has  even 
been  claimed,  especially  of  late,  that  there  may  be  a  "psychology 
without  a  soul,"  and,  indeed,  that  this  kind  of  psychology  is  alone 
worthy  of  being  considered  truly  scientific.  Further  objection  to 
the  same  definition  has  been  made  in  other  quarters,  because  it 


2  ^INTRODUCTION 

seeins^ito  regard  the  question  as  settled,  whether  there  may  not  be 
more  than  one  subject  (or  "ground")  of  the  manifold  phenomena 
called  psychical.  Recent  researches  into  so-called  "  subconscious- 
ness"  as  involving  mental  processes  which  go  on  "below  the 
threshold,"  and  theories  of  double  and  triple  selves,  have  served 
further  to  confuse  or  discredit  the  time-honored  concept  of  a 
soul,  or  mind,  as  a  permanent  and  quasi-independent  entity.  It 
would  be  aside  from  the  course  of  our  inquiries  to  consider  these 
objections  in  detail  at  this  time.  They  may  all  be,  for  the  present, 
excluded  by  stating  the  course  of  procedure  which  the  study  of  psy- 
chology from  the  physiological  point  of  view  seems  to  us  plainly  to 
recommend. 

Accordingly,  it  will  serve  our  purpose  best  to  define  our  science, 
in  at  least  a  preliminary  way,  by  ascribing  to  it  a  certain  more  or 
less  definite  sphere  of  phenomena.  We  shall,  therefore,  consider 
psychology  as  that  science  which  has  for  its  primary  subject  of 
investigation  all  the  phenomena  of  human  consciousness,  or  of  the 
sentient  life  of  man.  If  the  term  "sentience"  be  employed  as  pref- 
erable to  consciousness,  it  must  be  understood  as  equivalent  to 
consciousness  in  the  broader  sense  of  the  latter  word.  This  defini- 
tion, or  rather  description,  plainly  implies  an  acquaintance  experi- 
mentally with  certain  phenomena  that  cannot,  strictly  speaking, 
be  defined.  These  are  the  phenomena  of  consciousness;  and  one 
result  of  all  our  subsequent  investigations  will  be  to  show  us  that 
consciousness  and  its  primary  phenomena  can  never  be  defined. 

Nevertheless  it  would  be  very  inconvenient,  not  to  say  impos- 
sible, to  begin  and  continue  the  investigation  of  psychical  phenom- 
ena, using  only  roundabout  phrases  through  fear  of  implying  the 
real  existence  of  some  spiritual  entity  called  the  Soul  or  the  Mind. 
In  some  sort  there  cannot  be  any  description,  much  less  any  scien- 
tific study,  of  the  phenomena  of  consciousness  without  employing 
some  word  like  these.  In  all  languages,  and  in  the  constant  every- 
day use  of  them  all,  men  in  stating  and  describing  the  phenomena  of 
their  own  sentient  life  make  the  distinctions  involved  in  such  terms 
as  "I"  and  "me,"  and  place  in  a  kind  of  contrast  with  them  such 
other  terms  as  "thou"  and  "he"  or  "it."  In  all  the  earlier  part 
of  this  treatise  the  word  "mind"  will,  therefore,  be  employed  simply 
as  the  equivalent  of  the  subject  of  the  phenomena  of  consciousness. 
In  other  words,  whatever  all  men  mean  by  the  word  "I"  (the  em- 
pirical ego  of  philosophy),  whenever  they  say  I  think,  or  feel,  or 
intend  this  or  that;  and  whatever  they  understand  others  to  mean  by 
using  similar  language — thus  much,  and  no  more,  we  propose  at 
first  to  include  under  the  term  "  mind."  This  term  is  preferred  to 
the  word  "  soul,"  in  part  out  of  concession  to  the  prejudices  to  which 


INTRODUCTION  3 

reference  has  already  been  made,  and  in  part  because  it  seems  to 
admit  of  the  handling  which  it  is  proposed  to  give  to  it  subsequently, 
with  more  freedom  from  entangling  alliances  with  ethical,  social, 
and  religious  ideas.  In  brief,  we  wish  to  begin  and  continue  our 
investigation,  as  far  as'possible,  upon  purely  scientific  grounds. 

§  3.  In  accordance  with  what  has  already  been  said  concerning 
the  nature  of  psychology,  we  may  define  Physiological  Psychology 
as  the  science  which  investigates  the  phenomena  of  human  con- 
sciousness with  the  "physiological"  point  of  view  and  method  of 
approach;  or,  remembering  the  cautions  which  have  already  been 
expressed,  we  may  say  that  it  is  the  science  of  the  human  mind 
as  investigated  by  means  of  its  relations  to  the  human  physical  or- 
ganism. A  more  accurate  definition,  however,  requires  that  some- 
thing further  should  be  said  concerning  the  nature  and  method  of 
that  science  which  furnishes  the  adjective  to  our  compound  term. 
Human  Physiology  is  the  science  of  the  functions  (or  modes  of  the 
behavior)  of  the  human  physical  organism.  As  studied  at  present 
it  implies  an  acquaintance  with  the  fields  of  gross  and  special  micro- 
scopic anatomy  (histology),  of  embryology  and  the  general  doctrine 
of  development,  of  biology, — including  the  allied  phenomena  of 
plant  life, — of  molecular  physics  and  chemistry  as  related  to  the 
structure  and  action  of  the  bodily  tissues;  and  of  other  forms  of 
kindred  knowledge.  It  is  only  a  relatively  small  part  of  this  vast 
domain,  however,  with  which  Physiological  Psychology  has  directly 
to  deal;  for  it  is  only  a  part  of  the  human  organism  which  has  any 
direct  relation  to  the  phenomena  of  consciousness.  As  will  appear 
subsequently,  it  is  with  the  nervous  system  alone  that  our  science 
has  its  chief  immediate  concern.  Indeed  it  might  be  described — 
though  in  a  still  somewhat  indefinite,  but  more  full  and  complete 
way — as  the  science  which  investigates  the  correlations  that  exist 
between  the  structure  and  functions  of  the  human  nervous  mechan- 
ism and  the  phenomena  of  consciousness;  and  which  derives  there- 
from conclusions  as  to  the  laws  and  nature  of  the  so-called  mind, 
or  subject  of  these  phenomena. 

§  4.  Physiology  is  compelled,  from  its  very  nature  as  a  physical 
science,  to  regard  the  nervous  system  as  a  mechanism.  Physiologi- 
cal Psychology,  inasmuch  as  it  relies  so  largely  upon  physiology  for 
its  data  and  method  and  points  of  view,  is  also  required  to  consider 
this  system  in  the  same  way.  Those  unique  relations  in  which  the 
structure  and  functions  of  the  nervous  substance  of  the  body  stand 
to  the  phenomena  of  mental  life  cannot  deter  the  investigator  from 
assuming  toward  it  the  so-called  mechanical  point  of  view.  Physi- 
ology presejUs^psychology  with  a  description  of  this  nervous  sub- 
stance as  a  vast  and  complex  system  of  materialjnolecules,  which 


4  INTRODUCTION 

are  acted  upon  by  different  forms  of  the  energy  of  nature  outside 
(external  stimuli),  and  by  intimate  changes  in  the  contiguous  mole- 
cules of  the  other  substances  of  the  body  (internal  stimuli);  and 
which  behave  as  they  do  on  account  of  the  influences  thus  received, 
as  well  as  on  account  of  their  own  molecular  constitution  and  ar- 
rangement. But  all  this  is  the  description  of  a  material  mechanism. 
Indeed,  it  is  only  as  falling  under  this  general  conception  that  these 
molecules  admit  of  any  scientific  treatment  at  all. 

Whatever  is  to  be  said  further  upon  the  conception  of  the  nervous 
system  as  a  mechanism  must  appear  in  its  proper  place  in  the  order 
adopted  for  the  discussion  of  the  general  subject.  Physiological 
Psychology,  however,  can  scarcely  establish  itself  at  all  unless  it 
is  willing  to  receive  from  the  proper  one  of  the  two  sciences  which 
enter  into  it  the  conclusions  at  which  this  science  has  arrived  as  the 
result  of  the  most  successful  modern  researches.  As  far  as  the  ner- 
vous system  admits  of  being  subjected  at  all  to  scientific  treatment, 
for  the  purpose  of  attaining  a  more  complete  knowledge  of  the  nature 
of  its  functions,  it  is  necessarily  considered  as  a  complex  molecular 
mechanism.  We  shall,  then,  receive,  in  a  grateful  and  docile  man- 
ner, all  that  the  noble  science  of  human  physiology  has  to  teach  us, 
under  the  guidance  of  the  conception  of  a  mechanism,  both  directly 
concerning  the  manner  in  which  the  nervous  matter  of  the  human 
body  performs  its  wonderful  functions,  and  more  indirectly  concern- 
ing the  relations  in  which  these  functions  stand  to  the  facts  and  the 
development  of  man's  mental  life. 

§  5.  The  remark  just  made  introduces  the  truth  that  there  are 
many  indirect  relations,  which  need  investigation,  between  the  phe- 
nomena of  human  consciousness  and  the  constitution  and  functions 
of  the  human  nervous  organism.  Indeed,  a  large  portion  of  this 
treatise  will  make  little  use  of  explanations  derived  directly  from 
the  facts  and  laws  of  anatomy,  histology,  and  physiology.  The 
entire  Second  Part  falls  more  properly  under  the  head  of  Psycho- 
physics  or  Experimental  Psychology,  than  under  Physiological 
Psychology,  strictly  so  called.  Still  all  investigators  are  convinced 
that  the  phenomena  of  sensation,  reproduction  in  the  form  of  mem- 
ory or  imagination,  association  of  ideas,  and  thought-processes,  so 
far  as  they  have  any  basis  or  correlate  in  the  physical  world,  are 
either  immediately  or  indirectly  dependent  upon  the  structure  and 
functions  of  that  mechanism  which  human  physiology  investigates. 
Only,  in  the  great  majority  of  cases,  we  have  as  yet  no  knowledge 
of  precisely  how  this  mechanism  behaves,  so  interior,  hidden,  and 
subtle  are  its  relations  to  mental  phenomena.  After  making  this 
confession,  we  are  only  complying  with  a  fairly  well-established  usage 
in  giving  the  ti tie  "Physiological  Psychology"  to  the  entire  treatise. 


INTRODUCTION  5 

§  6.  Physiological  Psychology — it  is  by  this  time  apparent — par- 
takes of  the  nature  and  methods  of  two  sciences  that  differ  widely 
from  each  other.  One  is  a  science  which  involves  introspection; 
for  it  is  only  by  the  method  of  introspection  that  the  actual  and  pres- 
ent facts  of  human  consciousness  can  be  reached.  The  other  is  a 
physical  science,  and  involves  external  observation  to  determine  the 
external  facts  of  the  structure,  development,  and  functions  of  a 
physical  mechanism.  Two  sets  of  phenomena  must  then  be  exam- 
ined in  their  relations  to  each  other,  and,  so  far  as  possible,  the 
laws  (or  permanent  modes)  of  these  relations  pointed  out.  It  is 
due  to  this  fact,  in  part,  that  both  the  peculiar  difficulties  and  the 
peculiar  interest  and  value  of  psycho-physical  researches  are  so 
great. 

In  every  science  a  beginning  is  first  made  by  ascertaining  and 
comparing  together  all  the  important  phenomena;  the  laws,  or 
regular  modes  of  the  occurrence  of  the  phenomena  in  relation  to 
each  other,  are  then  investigated;  and  finally,  certain  conclusions 
are  drawn  concerning  the  nature  and  significance  of  those  real  be- 
ings which  reason  compels  us  to  assume  as  permanent  subjects  of 
the  different  classes  of  phenomena.  In  its  effort  to  establish  itself 
upon  a  scientific  basis,  Physiological  Psychology  has  no  choice  but 
to  follow  essentially  the  same  method  of  procedure.  In  its  case, 
however,  as  has  already  been  remarked,  the  phenomena  which  are 
to  be  ascertained  and  compared  belong  to  two  orders  that  obviously 
differ  greatly  from  each  other;  and  the  laws  which  it  is  sought  to 
discover  are  laws  which  maintain  themselves  between  these  two  or-  ^ 
ders  of  phenomena.  It  has  already  been  said  that  the  phenomena 
of  the  nervous  system,  like  all  physical  phenomena,  consist  in  changes 
in  the  constitution  and  mutual  relation  of  material  masses  and  mole- 
cules; and  that  the  psychical  phenomena  are  states  of  consciousness, 
constantly  shifting  modes  of  the  behavior  of  that  subject  which  we 
have  agreed — as  much  as  possible  without  involving  any  premature 
assumptions — to  call  the  Mind.  Still  the  above-mentioned  two 
orders  of  phenomena  are  obviously  to  a  large  extent  related  to  each 
other;  they  may,  in  fact,  be  said  to  be  correlated  in  a  unique  manner. 
The  constant  forms  of  this  correlation  constitute  the  laws  for  the 
discovery  of  which  Physiological  Psychology  undertakes  its  special 
researches.  It  endeavors  to  bring  the  two  orders  of  phenomena 
face  to  face,  to  look  at  them  as  they  stand  thus  related  to  each  other, 
and,  as  far  as  possible,  to  unite  them  in  terms  of  a  uniform  character,  ^ 
under  law. 

§  7.  It  might  seem  that  simply  to  attempt  the  accomplishment 
of  so  difficult  and  complicated  a  task  as  that  just  described  should 
satisfy  all  legitimate  demands.  And,  again,  we  remind  ourselves 


6  INTRODUCTION 

that  no  little  protest  has  of  late  been  made  against  any  introduc- 
tion of  metaphysics,  whether  in  the  form  of  assumptions  or  con- 
clusions, into  the  science  of  psychology;  especially  when  this  science 
is  studied  from  the  physiological  and  experimental  points  of  view. 
And  has  it  not  just  been  agreed  that  metaphysical  assumptions 
shall  prejudice  as  little  as  possible  our  statement  of  psychological 
facts  and  laws  ?  But  our  science,  like  every  other  science,  has  the 
right  to  form  and  announce  conclusions  as  to  the  real  nature  of 
the  subject-matter  which  it  investigates,  if  these  conclusions  seem 
to  follow  legitimately  from  its  discussions  of  phenomena  and  laws. 
It  has  even  a  right  to  indulge  in  well-founded  and  reasonable  spec- 
ulation. Such  things  are  not  necessarily  objectionable  when  in- 
dulged in  by  any  of  the  more  purely  physical  sciences.  Indeed, 
there  is  not  one  of  these  sciences  which  would  not  look  compara- 
tively bare  and  unattractive  if  wholly  stripped  of  its  more  or  less 
questionable  inferences,  its  metaphysical  assumptions,  its  guessings 
and  speculations. 

§  8.  The  remarks  immediately  foregoing  serve  to  indicate  what 
are  the  principal  divisions  of  this  work.  The  First  Part  will  con- 
sist of  a  description  of  the  structure  and  functions  of  the  Nervous 
System.  In  order  that  this  system  in  man's  case  may  be  the  better 
understood,  the  first  two  chapters  give  a  brief  description  of  its  place 
in  the  developmental  series.  As  has  already  been  said,  this  system 
will  be  considered  under  the  conception  of  a  mechanism,  and  as  far  as 
possible  without  much  direct  or  indirect  reference  to  the  phenomena 
of  consciousness  as  determined  by  introspection.  An  important 
exception  may  seem  to  be  made  in  the  case  of  the  chapters  which 
treat  of  the  "Localization  of  Cerebral  Function";  and  which 
thus  bring  forward  the  relations  that  have  recently  been  estab- 
lished in  fact  between  the  conditions  and  activities  of  the  supreme 
nervous  centres  and  the  phenomena  of  conscious  sensation  and 
volition. 

Again,  reference  is  constantly  made  throughout  this  Part  to  the 
phenomena  of  automatic  and  reflex  action  of  the  nervous  system 
as  a  whole;  and  so  of  necessity  to  certain  correlated  mental  phe- 
nomena. But  such  topics  seem  indissolubly  connected  with  a  satis- 
factory description  of  the  nervous  mechanism;  and  to  introduce 
them  here  avoids  much  otherwise  necessary  repetition. 

The  Second  Part  discusses  more  particularly  the  relations  which 
exist  between  the  quality,  quantity,  combination,  and  order  of  suc- 
cession in  time,  of  the  various  stimuli  which  act  upon  the  nervous 
system,  and  .the  kind,  magnitude,  composite  result,  and  time-re- 
lations of  the  mental  phenomena.  Hence  the  significance  of  the 
term  psycho-physics.  As  Physiological  Psychology  is  ordinarily 


INTRODUCTION  7 

and  legitimately  treated,  it  includes  these  more  specially  psycho- 
physical  researches. 

Besides  the  foregoing  groups,  or  classes,  certain  observations 
which  have  more  or  less  of  scientific  confirmation  and  value,  may 
be  made  regarding  the  physical  basis  of  the  feelings  and  volitions 
controlling  the  bodily  members,  and  of  the  higher  faculties  of  mem- 
ory, association  of  ideas,  etc. 

The  Third  Part  will  fitly  introduce,  at  the  close  of  the  psycho- 
physical  researches,  the  briefest  possible  presentation  of  such  con- 
clusions as  may  be  legitimately  gathered,  or  more  speculatively  in- 
ferred, concerning  the  nature  (considered  as  a  real  being)  of  the 
human  mind.  The  justification  of  the  order  and  extent  of  the  en- 
tire discussion,  and  especially  of  the  Third  Part  as  a  whole,  has 
already  been  given  to  some  extent;  the  rest  must  be  left  to  the  prog- 
ress and  result  of  the  discussion  itself. 

§  9.  It  has  already  been  said  that  the  peculiarity  of  Physiological 
Psychology,  considered  as  a  branch  of  the  general  science  of  mind, 
consists  largely  in  the  method  of  its  approach  to  its  subject.  At- 
tention must  now  be  more  specifically  called  to  this  method  as 
necessarily  partaking  of  the  methods  of  the  two  sciences  whose 
researches  it  undertakes  to  combine.  The  method  of  physiology, 
which  is  in  general  that  of  external  observation  as  employed  in  all 
the  physical  sciences,  should  be  applied  only  when  supplemented  by 
the  many  delicate  and  accurate  instruments  of  observation  now  at 
command,  and  guarded  and  checked  by  that  accumulation  of  expe- 
rience concerning  the  best  ways  of  studying  nature  and  concern- 
ing her  ways  of  working  which  the  whole  body  of  such  sciences 
has  made.  On  the  other  hand,  the  method  of  psychology  has  or- 
dinarily been  defined  as  solely  the  method  of  introspection  or  self- 
consciousness.  These  two  methods  are  obviously  very  different. 
It  would  not  be  strange,  then,  if  the  science  which  finds  it  neces- 
sary to  combine  the  two  should  experience  some  special  difficulty. 
This  difficulty  has,  however,  more  often  been  exaggerated  than  ex- 
plained and  (what  is  quite  possible)  for  the  most  part  removed. 

Our  present  purpose  does  not  require  that  we  should  examine  at 
length  the  question  whether  the  introspective  method  is  the  only 
one  possible  in  psychology.  Scarcely  more  is  necessary  than  the 
statement  of  the  bearing  of  this  question  upon  the  inquiries  it 
is  proposed  to  make.  There  should  in  general  be  no  mystery  or 
arrogant  assumption  about  the  use  of  such  words  as  "science"  and 
"  scientific  method."  Science  is  nothing  but  knowledge — real,  veri- 
fiable, and  systematic.  Scientific  method  is  nothing  but  the  way 
of  arriving  at  such  knowledge.  Now,  although  Physiological  Psy- 
chology brings  the  investigator  face  to  face  with  some  of  the  most 


8  INTRODUCTION 

interesting  and  distinctive  mysteries,  it  is  not,  as  a  science,  to  be 
regarded  as  especially  mysterious.  Inasmuch  as  its  specific  busi- 
ness is  to  ascertain  and  combine,  under  definite  laws,  two  widely 
differing  classes  of  facts  (facts  of  the  human  nervous  mechanism  and 
facts  of  human  consciousness),  it  is,  of  course,  compelled,  first  of  all, 
to  ascertain  both  kinds  of  facts.  The  phenomena  of  consciousness, 
as  primary  facts,  can  be  ascertained  in  no  other  way  than  in  and  by 
consciousness  itself.  There  is  no  way  of  directly  examining  con- 
sciousness but  the  way  of  being  conscious  one's  self.  On  the  other 
hand,  it  is  perfectly  obvious  to  students  of  psychology  and  of  its 
history  (on  grounds  which  need  not  be  stated  here)  that  the  scientific 
treatment  of  the  facts  of  consciousness  can  never  be,  to  any  satis- 
factory extent,  accomplished  by  introspection  alone.  For  psy- 
chology, in  order  to  make  valid  its  claim  to  be  a  science,  must  not 
merely  display  the  alleged  facts  of  individual  mental  experience; 
it  must  treat  these  facts  analytically,  must  resolve  them  into  their 
ultimate  factors,  and  trace  the  stages  of  their  development  from  what 
is  simpler  to  what  is  more  complex;  it  must  also  show  on  all  sides 
their  connections  and  causes,  thus  placing  the  phenomena  of 
the  mind  as  much  as  possible  in  interaction  with  the  rest  of  the 
world. 

The  following  statements  will,  accordingly,  be  found  to  hold 
good  concerning  the  method  of  Physiological  Psychology.  It  must 
employ  faithfully  the  methods  distinctive  of  both  the  two  sciences 
which  it  endeavors  to  combine.  Facts  as  to  the  structure  and 
functions  of  the  nervous  mechanism,  and  as  to  the  effect  upon  it  of 
various  kinds  of  physical  energy  acting  as  stimuli,  must  be  ascer- 
tained by  external  observation.  The  primary  facts  of  conscious- 
ness must  be  ascertained  from  consciousness  itself;  or,  since  they  have 
already  been  for  a  long  time  subjected  to  this  form  of  observation, 
and  tabulated,  compared,  and  classified,  they  may  be  accepted 
from  the  science  of  introspective  psychology.  Care  must  be  taken, 
however,  to  make  sure  that  all  alleged  psychical  facts  are  really 
facts;  but  upon  this  point,  again,  there  is  no  other  way  of  making 
sure  than  in  and  through  consciousness.  The  final  result  of  re- 
search will  doubtless  be,  not  only  to  supplement  and  explain,  but 
also  to  modify  and  correct,  the  previous  statements  of  psychologi- 
cal science  as  to  its  laws  and  inferences.  But  here,  as  in  other  sci- 
entific research,  we  shall  be  obliged  to  work  our  way  through  many 
mistakes,  obscurities,  and  other  obstacles,  progressively  nearer  the 
complete  and  verifiable  knowledge  of  the  truth. 

§  10.  What  has  already  been  indicated  will  become  more  evi- 
dent in  the  course  of  the  following  investigations — namely,  that 
we  are  seldom  or  never  able  to  proceed  directly  with  the  work  of 


INTRODUCTION  9 

comparing  the  immediate  physical  antecedents  or  consequents  of 
the  mental  phenomena  with  these  phenomena  themselves,  and  so 
of  drawing  conclusions  at  once  as  to  the  laws  by  which  the  two 
classes  of  facts  are  connected.  Such  immediate  antecedents  and 
consequents  are  hid  in  the  inexplorable  recesses  of  the  living  and 
molecularly  active  brain.  It  is  seldom,  indeed,  that  our  direct  ob- 
servation can  approach  within  the  tenth,  or  it  may  be  within  the 
hundredth,  remove  of  what  goes  on  in  these  recesses.  We  are 
obliged  to  examine  the  physical  phenomena  from  a  greater  distance 
and  in  a  more  indirect  way.  For  example,  physics  can  inform  us 
what  combinations  of  what  wave-lengths  of  the  vibration  of  ether 
fall  on  the  eye  when  a  certain  form  of  conscious  sensation,  which 
we  call  "yellow"  or  "red"  or  "blue,"  arises;  physiology  can  locate 
the  nervous  elements  of  the  retina  upon  which  the  waves  fall,  can 
conjecture  something  as  to  the  chemical  changes  there  produced, 
and  trace  doubtfully  the  paths  along  which  the  resulting  nervous 
impulses  rise  to  the  brain  and  diffuse  themselves  over  certain  of 
its  areas;  psycho-physics  can  tell  approximately  the  relations  in 
which  the  varying  quantities  of  the  stimulus  stand  to  the  resulting 
degrees  of  the  sensations.  But  in  all  this  we  are  still  at  a  great  dis- 
tance from  the  enjoyment  of  those  opportunities  which  would  seem 
necessary  to  make  the  science  of  Physiological  Psychology  as  com- 
prehensive and  exact  as  could  readily  be  wished.  As  a  rule,  certain 
kinds  and  amounts  of  physical  energy,  more  or  less  definitely  meas- 
urable, are  known  to  be  acting  on  the  peripheral  parts  of  the  body, 
and  the  next  series  of  observed  facts  is  the  emergence  in  conscious- 
ness of  a  psychical  experience  quite  unlike  all  kinds  of  physical  en- 
ergy. To  be  sure,  Fechner's l  conception  of  psycho-physics  is  that 
it  treats  those  "  physical  activities  which  are  the  bearers  ( Trager)  or 
conditions  of  the  psychical,  and  accordingly  stand  in  direct  func- 
tional relation  with  them";  or  again,  "psycho-physics  is  an  exact 
doctrine  of  the  relations  of  function  or  dependence  between  body 
and  soul — of  the  universals  that  lie  between  the  bodily  and  spirit- 
ual, the  physical  and  psychical  world."  But  the  course  of  investi- 
gation will  make  clear  the  fact  that  of  such  physical  activities  we 
have  little  or  no  assured  knowledge;  although  we  have  the  best  of 
grounds  for  believing  that  such  activities  exist,  and  that  they  stand 
in  important  relations  under  law  with  the  facts  of  the  conscious 
psychical  life. 

§  11.  If  the  correctness  of  the  remarks  last  made  be  admitted,  the 
inquiry  may  be  raised:  What  justification  has  this  so-called  sci- 
ence of  Physiological  Psychology  for  the  large  claims  which  it  has 
made  of  late;  and,  indeed,  what  right  has  it  to  exist  as  a  special 
lElemente  d.  Psychophysik,  pp.  8  and  10  (Leipzig,  1860). 


10  INTRODUCTION 

discipline  at  all?  The  full  answer  to  the  call  for  self-justification 
must  be  made  by  the  actual  achievements  of  the  science  itself. 

The  history  of  modern  investigation,  and  the  conclusions  of  the 
modern  science  of  man,  both  physical  and  psychological,  emphasize 
the  necessity  of  studying  his  nature  and  development  as  that  of  a 
living  unity.  Such  science  shows  man  to  be  at  the  head  of  a  series 
of  physical  and  psychical  existences;  he  cannot  be  understood  as 
he  is,  in  his  whole  nature  and  in  his  place  within  nature  at  large, 
without  taking  both  sides  of  this  living  unity  into  account.  For 
man  is  known  to  himself  as  body  and  mind — and  not  as  bodiless 
spirit  or  a  mindless  congeries  of  moving  molecules.  That  the  struct- 
ure and  functions  of  the  body,  especially  of  the  nervous  mechan- 
ism, and  the  activities  of  the  mind,  are  extensively  and  intimately 
correlated,  is  a  fact  beyond  all  doubt.  It  is  the  particular  task  of 
Physiological  Psychology  to  show  in  what  manner,  and  to  what  ex- 
tent, such  correlation  exists.  .Moreover,  there  are  few  questions 
more  interesting,  from  a  philosophical  and  an  ethical  point  of  view, 
than  such  as  the  following:  What  is  the  nature  of  mind,  considered 
in  the  light  of  its  correlations  with  the  body?  and,  Do  the  so-called 
physiological  and  the  so-called  psychical  phenomena  belong  to  one 
subject,  or  to  more  than  one  ?  But  these  and  similar  questions  can 
be  scientifically  answered  only  by  giving  a  speculative  treatment 
to  the  conclusions  of  psycho-physical  investigation. 

In  brief,  it  may  be  said  that  introspective  psychology,  important 
as  its  results  have  been,  and  indispensable  as  its  method  is,  has 
shown  its  incompetency  to  deal  with  many  of  the  most  interesting 
inquiries  which  it  has  itself  raised.  On  the  other  hand,  psychology 
as  pursued  by  the  experimental  and  physiological  method  has  al- 
ready thrown  a  flood  of  fresh  light  upon  many  of  these  inquiries. 
We  may  affirm  with  Wundt,1  without  fear  of  successful  contradic- 
tion: "Psychology  is  compelled  to  make  use  of  objective  changes 
in  order,  by  means  of  the  influences  which  they  exert  on  our  con- 
sciousness, to  establish  the  subjective  properties  and  laws  of  that 
consciousness/'  On  this  fact  and  on  the  real  achievements  of  the 
method  we  confidently  rest  its  claims  to  serious  and  permanent  con- 
sideration. 

'Art.,  "Ueber  psychophysischen  Methoden,"  Philosophische  Studien  (1881), 
Heft  1,  p.  4. 


PART  FIRST 
THE   NERVOUS   MECHANISM 


CHAPTER  I 

THE  PLACE  OF  THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 

§  1.  It  is  essential  to  the  development  of  a  physiological  psy- 
chology that  the  part  played  by  the  nervous  system  in  animal  life 
and  the  manner  of  its  working  should  be  understood,  as  far  as  pres- 
ent knowledge  permits.  As  has  already  been  explained  in  the  In- 
troduction, we  wish  to  study  man's  mental  life  in  its  relation  to  the 
nervous  system  and,  in  general,  to  the  life  of  the  organism;  we  wish 
to  understand  the  functions  of  the  nervous  system  in  their  relations 
to  mental  phenomena.  In  such  a  study,  the  comparative  method 
should  prove  of  much  assistance.  For  both  the  nervous  structures 
and  the  mental  activities  of  man  are  so  enormously  complex  that 
the  discovery  of  their  relations  is  of  necessity  a  very  difficult  task. 
If,  however,  we  turn  to  animals  of  a  lower  grade,  with  much  less 
complexity  of  nervous  organization  and  functions,  we  may  hope  to 
glean  some  facts,  of  a  fundamental  sort,  regarding  the  most  gen- 
eral relation  of  structure  to  behavior.  We  should,  at  the  outset  of 
such  a  study,  lay  aside  for  the  time  our  psychological  preoccupa- 
tion with  such  complex  mental  performances  as  imagination,  rea- 
soning, and  will,  and  limit  ourselves  to  an  objective  examination  of 
the  simpler  behavior  of  lower  types  of  animal  which  present  little 
indication  of  these  complex  activities.  We  should,  at  the  same  time, 
banish  any  preconceived  notion,  founded  on  human  experience, 
that  the  nervous  system  is  essentially  the  servant,  or  the  organ,  of 
mental  life,  and  look  for  its  primary  function  in  animals  that  possess 
the  most  rudimentary  nervous  structures,  and  for  its  increase  in 
function  with  increase  in  the  complication  of  its  structure.  We 
shall,  of  course,  be  unable  to  present  in  this  book  a  full  account  of 
the  comparative  anatomy  of  the  nervous  system  and  of  the  behavior 
of  different  classes  of  animals— both  of  which  studies  have  been 
zealously  prosecuted  of  late  years — but  must  limit  ourselves  to  a 
brief  sketch. 

§  2.  The  primary  division  of  animals  distinguishes  two  classes: 
the  protozoa  and  the  metozoa.  The  protozoa  are  described  as 
"unicellular";  while  the  metozoa  are  composed  of  a  number,  usu- 
ally a  great  multitude,  of  cells.  A  "cell,"  in  the  biological  sense,  is 
a  microscopic  bit  of  semi-fluid  matter,  bounded  either  by  a  definite 

13 


14    THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 

membrane  or  at  least  by  a  "surface  of  separation,"  which  has  the 
important  property  of  resisting  free  diffusion  of  dissolved  sub- 
stances. Though  cells  will  live  either  when  immersed  in  the  water 
or  else,  as  in  higher  animals,  when  bathed  in  the  fluids  of  the  body, 
substances  dissolved  within  the  cell  do  not  freely  pass  out;  nor  do 
dissolved  substances  from  without  freely  pass  in,  through  the  sur- 
face of  the  cell.  There  is,  of  course,  much  interchange  of  materi- 
als between  the  inside  and  the  outside  of  the  cell;  such  interchange, 
however,  is  not  free,  but  selective.  Certain  substances  can  pass  in, 
others  out.  No  property  of  the  cell  is  more  necessary  to  its  life 
than  this;  for,  if  the  cell  boundary  interposed  no  barrier  to  free  dif- 
fusion, the  cell  would  promptly  lose  its  individuality  and  be  merged 
in  the  surrounding  medium.  The  semi-fluid  matter  of  the  cell, 
called  "protoplasm,"  consists  of  a  large  proportion  of  water,  in 
which  are  dissolved  salts  of  sodium,  potassium,  and  calcium,  as  well 
as  the  highly  complex  compounds  of  carbon,  nitrogen,  hydrogen, 
oxygen,  and  sulphur,  which  are  called  proteins.  Other  organic  com- 
pounds, and  other  salts,  may  also  be  present.  Within  this  mass  of 
protoplasm  is  a  smaller  mass  called  the  nucleus,  which  appears  to 
be  more  nearly  solid  than  the  protoplasm,  and  which  has  a  some- 
what different  chemical  composition,  inasmuch  as  its  protein  con- 
tains phosphorus,  in  addition  to  the  other  elements  mentioned. 
Some  cells,  indeed,  have  no  nucleus,  but  these  do  not  possess  the 
power  of  reproducing  their  kind,  and  it  is  doubtful  if  they  should 
be  regarded  as  true  cells,  in  the  full  sense  of  the  word.  The 
nucleus,  the  protoplasm,  and  the  surface  of  separation,  may  there- 
fore be  regarded  as  the  essential  parts  of  every  living  cell. 

§  3.  As  an  example  of  a  unicellular  animal  we  may  take  the 
amoeba,  a  minute  spherical  creature,  which  lives  in  stagnant  water, 
feeding  on  the  organic  matter  contained  in  the  water.  It  maintains 
its  own  chemical  composition,  which  is  different  from  that  of  the 
water;  it  takes  in  organic  matter  from  the  water,  changes  or  "as- 
similates" it,  and  so  grows;  and  it  excretes  waste  products.  Besides 
these  distinctly  chemical  processes,  it  shows  the  phenomenon  of  re- 
production. When  it  has  grown  to  a  sufficient  size,  it  divides  into 
two  "daughter  cells,"  each  of  which  is  an  amoeba.  It  has  further 
the  power  of  motility;  its  movements  being  of  two  opposed  sorts, 
one  consisting  in  the  bulging  outward  of  some  part  of  its  surface 
into  a  temporary  arm  or  branch,  while  the  other  consists  in  the  draw- 
ing in  of  these  temporary  arms  and  the  resumption  of  the  spher- 
ical shape.  Certain  stimuli  cause  it  to  exhibit  one  of  these  move- 
ments, and  other  stimuli  cause  it  to  exhibit  the  other.  A  bit  of 
food  in  its  neighborhood,  that  sends  out  particles  through  the  water, 
acts  as  a  stimulus  to  the  putting  forth  of  a  branch;  the  amoeba 


DESCRIPTION  OF  THE  AMOEBA  15 

bulges  out  on  the  side  toward  the  food,  and  then  slowly  flows  into 
the  protruding  arm,  thus  advancing  toward  the  food.  When  the 
bit  of  food  is  reached,  two  or  more  arms  bulge  out  around  it,  and 
unite  on  the  further  side,  enclosing  the  food  within  the  amoeba's 
body.  On  the  other  hand,  a  sudden  contact  with  a  solid  body,  or  a 
jar  to  the  water,  acts  as  a  stimulus  to  the  drawing  in  of  the  tempo- 
rary arms  (see  Fig.  1). 

Such  is  animal  behavior  when  observed  and  described  in  its  low- 
est terms.  If  we  leave  aside  the  chemical  and  reproductive  ac- 
tivities of  the  amoeba,  we  may  say  that  its  behavior  exhibits  the 
powers  of  motility,  irritability  or  sensitivity,  and  conductivity.  "  Ir- 


Fio.  1.— The  Amoeba.     (Verworn.)     Four  stages  in  the  ingestion  of  a  food  particle. 


ritability"  refers  to  the  fact  that  the  animal  moves  in  response  to 
forces  acting  on  it;  these  forces  do  not  actually  move  the  animal,  as 
the  current  of  the  water  moves  it,  but  they  arouse  its  inherent  power 
of  motion.  A  force  acting  to  arouse  an  inherent  power  of  an  animal 
is  called  a  stimulus;  and  the  movement  so  aroused  is  called  a  re- 
sponse or  "reaction"  to  stimulus.  The  stimulus  may  be  said  to 
"discharge"  the  movement,  as  the  blow  of  a  trigger  discharges  a 
cartridge.  "Conductivity"  refers  to  the  fact  that  the  part  of  the 
animal's  body  which  moves  need  not  be  the  same  as  the  part  which 
receives  the  stimulus;  there  must,  therefore,  be  a  conduction  or 
transmission  of  the  excitement  through  the  body.  The  stimulus 
acts  directly  on  one  side  of  the  body,  which  responds  by  bulging 
outward;  but  the  response  spreads  to  remoter  parts,  till  all  of  the 
protoplasm  is  in  motion.  It  is  not  the  external  stimulus  which  is 
transmitted,  but  some  sort  of  activity  lying  within  the  body  itself. 

§  4.  In  the  amceba  there  are  no  organs  to  take  care  of  the  differ- 
ent functions;  no  part  of  the  protoplasm,  more  than  any  other,  is 
specially  concerned  with  digestion,  or  with  motion,  or  with  receiving 
stimuli,  or  with  conduction.  There  is  a  complete  absence  of  di- 
vision of  labor,  of  specialization  or  differentiation  of  parts.  Every 
part  of  the  surface  can  receive  stimuli  and  be  thrown  into  activity 
by  the  stimuli;  every  part  of  the  protoplasm  can  conduct  the  exci- 
tation to  adjoining  parts;  and  every  part  can  respond  by  motion. 


16    THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 

On  the  other  hand,  the  progress  from  lower  to  higher  types  of 
animal  is  marked,  not  only  by  increase  in  size,  but  by  increasing 
specialization  of  the  parts.  Every  multicellular  animal  begins  its 
individual  life  as  a  single  cell.  This  divides  into  two;  and  the  daugh- 
ter cells  divide  in  their  turn,  and  so  on,  as  will  be  more  fully  de- 
scribed in  the  following  chapter.  The  many  cells  generated  in 
this  manner  remain  adherent  to  one  another;  and  as  their  multi- 
plication proceeds,  it  is  seen  that  they  begin  to  manifest  character- 
istic differences  one  from  another.  They  differ  both  in*  visible 
appearance  and  also  in  their  powers.  Comparatively  few  retain 
full  reproductive  power,  so  as  to  be  capable  of  giving  rise  to  a  new 
individual.  Some  groups  of  cells  become  specialized  in  their  chem- 
ical powers,  and  produce  powerful  digestive  fluids.  Others  become 
specialized  in  the  power  of  motion  and  develop  this  power  to  a  much 
higher  degree  than  that  possessed  by  the  amoeba.  Some  groups  of 
cells,  lying  for  the  most  part  on  the  surface  of  the  animal's  body,  be- 
come specialized  in  the  line  of  irritability;  that  is  to  say,  they  become 
highly  sensitive  to  specific  stimuli,  such  as  contact  and  jarring,  or 
light,  or  certain  chemical  substances.  Still  other  groups  of  cells 
develop  to  a  high  degree  the  power  of  conduction.  The  cells  which 
have  specially  developed  powers  of  receiving  stimuli  are  called  "  re- 
ceptors"; those  with  special  powers  of  conduction  may  be  called 
"conductors";  and  those  with  special  powers  of  movement,  along 
with  others  which  respond  to  stimuli  with  chemical  or  (in  certain 
species  of  animals)  electrical  effects,  are  called  "effectors."  The 
receptors  form  the  essential  part  of  the  sense-organs ;  the  conductors 
of  the  nerves;  and  the  effectors  of  the  muscles  and  other  organs 
which  produce  effects  of  importance  to  the  animal.1 

§  5.  In  a  general  way,  the  specialization  both  of  receptors  and  also 
of  effectors,  in  different  orders  of  animals,  keeps  pace  one  with  the 
other.  Where,  on  the  one  hand,  the  receptors  are  adapted  to  a 
great  variety  of  stimuli,  the  effectors  are,  on  the  other,  numerous 
and  capable  of  a  great  variety  of  effects.  Perhaps,  on  the  whole,  in 
the  lower  forms  of  animal  life,  the  development  of  effectors  keeps 

1  The  use  of  the  term  "receptors"  in  place  of  the  more  familiar  "sense-organs" 
is  justified  by  the  fact  that  a  sense-organ,  such  as  the  eye  or  ear,  contains  many 
accessory  structures  in  addition  to  the  sensitive  cells  which  receive  the  stimuli; 
and  by  the  further  consideration  that  "sensation"  properly  implies  conscious- 
ness; and  we  do  not  know,  or  wish  to  imply,  that  all  animals  provided  with 
receptors  are  conscious;  while  on  the  other  hand  we  do  know,  in  the  case  of 
human  beings,  that  many  stimuli  acting  on  receptors  in  the  internal  organs, 
and  producing  reactions,  do  not  give  rise  to  distinguishable  sensations.  The 
use  of  the  term  "effectors"  is  justified  by  the  frequent  need  of  including  under  a 
single  term  glands  and  other  organs  which,  as  well  as  muscles,  often  execute  the 
animal's  response  to  a  stimulus. 


SPECIALIZATION  OF  RECEPTORS  AND  EFFECTORS    17 


somewhat  ahead  of  the  development  of  receptors.  Every  effector 
cell  is  a  receptor  as  well,  to  this  extent,  at  least,  that  it  can  be  aroused 
by  the  direct  application  to  it  of  what  are  called  the  "  general  stim- 
uli," such  as  mecnanical  jars,  sudden  changes  of  temperature,  electric 
shocks,  and  certain  chemical  agents.  In  sponges,  which  are  re- 
garded as  the  lowest,  or  least  differentiated,  among  the  metazoa,  there 
are  two  sorts  of  motor  cell,  one 
of  which,  lying  along  the  sides  of 
the  pores  that  run  in  from  the 
exterior  of  the  sponge,  is  pro- 
vided with  vibratile  hairs  or 
"cilia";  these,  continually  lash- 
ing inward,  produce  currents  of 
the  sea  water  in  through  the 
pores  to  the  interior  cavity  of  the 
sponge.  The  other  motor  cells 
form  a  ring  or  "sphincter" 
around  the 'mouth  of  the  cavity 
at  the  top  of  the  sponge,  and  by 
contracting  close  the  mouth  and 
check  the  circulation  of  water 
through  the  sponge.  This  oc- 
curs in  response  to  stimuli  ap- 
plied directly  to  the  sphincter. 
But  the  cilia  in  the  pores  are  not 
checked  in  their  activity  by  this 
stimulus  when  thus  applied;  they 
still  continue  to  lash  the  water, 
though  without  effect,  since  the 
outlet  is  closed.  The  sponge, 
then,  show,s  some  specialization 
of  effectors,  but  little  if  any  of 
receptors,  and  none  of  a  con- 
ducting mechanism  which  would 

bring  the  cilia  into  harmonious  action  with  the  sphincter.  In  other 
words,  the  sponges  possess  no  nervous  tissue,  and  show  no  con- 
duction from  one  cell  or  group  of  cells  to  another;  in  general, 
they  exhibit  an  almost  complete  lack  of  co-ordination  (see  Fig.  2). 
§  6.  A  nervous  system  is  met  in  its  most  elementary  form  in  the 
ccelenterates,  of  which  the  jelly-fish  may  be  taken  as  an  example. 
The  body  of  the  jelly-fish  may  be  roughly  described  as  consisting  of 
the  "umbrella,"  which  lies  uppermost  in  the  usual  position  of  the 
jelly-fish,  and  is  a  gelatinous  mass,  with  no  power  of  sensitivity  or 
motility.  The  motor  organs  lie  partly  in  a  circular  band  which  is 


FIG.  2. — Diagram  of  a  Sponge.  (Parker.) 
The  narrow  pores  on  the  side  are  lined 
with  cilia,  lashing  the  water  inward;  the 
large  opening  at  the  top  is  surrounded  by 
the  sphincter. 


18    THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 


attached  around  the  rim  of  the  umbrella,  and  partly  in  the  tenta- 
cles which  hang  down  at  intervals  from  the  rim,  and  in  the  mouth 
stalk  suspended  from  the  centre  of  the  animal.  As  to  sense-organs, 
the  tentacles  are  sensitive,  and  so  is  the  circular  band;  and  special- 
ized receptors  of  several  kinds  are  arranged  at  intervals  around  the 
rim.  There  is  also  a  simple  form  of  nervous  system,  lying  princi- 
pally in  the  circular  band,  and  composed  of  a  special  form  of  cells 

with  long  slender  branches.  The 
uniting  or  "anastomosing"  of 
each  cell  with  the  branches  of 
neighboring  cells  forms  a  network 
which  extends  all  through  the 
circular  band  around  the  rim, 
and  also  into  the  tentacles. 
Branches  of  this  "nerve-net" 
pass  to  the  specialized  receptors 
along  the  rim,  and  to  all  parts  of 
the  sensitive  surface  and  to  all 
parts  of  the  muscular  tissue  (see 
Fig.  3).  Thus  every  receptor  and 
every  muscle-cell  is  connected 
with  the  nerve-net,  and  all  parts 
of  the  net  are  mutually  connected, 
so  that  the  net  forms  a  universal 
medium  of  connection  between 
receptors  and  effectors.  The  in- 
ternal structure  of  the  cells  and 
branches  forming  the  nerve-net 

is  highly  specialized,  the  principal  feature  being  the  presence  of 
fine  fibrils — "  neurofibrils  " — extending  along  the  branches  from  one 
cell  to  another  or  from  one  branch  into  another. 

§  7.  To  understand  the  function  of  the  nerve-net  in  these  ani- 
mals, their  behavior  under  natural  and  experimental  conditions 
should  be  briefly  considered.  The  swimming  action  of  the  jelly- 
fish consists  of  rhythmical  contractions  of  the  musculature  in  the 
circular  band,  all  portions  of  which  act  in  unison.  This  rhythmical 
movement  is  aroused  by  influences  proceeding  from  the  specialized 
receptors  in  the  rim,  for  if  these  are  all  cut  off  the  movements  cease. 
They  do  not  cease,  if  all  but  one  are  cut  off,  but  remain  the  same, 
no  matter  which  one  has  been  retained.  This  fact  shows  an  absence 
of  specialization  in  the  reactions  to  different  receptors.  Any  small 
portion  of  the  body,  containing  a  receptor  along  with  some  of  the 
muscle  and  nerve-net,  will  execute  the  same  rhythmic  movements, 
showing  that  any  part  of  the  nerve-net  can  do  the  same  sort  of  work 


FIQ.  3.— The  Jelly-Fish  (schematic).  (Bethe.) 
V,  umbrella;  B,  circular  band,  containing 
N,  the  ring  of  nervous  tissue;  R,  recep- 
tor; T,  tentacle;  M,  mouth  stalk. 


NERVE-NET  TYPE  OF  NERVOUS  SYSTEM 


19 


as  the  whole  net.  If  all  but  one  of  the  special  receptors  are  removed, 
and  then  a  further  cut  is  made  through  the  circular  band  on  one 
side  of  the  remaining  receptor,  the  whole  band  will  even  then  exe- 
cute its  rhythmic  movements;  and  it  makes  no  difference  on  which 
side  of  the  receptor  the  division  is  made.  This  shows  that  the  con- 
duction is  equally  good  in  both  directions.  Moreover,  even  if  many 
cuts  are  made  part-way  through  the  circular  band,  in  such  a  manner 
as  to  leave  the  band  of  a  zig-zag  shape,  the  rhythmic  contractions 
will  still  spread  from  the  remaining  receptor  throughout  the  band, 
provided  only  that  the 
divisions  are  not  com- 
plete. This  shows  that 
the  conduction  can  go 
around  corners  and  in 
every  direction  through 
the  circular  band.  That 
it  is  the  nerve-net  which 
supplies  the  means  of 
conduction  is  shown  by 
experiments  in  which  a 
complete  separation  is 
made  between  two  parts 
of  the  muscular  tissue, 
without  division  of  the 

nerve-strands  connecting  the  two  parts;  the  conduction  is  not  inter- 
fered with  so  long  as  the  nervous  connections  are  left  (see  Fig.  4). 

One  further  fact  should  be  brought  forward  to  complete  the  de- 
scription of  the  conductive  process.  If  a  tentacle  is  gently  stimu- 
lated by  lightly  touching  it  with  a  glass  rod,  it  responds  by  a  slight 
muscular  contraction  confined  to  the  part  touched.  If  the  touch 
is  a  little  heavier,  the  muscular  reaction  involves  a  larger  part  of 
the  tentacle.  If  the  touch  reaches  the  intensity  of  a  slight  blow, 
the  whole  tentacle  responds;  and  if  the  blow  is  made  stronger  and 
stronger,  the  reaction  spreads  to  other  tentacles,  to  the  foot,  and 
finally  to  the  swimming  muscles.1 

§  8.  The  facts  have  now  been  presented  with  sufficient  detail  to 
permit  of  a  proper  conception  of  the  nervous  system  of  the  jelly- 
fish and  other  coelenterates.  Anatomically,  it  is  a  diffuse  network, 
continuous  throughout,  and  connected  with  all  receptors  and  with 
all  motor  organs.  Experiment  shows  it  to  have  no  functional  cen- 
tre, all  parts  of  it  being  equivalent.  It  conducts  alike  in  all  direc- 
tions, and  serves  to  bring  about  a  general  contraction  of  the  mus- 

1  See  A.  Bethe,  Allgemeine  Anatomic  und  Physiologic  des  Nervensy stems,  1903, 
p.  110. 


FIG.  4.— The  Nerve -Net  (somewhat  schematic). 
(Bethe.)  R,  receptor;  M  F,  part  of  circular  band 
free  from  muscle. 


20    THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 

culature  when  any  receptor  is  excited.  The  observations  on  the 
tentacles,  however,  show  a  stronger  conduction  to  neighboring  than 
to  more  distant  parts;  inasmuch  as  the  spread  of  the  reaction  oc- 
curs only  within  narrow  limits  when  the  stimulus  is  feeble,  and  ex- 
tends more  and  more  widely  as  the  strength  of  the  stimulus  is  in- 
creased. Universal  and  indiscriminate  conduction,  limited  only  by 
a  dying  out  of  the  influence  conducted  with  the  distance  traversed,  is 
the  characteristic  of  this  simplest  type  of  nervous  system. 

§  9.  This  ccelenterate  type  of  nervous  system  may  be  called  the 
"nerve-net  type,"  because  of  the  branches  connecting  the  nerve- 
cells;  it  is  also  called  the  "diffuse  type,"  because  of  its  being  spread 
widely  through  the  epithelium,  and  also  because  of  its  diffuse  con- 
duction. The  first  distinct  sign  of  centralization  is  met  in  the  ner- 
vous system  of  the  flatworms.  In  them  a  large  proportion  of  the 
nerve-cells  is  located  in  two  bunches  or  ganglia,  situated  at  the  for- 
ward or  mouth  end  of  the  animal.  The  centralization  here  is  far 
from  complete;  for  many  nerve-cells  lie  diffusely  at  the  surface,  and 
these  cells  are  united  by  their  branches  into  a  network,  as  in  the 
jelly-fish.  In  mollusks  the  condition  is  much  the  same  as  in  the  flat- 
worms;  although  rather  more  concentration  into  ganglia  is  visible. 
The  nerve-net  is  still  complete  enough  to  make  possible  co-ordi- 
nated contractions  of  the  muscles  of  the  whole  body,  even  after  the 
removal  of  the  "centres"  or  ganglia.  The  significance  of  the  cen- 
tral system  is  seen,  however,  from  the  fact  that  reactions  of  parts  dis- 
tant from  the  point  of  stimulation  are  much  more  prompt  when  the 
ganglia  are  intact  than  when  they  are  removed.  The  essential 
point  of  advantage  in  the  ganglionic  system  of  these  creatures  ap- 
pears to  be  that  the  ganglia  can  be  connected  with  the  muscles  and 
receptors  by  long  strands  of  nerve,  and  not  simply  by  the  short 
branches  which  connect  the  cells  of  the  nerve-net.  These  long 
strands  thus  afford  more  rapid  conduction  between  distant  parts 
than  is  provided  by  the  net. 

§  10.  In  contrast  with  the  diffuse  nerve-net  of  the  coelenterates, 
and  the  partially  modified  net  of  the  flatworms  and  mollusks,  the 
nervous  system  of  the  higher,  segmented  worms  or  annelids,  and  of 
the  arthropods  (crabs,  insects,  etc.)  and  vertebrates,  may  be  called 
a  centralized  system.  Nerve-nets  are  not  indeed  entirely  absent 
from  these  higher  forms  of  animal  life;  they  persist  in  certain  se- 
cluded and  protected  situations,  such  as  the  walls  of  the  blood-ves- 
sels, heart,  stomach,  and  intestines  of  vertebrates,  including  man. 
But  the  receptors  of  the  external  surface  of  the  body,  by  which  it 
is  brought  into  relation  with  its  environment,  and  the  muscles  that 
move  the  body  or  its  parts  and  so  produce  external  effects,  are  no 
longer,  in  these  higher  forms,  connected  with  each  other  by  a  diffuse 


SEGMENTED  TYPE  OF  NERVOUS  SYSTEM 


21 


net  of  anastomosing  nerve-cells.  Instead,  the  receptors  are  con- 
nected by  long  fibres  with  masses  of  nerve-cells  lying  along  the 
middle  line  of  the  body  (these  animals  all  being  bilaterally  sym- 
metrical), and  the  muscles  are  likewise  connected  by  similar  fibres 
with  the  central  masses  of  cells.  The  fibres  which  connect  the  cen- 
tres with  the  receptors  and  effectors  are  characterized  by  fibrils 
running  lengthwise  within  them,  and  are  true  nerve-fibres,  as  in  the 
nerve-net.  Such  fibres,  however,  do  not  anastomose  with  each  other 


CG 


FIG.  5. — Diagram  of  the  Nervous  Systems  of  (4)  the  Flatworm,  of  (B)  the  Earth- 
worm, and  (C)  the  Mollusk.     (Bethe.)     CG,  cerebral  ganglion. 

outside  of  the  centres,  but,  where  they  are  gathered  into  the 
bundles  which  are  called  nerves,  each  maintains  its  individuality, 
somewhat  as  the  wires  bound  together  in  a  telephone  cable 
remain  separated  from  each  other.  Connections  between  the 
fibres  from  receptors  and  those  to  effectors  are  made  only  in  the 
centres. 

Further:  the  manner  of  communication  within  the  centres  them- 
selves differs  from  that  observed  in  the  nerve-net.  In  the  nerve-net, 
the  branches  of  different  cells  unite;  in  the  nerve-centres,  there  seems 
to  be  no  union  of  the  branches  of  different  cells ;  at  least,  this  is  true 
of  all  the  stouter  branches.  If  there  is  any  real  continuity  between 
different  cells  in  the  nerve-centres,  it  must  be  due  to  passage  from 
one  to  another  of  extremely  fine  strands — extremely  fine  even  as 
tested  by  the  highest  powers  of  the  microscope.  But  the  very  fine 


22    THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 

fibrils,  which  course  through  the  nerve-cells  and  along  their  branches, 
would  seem,  from  the  observations  of  some  good  authorities,  to 
have  a  certain  independence,  and  to  pass  freely  from  one  nerve-cell 
to  another,  forming  between  the  cells  an  exceedingly  tenuous  and 
intricate  network,  a  network  of  much  more  delicate  strands  than 
those  which  form  the  nerve-net.  The  existence  of  such  an  extra- 
cellular network  of  fibrils  has  been  made  out,  with  some  probability, 
in  worms  and  perhaps  in  crabs;  in  vertebrates,  however,  it  has  not 
been  demonstrated  to  the  satisfaction  of  those  best  qualified  to 
judge;  and  many  authorities  are  disposed  to  believe  that  no  complete 
continuity  exists  between  the  different  cells,  but  only  a  close  con- 
tiguity of  their  fine  branches.  The  evidence  on  this  point  will  be 
presented  in  a  later  chapter  on  the  nerve-cell.  Whatever  may  be 
the  truth  of  this  difficult  question,  there  is  no  doubt  that  the  path 
of  communication  existing  in  the  centres  of  the  higher  animal  forms 
— including  the  worms — is  less  diffuse  and  indiscriminate  than  that 
which  obtains  in  the  primitive  nerve-net.  There  is  less  of  wide- 
spread distribution  of  the  influence  coming  from  a  receptor  to  all 
the  musculature.  In  other  words,  the  influence  is  confined  within 
narrower  limits,  and  accordingly  more  specialization  and  greater 
variety  of  movements  result.  This  is  one  cardinal  feature  of  the 
centralized  form  of  a  nervous  system.  Another  is  the  existence  of 
long  fibres,  which  afford  quicker  connection  between  distant  parts 
of  the  body  than  occurs  through  the  primitive  nerve-net. 

§  11.  From  these  general  considerations  we  may  turn  to  a  brief 
sketch  of  the  nervous  system  and  behavior  of  the  annelids,  taking 
as  a  simple  example  the  earthworm  (see  Fig.  5).  Like  other  anne- 
lids, and  like  the  arthropods  and  vertebrates  as  well,  the  body  of  the 
earthworm  is  built  on  a  segmental  plan,  the  segments  succeeding 
one  another  along  the  length  of  the  animal,  and  being  visible,  in  the 
earthworm,  in  the  ringed  appearance  of  the  animal's  surface.  Each 
segment,  except  at  the  front  and  back  ends  of  the  worm,  is  much  like 
every  other,  and  each  is  fairly  complete  in  itself.  Each  has  a  ring 
of  muscle,  little  protruding  bristles  which  are  of  use  in  locomotion, 
and  a  set  of  simple  sense-organs.  From  the  sense-organs  and  from 
the  muscles  nerve-fibres  run  to  pairs  of  ganglia  within  the  segment, 
one  of  each  pair  lying  on  the  right  and  one  on  the  left.  The  nerve- 
fibres  connecting  with  the  receptors  are  branches  of  the  receptive 
cells;  those  connected  with  the  muscles  are  branches  of  cells  lying 
within  the  ganglia.  Connections  within  the  ganglia  are  formed,  as 
above  described,  between  the  fibres  from  the  receptors  and  those 
issuing  to  the  muscles;  and  thus  a  local  reflex  path  is  provided,  by 
means  of  which  stimuli  affecting  the  receptors  of  the  segment  arouse 
activity  in  the  muscles  of  that  segment  (see  Fig.  6). 


SEGMENTED  TYPE  OF  NERVOUS  SYSTEM 


23 


Local  reflexes  can  be  obtained  from  single  segments  cut  out  of  a 
worm  of  this  species.     But  the  segments  are  not  entirely  independent 


FIG.  6.— Nerve-Cells  and  Fibres  in  Ganglia  of  the  Earthworm.  (Retzius.) 
Only  a  few  out  of  many  cells  and  fibres  are  shown.  A  motor  fibre 
is  indicated  by  an  outward-pointing  arrow,  and  a  sensory  fibre  by 
an  inward-pointing  arrow. 

of  one  another,  nor  are  their  ganglia  without  mutual  connections. 
Between  each  ganglion  and  that  of  the  segment  next  in  front  or  be- 


24    THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 

hind  is  a  strand  of  nerve-fibres;  so  that  the  ganglia  form  a  connected 
chain  extending  along  the  worm  near  its  ventral  or  under  surface. 
The  nerve-fibres  which  compose  the  longitudinal  strands  connect- 
ing the  ganglia  are,  for  the  most  part,  derived  neither  from  the  re- 
ceptor cells  nor  from  the  nerve-cells  which  send  out  branches  to  the 
muscles,  but  from  another  class  of  nerve-cells,  which  lie  in  the 
ganglia  and  send  their  branches  in  a  longitudinal  direction  from  one 
ganglion  to  the  next  or  to  the  next  but  one  or  two.  These  central 
fibres  connect  neighboring  ganglia  with  each  other,  and  by  the  series 
of  them  in  successive  ganglia  the  whole  chain  is  made  a  continuous 
conducting  medium.  There  are  also  a  few  much  longer  fibres,  which 
connect  distant  ganglia  directly  with  each  other.  At  the  forward 
end  of  the  worm  there  is  somewhat  of  a  departure  from  the  regular 
segmental  scheme;  a  ring  of  nervous  tissue  extends  from  the  fore- 
most pair  of  ganglia  around  the  gullet  and  joins  these  ganglia  to  a 
single  ganglion  lying  on  the  dorsal  side,  which  is  called  the  cerebral 
ganglion  or  "  brain."  In  the  earthworm  this  so-called  brain  is  not 
highly  developed. 

Experiment  shows  that  the  chain  of  ganglia  is  the  only  path  of 
communication  between  the  receptors  and  effectors  of  the  worm. 
If  the  ganglia  of  a  segment  are  cut  out,  that  segment  is  paralyzed: 
its  muscles  no  longer  respond  to  stimuli  applied  to  any  receptor, 
and  its  own  receptors,  on  being  stimulated,  give  rise  to  no  reac- 
tion in  any  part  of  the  body.  If  the  longitudinal  cord  connecting 
the  ganglia  is  severed  at  any  point,  no  stimulus  applied  to  the  part 
of  the  worm  forward  of  the  break  arouses  any  response  in  the  part 
lying  behind  the  break;  and  vice  versa;  the  waves  of  muscular  con- 
traction that  sweep  backward  over  the  animal  in  its  creeping,  stop 
at  the  point  where  the  cord  is  severed,  and  all  co-ordination  be- 
tween the  front  and  rear  parts  is  abolished.  We  find  in  the  earth- 
worm three  main  classes  of  movement:  (1)  local  reflexes,  which 
can  be  carried  out  by  the  receptors,  effectors,  and  connecting  nerve- 
fibres  of  a  single  segment;  (2)  slowly  moving  waves  of  muscular  con- 
traction, the  impulse  to  which  is  probably  transmitted  from  segment 
to  segment  by  the  short  fibres  of  the  longitudinal  cord,  and  (3) 
sudden  jerks  of  the  whole  worm,  which  are  probably  due  to  con- 
duction along  the  few  long  central  fibres. 

§  12.  As  compared  with  the  lowly  earthworm,  the  higher  forms 
of  annelid,  and  crustaceans  and  insects,  while  preserving  the  same 
general  type  of  segmented  nervous  system,  show  a  superior  nervous 
development  in  many  respects.  There  is  an  increase  in  the  number 
of  cells  in  the  ganglia,  which  is  partly  to  be  explained  by  the  greater 
complexity  of  the  sensory  and  motor  apparatus.  There  is  a  still 
greater  increase  in  the  amount  of  branching  of  the  nerve-cells  within 


CENTRALIZED  TYPE  OF  NERVOUS  SYSTEM  25 

the  ganglia,  which  probably  means  a  higher  organization  of  the  con- 
nections between  fibres.  There  is  a  great  increase  in  the  number 
of  long  fibres  within  the  ganglion  chain,  and  a  high  development  of 
the  cerebral  ganglion.  Both  of  the  last-mentioned  changes  are  to 
be  interpreted  in  the  light  of  the  development  of  the  head,  which  in 
the  earthworm  is  not  much  of  an  affair,  but  which  becomes  richly 
equipped  with  various  organs  in  crabs  and  insects.  The  head  is 
first  of  all  the  mouth  end  of  an  animal,  and  as  such  it  is  natural  that 
it  should  go  first  in  locomotion,  and  that  it  should  have  a  certain 
predominance  over  the  rest  of  the  body.  Around  the  mouth  are  de- 
veloped motor  organs  to  serve  it,  and  sense-organs  concerned  with 
the  finding  and  testing  of  food;  and  thus  the  importance  of  the  head 
is  progressively  increased. 

The  nervous  connections  must  keep  pace  with  the  development 
of  receptors  and  effectors,  and  thus  it  is  that  the  cerebral  ganglion 
grows  in  size,  and  becomes  more  and  more  dominant  over  the  ganglia 
lying  behind.  It  is,  further,  to  be  expected  that  such  highly  special- 
ized receptors  as  the  eye  should  appear  in  this  same  region.  The  eye 
is  the  type  of  a  "distance  receptor";  its  importance  lies  in  the  fact 
that  it  is  excited  by  objects  at  a  greater  or  less  distance  from  the 
surface  of  the  body.  With  the  appearance  of  distance  receptors 
comes  a  great  increase  in  the  range  of  the  environment  that  can  be 
reacted  to;  and  thus  the  life  of  the  animal  comes  to  be  largely  dom- 
inated by  these  organs.  Since  the  distance  receptors  are  located  in 
the  head,  the  importance  of  the  cerebral  ganglion  and  its  connections 
is  still  further  increased;  and  to  this  is  to  be  ascribed,  not  only  the 
increase  in  the  size  of  the  ganglion,  but  the  increase  in  the  number 
of  long  fibres  leading  backward  from  it,  and  bringing  distant  mus- 
cles into  quick  communication  with  the  sense-organs  of  the  head. 

§  13.  The  vertebrates — fishes,  amphibia,  reptiles,  birds,  and  mam- 
mals— are,  like  annelids,  crustaceans,  and  insects,  built  on  a  seg- 
mental  plan,  the  segmentation  appearing  clearly  in  the  vertebral  col- 
umn or  backbone.  The  nervous  system  in  vertebrates  shows  some 
of  the  segmental  origin,  in  the  regular  succession  of  the  nerves  which 
issue,  a  pair  at  each  vertebra,  along  the  length  of  the  trunk.  The 
central  organs  of  vertebrates  are  located  inside  the  vertebrae  and 
skull,  i.  e.,  in  the  spinal  cord  and  brain,  and  these  show  very  little 
sign  of  segmentation.  In  fact,  one  of  the  characteristics  of  the  verte- 
brate nerve-centres  is  their  visible  continuity  and  absence  of  apparent 
segmentation.  It  is  much  as  if  the  ganglia  of  the  annelid's  nerve- 
chain  had  grown  together  into  a  continuous  cord.  A  second  differ- 
ence between  the  vertebrate  nervous  system  and  that  of  the  annelids, 
etc.,  is  seen  in  the  position  of  the  spinal  cord.  The  nerve-chain  of 
invertebrates  lies  near  the  ventral  surface,  the  digestive  tract  lying 


26    THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 

above  it,  while  in  the  vertebrates  the  cord  lies  near  the  dorsal  sur- 
face and  above  the  digestive  tract.  The  cerebral  ganglion  of  the 
earthworm,  it  will  however  be  remembered,  lies  dorsal  to  the  mouth, 
and  this  corresponds  to  the  brain  of  vertebrates. 

A  third  point  of  difference  between  the  vertebrate  and  inverte- 
brate systems  concerns  the  location  of  the  cells,  the  branches  of 
which  connect  the  receptors  with  the  centres.  In  invertebrates,  it 
is  the  receptive  cells,  lying  at  the  periphery,  the  branches  of  which 
pass  into  the  centres;  but  in  vertebrates  the  receptor  cells  do  not 
themselves  provide  the  fibres  for  their  connections,  but  are  sup- 
plied by  fibres  which  grow  out  from  special  ganglia  that  lie  close 
to  the  spinal  cord  and  brain,  and  are  enclosed  in  the  same  bony  cov- 
ering. The  receptive  cells  of  the  sense  of  smell  form  an  exception, 
and  send  branches  of  their  own  into  the  brain,  preserving  the  in- 
vertebrate condition.  The  nerve-fibres  in  vertebrates  very  com- 
monly show  a  more  complicated  structure  than  in  invertebrates,  in 
that  the  branch  of  the  nerve-cell  becomes  enveloped  by  a  sheath  of 
"  myelin,"  a  fat-like  substance  the  significance  of  which  is  not  quite 
clear,  but  will  be  discussed  in  a  later  chapter.  On  the  whole,  and 
with  exceptions,  the  nerve-centres  of  vertebrates  may  be  considered 
as  more  advanced  in  development  than  those  of  invertebrates;  and 
this  is  seen  especially  in  the  richer  provision  of  long  fibres  connect- 
ing distant  parts  of  the  system.1 

§  14.  The  nerve-centres  of  vertebrates  may  be  considered  as  con- 
sisting of  (1)  a  fundamental  system,  comprising  the  cord  and  the 
brain-stem,  and  (2)  accessory  organs  developing  as  outgrowths  of 
the  brain-stem,  the  chief  of  these  being  the  cerebellum  and  the  cere- 
brum. The  development  of  the  accessory  structures  is  very  unequal 
in  different  forms  of  vertebrate  animals ; — the  size  of  the  cerebellum 
being  closely  related  to  the  animal's  powers  of  locomotion,  and  the 
size  of  the  cerebrum  with  his  powers  of  learning  new  and  specific 
adaptations.  The  fundamental  system  is,  on  the  other  hand,  fairly 
constant  throughout  the  vertebrate  series.  This  is  especially  true 
of  the  spinal  cord,  the  size  of  which  seems  to  depend  almost  wholly 
on  the  size  of  the  animal,  i.  e.,  on  the  bulk  of  the  receptor  and  ef- 
fector organs  with  which  it  is  connected.  Thus,  though  the  brain 
of  an  ox  is  much  smaller  than  that  of  a  man,  the  spinal  cord  of  the 
former  is  both  longer  and  thicker. 

The  fundamental  system  (compare  Fig.  7),  as  most  clearly  seen  in 
the  spinal  cord,  consists  of  the  above-mentioned  sensory  ganglia,  the 
cells  of  which  provide  fibres  connecting  the  receptors  of  the  trunk 

1  The  following  treatment  of  the  vertebrate  nervous  system  is  based  mostly  on 
L.  Edinger,  "  Vorlesungen  iiber  den  Bau  der  nervosen  Zentralorgane,"  Band  2, 
Vergleichende  Anatomie  des  Gehirns  (Leipzig,  1908). 


NERVE-CENTRES  OF  VERTEBRATES 


27 


N.  VIII 


and  limbs  with  the  spinal  cord;  of  nerve-cells  lying  within  the  cord, 
branches  of  which  pass  out  to  the  muscles  and  other  effector  organs ; 
and  of  numerous  central  units—  cells  whose  branches  do  not  extend 
beyond  the  centres,  but  run  up  or  down  or  across  from  one  side  of 
the  cord  to  another,  and  so  bring  its  different  parts  into  connection. 
Most  of  these  central  fibres 
are  short;  but  there  are  some 
sets  of  long  ones,  which  are 
present  in  all  vertebrates,  and 
which  connect  the  cord  di- 
rectly with  the  brain  stem. 

§  15.  The  brain-stem 
shows  the  same  basal  char- 
acteristics as  the  cord,  but 
its  structure  is  greatly  com- 
plicated by  the  presence  of 
the  important  receptors 
which  are  located  in  the 
head,  and  connected  with 
this  part  of  the  central  sys- 
tem. Where  a  given  set  of 
receptors  is  highly  developed 
and  much  used,  the  part  of  N.  ix,  x,  xi 
the  brain-stem  which  receives 
fibres  from  it  is  itself  highly 
developed.  The  receptors 
of  the  vertebrate  head  can 
be,  though  only  roughly,  in- 
dicated by  mentioning  the  FIG.  7.— Nerve-centres 
human  nose,  eye,  ear, 
mouth,  and  facial  skin.  Of 
these,  the  development  of  the  eye  is  perhaps  the  most  equal  through- 
out the  vertebrates.  The  sense  of  smell  is  much  more  important 
in  some  forms  than  in  others,  and  the  associated  parts  of  the  brain 
are  correspondingly  unequal.  The  skin  of  the  face,  especially  of 
the  snout,  and  of  the  front  of  the  mouth  and  nose,  is  much  more 
important  as  a  receptive  surface  in  some  species  of  animals  than  in 
others;  and  the  "trigeminus"  nerve,  which  supplies  the  cutaneous 
receptors  of  the  face,  varies  correspondingly.  The  ear  of  land- 
living  forms  is  the  representative  of  a  much  more  extensive  system 
of  receptors  in  fishes,  which  is  excited  by  vibrations  and  currents 
in  the  water;  this  constitutes  the  so-called  "lateral  line"  system, 
the  incoming  fibres  from  which  give  rise  to  a  considerable  expansion 
of  the  brain-stem  at  their  place  of  entrance.  Again,  the  sense  of 


Olf.  nerve 


Olf.  lobe 


>  Cerebrum 


Interbrain 


Optic  lobe 
Cerebellum 


Bulb 


Spinal  cord 


of  the  Frog.  (Ecker- 
Wiedersheim-Gaupp.)  The  brain  and  upper  part 
of  the  cord  are  shown  from  the  dorsal  side. 


28    THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 

taste  in  man  is  provided  with  a  comparatively  slight  outfit;  but  in 
fishes  it  has  an  extensive  system  of  chemo-receptors,  which  lie  not 
only  within  the  mouth  and  throat,  but  on  the  external  surface  of 
the  head  and  even  of  the  trunk;  and  these  receptors  also,  being  con- 
nected with  a  certain  region  of  the  brain-stem,  are  the  occasion  of  a 
complex  local  development  in  that  region.  Thus  it  comes  about 
that  the  brain-stem  in  vertebrates  is  not  only  larger,  but  less  uni- 
form in  size  than  the  spinal  cord. 

Another  difference  between  these  two  parts  of  the  fundamental 
nervous  system  lies  in  the  more  extensive  development,  within  the 
brain-stem,  of  the  short  central  fibres,  which  connect  the  neighboring 
parts  of  the  system.  The  impressions  received  by  the  various  sense- 
organs  of  the  head  are  so  important  in  regulating  the  vital  functions 
of  the  animal  that  it  is  but  natural  to  find  extensive  paths  of  com- 
munication leading  from  the  nerves  of  these  senses  in  various  di- 
rections. The  impressions  received  by  the  internal  ear  (and  by  the 
lateral  line  in  fishes)  are  important  in  governing  locomotion  and 
preserving  equilibrium,  and  in  maintaining  the  "tone"  and  readi- 
ness for  action  of  the  muscles.  The  sense  of  taste  is  important  in 
regulating  digestion.  The  impressions  made  on  receptors  in  the 
lungs — the  nerve-fibres  of  which  also  enter  the  brain-stem — are  im- 
portant in  breathing.  The  "centres"  for  the  digestive  organs,  and 
for  respiration  and  the  closely  allied  function  of  circulation,  are  lo- 
cated in  the  brain-stem;  and  one  of  the  important  centres  for  loco- 
motion is  located  there  as  well.  (By  a  so-called  "centre"  is  to  be 
understood,  anatomically,  a  set  of  connections  between  incoming 
and  outgoing  fibres,  by  which  receptors  can  influence  the  right 
effectors.  Such  connections  are  largely  provided  by  the  central 
fibres,  which  accordingly  abound  in  the  region  concerned  with  the 
vital  functions.) 

The  brain-stem,  like  the  cord,  does  not  belong  wholly  to  the 
fundamental  system,  but  is  invaded  by  fibres  belonging  to  the  acces- 
sory systems  of  the  cerebrum  and  cerebellum.  In  the  higher  mam- 
mals, the  fundamental  system  is  quite  overgrown  by  fibres  belong- 
ing to  these  accessory  systems.  These  are  long  fibres,  in  contrast 
with  the  short  interconnections  of  the  fundamental  system.  The 
higher  the  development  of  the  accessory  organs,  the  greater  is  the 
proportion  of  long  fibres  in  the  brain-stem  and  the  cord;  and  the 
more  the  fundamental  system  retires  into  the  background.  No 
doubt,  however,  it  always  remains  fundamental;  it  is  played  upon 
by  the  accessory  organs,  between  which  and  the  effectors  it  always 
intervenes  (compare  Fig.  8). 

§  16.  As  already  stated,  the  cerebellum  varies  in  size  according 
to  the  locomotor  powers  of  the  animal,  so  that  within  closely  related 


STRUCTURE  OF  THE  CEREBELLUM 


29 


groups,  such  as  the  lizards,  the  tortoises,  or  the  eels,  those  species 
which  are  active  swimmers  have  a  much  larger  development  of  this 
organ  than  those  which  crawl.  Birds,  with  their  high  powers  of 
locomotion  in  three  dimensions,  have  a  large  cerebellum;  in  mammals 
also  this  organ  is  large — in  man  specially  large,  perhaps  on  account 
of  his  upright  position  and  the  special  demands  which  this  entails 
on  the  muscles  for  locomotion  and  the  maintenance  of  equilibrium. 


£    BIRO 


F-  MAMMAU 


Fia.  8. — Longitudinal  Sections  through  the  Brains  of  Different  Classes  of  Vertebrates. 
(Edinger.)     The  cerebellum  is  represented  in  black. 

The  cerebellum  receives  fibres,  directly  or  indirectly,  from  a  large 
share  of  the  receptors  of  the  whole  body,  including  the  head.  The 
fibres  which  conduct  outward  from  the  cerebellum  run  mostly  to 
groups  of  cells  in  the  brain-stem;  and  the  fibres  from  these  groups 
which  belong  to  the  fundamental  connecting  or  co-ordinating  system, 
undoubtedly  spread  the  influence  of  the  cerebellum  to  the  cells  which 
directly  control  the  muscles.  The  development  of  the  cerebellum 
is  not,  however,  entirely  independent  of  that  of  the  cerebrum,  since 
in  mammals  there  is  a  mass  of  fibres  coming  from  the  cerebrum 
and  making  connections  with  the  cerebellum;  and  a  large  addition 
to  the  cerebellum,  on  each  side,  seems  to  result  from  this  connection 
with  the  cerebrum. 


30    THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 

§  17.  Before  we  pass  to  the  other  principal  accessory  organ,  the 
cerebrum,  we  should  pause  to  recognize  the  existence  of  the  in- 
creased development,  which  occurs  in  vertebrates  near  the  forward 
end  of  the  brain-stem,  in  the  part  called  the  mid-brain.  This  part 
reaches  a  great  size  in  many  vertebrates,  and  is  in  fact  the  best-de- 
veloped part  of  the  brain  in  fishes  and  amphibia;  in  reptiles  and 
birds  as  well  it  is  very  prominent.  The  most  elementary  fact  re- 
garding this  part  is  that  it  receives  fibres  from  one  of  the  most  im- 
portant of  all  receptors,  the  eye;  and  that  it,  in  turn,  sends  out 
motor  fibres  to  the  muscles  of  the  eye.  The  swollen  portions  of  the 
brain-stem  which  lie  on  the  back  of  the  mid-brain  are  accordingly 
called,  except  in  mammals,  the  "optic"  lobes.  The  fibres  of  the 
optic  nerve  here  terminate  in  close  connection  with  large  collec- 
tions of  variously  formed  cells,  some  of  which  send  forth  only  short 
branches,  and  serve  to  afford  rich  connections  within  this  mass  of 
cells,  while  others  send  out  long  fibres  that  run  in  large  measure 
downward  to  the  lower  parts  of  the  brain-stem,  and  to  the  cord, 
and  thus  carry  the  influence  of  the  eye  to  effectors  throughout  the 
body.  But  the  optic  lobes  are  not  exclusively  concerned  with  the 
eye;  for  fibres  can  be  traced  to  them  also  from  the  cord  and  brain- 
stem;  by  this  means  they  receive  impressions  from  the  other  sense- 
organs  of  the  body,  and  especially  from  the  ears.  Thus  the  optic 
lobes  come  to  be  a  general  receiving  and  distributing  centre;  and 
in  animals  which  have  not  developed  the  cerebrum  to  any  great  ex- 
tent, the  optic  lobes  are  probably  the  dominant  part  of  the  whole 
nervous  system. 

§  18.  The  part  of  the  brain-stem  which  lies  next  forward  of  the 
mid-brain  is  called  the  inter-brain  or  thalamus;  it  is  constantly  pres- 
ent throughout  the  vertebrate  series,  but  varies  greatly  in  extent; 
and  in  mammals  it  is  quite  definitely  an  adjunct  of  the  cerebrum, 
since  it  contains  masses  of  cells  which  are  relay  stations  in  the  path- 
ways of  impressions  from  all  the  receptors  to  the  cerebrum.  Thus, 
the  principal  connections  of  the  eye,  which  in  other  vertebrates  are 
almost  exclusively  with  the  optic  lobes,  become  in  mammals  mostly 
transferred  to  the  thalamus,  from  which  fibres  run  to  the  cortex  of 
the  cerebrum.  But  besides  this  cerebral  part  of  the  thalamus,  there 
is  in  all  forms  of  vertebrates  a  more  primitive  system  of  cells  and 
fibres,  connecting  largely  with  the  portion  of  the  brain  still  further 
forward  (the  olfactory  region).  The  thalamus  is  not,  however, 
the  direct  or  immediate  centre  of  smell;  and  in  fact  its  significance 
is  difficult  to  discover.  The  thalamus  has  several  gland-like  off- 
shoots, and  in  some  of  the  lower  vertebrates,  some  of  these  appear 
to  act  as  sense-organs.  The  "  parietal  organs,"  which  are  offshoots 
of  the  thalamus,  appear  in  some  forms  to  serve  as  eyes,  which  are 


INTER-BRAIN  AND  END-BRAIN  31 

fairly  well  developed,  though  not  by  any  means  as  highly  developed 
as  the  eyes  customarily  so  called.  It  is  possible  that  this  part  of 
the  brain-stem  is  primitively,  like  the  mid-brain,  the  local  centre  for 
certain  sense-organs  which  in  most  vertebrates  have  gone  out  of 
function;  and  that  later,  the  thalamus  was  taken  possession  of,  so 
to  speak,  by  the  adjoining  cerebrum. 

What  lies  forward  of  the  thalamus  is  called  the  fore-brain  or  end- 
brain.  It  consists  of  two  parts:  (1)  the  basal  portion,  which  be- 
longs to  the  fundamental  system  of  the  brain-stem,  and  (2)  the  ex- 
pansions to  the  side  and  back,  which  form  the  greatest  part  of  the 
whole  brain  in  mammals,  but  in  many  lower  vertebrates  are  of  very 
small  dimensions.  The  basal  portion  of  the  fore-brain  is  again  read- 
ily divisible  into  two  parts:  the  "corpus  striatum,"  and  the  olfactory 
lobe  and  bulb.  The  striatum,  which  lies  very  near  the  thalamus,  is, 
probably,  closely  related  to  it  in  function,  though  we  must  admit  that 
on  this  point  our  knowledge  is  far  from  complete.  The  function 
of  the  olfactory  bulb,  which  lies  at  the  very  front  of  the  whole  organ, 
is  clear  from  its  direct  connection  with  the  incoming  fibres  from  the 
olfactory  receptors  in  the  nose.  And  the  olfactory  lobe,  further, 
is  directly  connected  with  the  bulb,  and  must  also  be  regarded  as 
part  of  the  olfactory  system.  A  portion  of  it,  however,  receives 
fibres  from  the  skin  of  the  face  and  mouth,  and  is  perhaps,  as  sug- 
gested by  Edinger,1  a  centre  for  the  "  oral  sense,"  which  in  animals 
that  use  the  snout  or  tongue  as  an  exploratory  organ,  must  be  of 
much  service  in  directing  their  behavior.  There  is  no  doubt  that, 
as  the  mid-brain  is  primarily  the  central  mechanism  of  the  eye,  the 
fore-brain  is  primarily  the  local  centre  for  the  sense  of  smell.  In  the 
fishes,  it  seems  to  be  little  else;  only  the  basal  portion  develops,  the 
dorsal  remaining  (except  for  one  small  portion)  a  mere  membrane, 
without  nerve-cells  or  fibres  in  it.  In  amphibia,  however,  the  dorsal 
portion,  called  the  "pallium"  or  mantle,  begins  to  show  nervous 
structures,  and  in  reptiles,  this  is  still  more  markedly  the  case;  in 
birds  more  yet;  and  in  mammals  the  pallium  completely  overshad- 
ows the  primitive  basal  structures  of  the  fore-brain.  In  amphibia  the 
nerve-cells  of  the  pallium  are  irregularly  arranged,  but,  beginning 
with  the  reptiles,  there  is  a  true  bark,  or  "  cortex,"  on  the  outer  sur- 
face of  the  pallium,  consisting  of  cells  arranged  in  a  definite  fashion, 
and  with  an  extraordinary  richness  of  fine  branches. 

§  19.  At  its  first  appearance,  in  reptiles,  the  cortex  is,  to  judge 
from  its  fibre  connections,  an  adjunct  of  the  primary  olfactory  and 
oral  sense-centres  which  lie  in  the  adjacent  basal  portion  of  the  fore- 
brain.  This  oldest  part  of  the  pallium  is  called  the  "  archi-pallium," 
in  distinction  from  the  "  neo-pallium,"  the  connections  of  which  are 
1  L.  Edinger,  op.  cit.,  p.  261. 


32    THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 

with  more  distant  parts  of  the  brain-stem  and  cord  (see  Fig.  9). 
In  birds,  the  connections  with  the  optic  lobes  become  prominent; 
and  in  mammals,  through  the  intermediary  of  the  thalamus,  the 
cortex  comes  into  connection  with  all  the  receptors  of  the  body, 
and  also  sends  outgoing  fibres  further  and  further  back  along  the 


FIG.  9. — Archipallium  and  Neopallium  in  the  Brain  of  the  Calf. 
(Edinger.)  In  the  right  half  of  the  figure,  the  archipallium  (un- 
shaded) is  separated  from  the  partially  shaded  neopallium  by  the 
fissura  limbica. 

brain-stem  and  cord,  thus  assuming  control  over  more  and  more  of 
the  fundamental  system.  With  the  increase  in  the  neopallium,  the 
older,  olfactory  portion  of  the  cortex  is  left  behind  near  the  base 
and  median  line  of  the  cerebrum,  while  the  neopallium  spreads  to 
the  sides,  and  upward,  forward,  and  backward.  The  archi- 
pallium still  forms  a  distinguishable  portion  of  the  brain,  even  of 
man,  and  is  probably  still  concerned,  as  in  reptiles,  with  smell  and 
related  senses. 


RELATION  OF  SIZE  TO  INTELLIGENCE  33 

Among  the  mammals,  the  advance  of  the  brain  from  the  lower 
to  the  higher  forms  consists  in  the  development  of  the  neopallium. 
This  advance  is  marked  by  the  increase  of  the  extent  of  the  cortex, 
by  the  growing  richness  of  cells  and  their  fine  branches  within  the 
cortex,  and  by  the  increase  of  long  fibres  connecting  different  parts 
of  the  cortex  with  each  other  and  with  the  cord,  the  brain-stem, 
and  the  cerebellum.  The  increase  in  total  extent  of  the  cortex  is 
provided  for  partly  by  expansion  of  the  skull  and  partly  by  the  fold- 
ing of  the  surface  of  the  brain  into  "fissures,"  which  are  few  in  the 
lower  mammals,  but  numerous  in  many  of  the  higher  forms.  The 
increase  of  long  fibres  in  the  higher  forms  is  as  marked  as  the.  in- 
crease in  the  extent  of  the  cortex.  It  will  be  recalled  that,  even 
among  the  worms,  an  advance  in  organization  was  marked  by  an 
increase  in  the  long  central  fibres,  bringing  distant  parts  of  the  sys- 
tem into  direct  connection.  This  same  principle  appears  in  compar- 
ing the  brains  of  different  vertebrates.  Man,  of  all  vertebrates, 
possesses  the  greatest  proportion  of  long  fibres,  both  those  leading 
into  or  out  of  the  cerebrum,  and  those  connecting  its  own  non-adja- 
cent parts.  In  particular,  the  supply  of  fibres  connecting  the  right 
with  the  left  hemispheres  of  the  cerebrum  shows  a  great  advance 
from  lower  mammals  to  higher,  and  in  man  the  cross  connection 
between  the  hemispheres,  called  the  "corpus  callosum,"  is  a  prom- 
inent feature  of  the  brain. 

§  20.  In  a  general  way,  the  intelligence  of  different  vertebrates 
is  somewhat  proportional  to  the  size  of  their  brains,  and  especially 
to  their  development  of  cerebral  cortex.  We  have,  indeed,  as  yet 
no  accurate  measures  of  the  intelligence  of  different  animals,  with 
the  exception  of  a  few  which  have  been  subjected  to  experimental 
test.  The  monkey  is  more  intelligent  than  the  cat  or  dog,  as  tested 
by  the  speed  of  learning  or  the  number  and  variety  of  associations 
formed.1  The  chimpanzee  and  other  man-like  apes  seem  to  be 
superior  to  the  smaller  monkeys;  and  the  primates,  in  general,  seem 
to  surpass  most  other  animals.  Some  mammals  which  possess 
markedly  large  brains,  and  high  cortical  development — such  as  the 
seal,  porpoise,  walrus,  and  whale — are  little  known  as  respects 
their  behavior  and  intelligence. 

One  thing  is  reasonably  certain:  the  brain-weight  of  a  species, 
or  of  a  breed,  is  partly  dependent  on  the  size  of  the  body.  Larger 
breeds  have  larger  brains,  but  the  relative  size  of  the  brain  is  greater 
in  the  smaller  breeds.  There  is  apparently  a  purely  somatic  factor 
in  the  determination  of  brain  development,  and  this  factor  must  be 
allowed  for  before  the  true  relation  between  brain-size  and  intelli- 

1  Thorndike,  "The  Mental  Life  of  the  Monkeys,"  Psychol.  Rev.,  Monogr.  Suppl. 
15,  1901. 


34    THE  NERVOUS  SYSTEM  IN  THE  ANIMAL  KINGDOM 


gence  can  be  seen.  Suggestions  have  been  made  toward  the  de- 
termination and  elimination  of  this  somatic  factor,  but  there  are 
still  many  elements  of  uncertainty  in  the  calculation. 

The  following  table  of  approximate  brain-weights  and  body- 
weights  of  mammals  is  selected  from  the  much  more  extensive  tables 
of  Warnecke.1 


Brain-Weight 
in  Grams 

Body-Weight 
in  Grams 

Mouse        .         .... 

0  4 

20 

Squirrel     

6 

400 

Cat  

30 

3,500 

Beaver 

35 

20000 

Kangaroo 

65 

45,000 

#T 

Monkey  (macaque)  . 
Dog   (very  large)   .     .     . 
Sheep    

100 
120 
130 

5,000 
46,000 
50,000 

y$* 

.  >3P 

**t 

Lion      

220 

120,000 

Seal 

300 

26000 

Bear 

400 

200  000 

«o 

Gorilla  

400 

90,000 

Moose   

435 

200,000 

3Zn 

Cow 

450 

175  000 

f 

Porpoise 

500 

55,000 

X 

Hippopotamus    .... 
Horse    

580 
600 

1,750,000 
300,000 

>4V 

Giraffe 

680 

530  000 

Man           .              ... 

1  400 

70,000 

]  y^ytf 

Elephant  

5,000 

2,500,000 

Whale 

7000 

70  000  000 

§  21.  Within  the  order  of  primates,  we  find  a  large  range  of  brain- 
size,  the  cerebrum  of  the  common  tailed  monkeys  (e.  g.,  macacus 
rhesus)  weighing  about  80-100  grams,  that  of  the  anthropoid  apes 
(chimpanzee,  gorilla,  orang-outang)  running  to  about  400  grams, 
while  that  of  man  averages  about  1,400  grams.  Remains  of  extinct 
forms  of  primates,  showing  a  cerebral  development  intermediate 
between  that  of  the  gorilla  and  that  of  man,  have  been  unearthed 
in  only  a  few  cases;  and  there  has  been  considerable  difference  of 
judgment  regarding  the  interpretation  to  be  made  of  these  inter- 
vening forms.  On  the  whole,  it  seems  allowable  to  recognize  at 
least  one  intermediate  form  between  the  anthropoid  apes  and  man 
as  he  exists  to-day.  This  intermediate  form  (Pithecanthropus 

1  Journal  f.  Psychol.  u.  Neurol.,  1908,  XIII,  355.  A  collection  of  brain-weights 
of  birds,  by  the  same  author,  is  to  be  found  in  the  same  journal,  1907,  IX,  93. 


ESSENTIAL  FUNCTION  OF  NERVOUS  TISSUE         35 

erectus),  as  represented  by  a  single  specimen  found  in  Java,1  is 
reckoned  with  the  apes  rather  than  with  man,  but  has  a  skull  capac- 
ity, as  well  as  other  anatomical  peculiarities,  that  separate  him  from 
existing  apes,  and  indicate  a  brain-weight  of  perhaps  600-700  grams. 
The  paucity  of  the  evidence,  however,  must  continue  to  throw  doubt 
on  the  conclusion. 

A  considerable  number  of  specimens  of  great  antiquity  have  been 
unearthed,  which  are  undoubtedly  the  relics  of  the  human  species. 
In  regard  to  skull  capacity,  these  oldest  surviving  specimens  of 
inhabitants  of  Europe  do  not  differ  much,  if  any,  from  the  present 
inhabitants.  In  countries  like  Egypt,  also,  in  which  skulls  are  found 
representing  several  thousand  years  of  history,  there  is  no  sign  of 
an  increase  in  brain-size.  Among  existing  races,  there  are  racial 
differences  in  the  average  brain- weight;  but  these  differences  are 
small  compared  with  the  wide  range  of  variation  between  indi- 
viduals of  the  same  race.  For  example,  the  average  brain-weight 
of  negroes  is  perhaps  two  ounces  less  than  that  of  Europeans;  but 
the  difference  between  individuals  of  either  race  may  amount  to  as 
much  as  twenty-five  ounces.  It  should  be  added  that  the  differ- 
ences in  mental  capacity  of  different  races  appear  much  smaller  to 
the  ethnologist,  who  knows  different  races,  than  they  appear  to 
those  members  of  any  race  who,  not  having  studied  other  races 
scientifically,  judge  in  accordance  with  race  prejudice  and  pride 
rather  than  in  an  objective  manner.  Even  this  difference  may  be 
chiefly  due  to  the  fact  that,  in  the  more  highly  civilized  races,  the 
large  amount  and  variety  of  the  educative  processes  develops 
(that  is,  occasions  a  growth  in  size  of)  many  of  the  nerve  elements 
which  continue  relatively  undeveloped  in  the  less  civilized  races. 

§  22.  A  review  of  the  foregoing  sketch  of  the  varieties  of  the 
nervous  system  in  the  animal  kingdom  leaves  no  doubt  that  the 
most  elementary  and  essential  function  of  nervous  tissue  is  to  provide 
lines  of  conduction  between  receptors  and  effectors.  No  other  possi- 
ble function  of  the  nervous  system  would  have  any  biological  utility 
without  this.  The  progress  from  the  diffuse  nerve-net  of  the  jelly- 
fish to  the  highly  organized  system  of  mammals  is  marked,  first  by 
centralization;  second,  by  the  appearance  of  numbers  of  central  or 
co-ordinating  cells;  third,  by  the  dominance  of  the  system  by  the 
receptors  of  the  head;  and  fourth,  by  an  increase  in  the  plasticity  or 
modifiability  of  the  system,  or  of  certain  parts  of  it.  Conduction, 
co-ordination,  integration,  and  "learning"  (this  word  in  a  figurative 
sense)  may  be  assigned  as  the  functions  of  the  nervous  system;  and 
it  is  probable  that,  from  the  stand-point  of  inner  mechanics,  all  of 
these  are  specializations  of  the  primary  function  of  conduction. 
1  Discovered  and  described  by  E.  Dubois. 


CHAPTER  II 

THE  DEVELOPMENT  OF  THE  NERVOUS  SYSTEM  IN  THE 
INDIVIDUAL1 

§  1.  The  study  of  the  growth  of  the  nervous  system  in  embry- 
onic life  and  childhood  is  useful  in  two  ways:  it  throws  some  light 
on  the  functions  of  the  various  parts,  and  it  is  of  great  aid  in  gain- 
ing clear  conceptions  of  the  highly  complicated  structure  of  the 
central  organs.  These  organs  first  appear  in  relatively  simple 
forms  and  relations,  and  develop  gradually  in  complexity;  an  under- 
standing of  the  simpler  facts  of  the  embryonic  brain  is  thus  a  good 
point  of  starting  for  the  study  of  its  fully  developed  condition. 

In  unicellular  organisms  reproduction  consists  simply  in  division. 
The  adult,  full-grown  cell  divides  into  two  cells  of  half  its  size,  which 
may  be  called  young  cells,  or  "daughter  cells";  and  these  in  their 
turn  grow  and  subsequently  divide.  In  all  but  the  simplest  forms 
of  living  beings,  there  exists  a  differentiation  among  the  numerous 
cells  that  compose  the  body;  most  of  them  form  organs  which, 
from  the  stand-point  of  reproduction  and  the  perpetuation  of  the 
species,  are  accessory,  and  simply  serve  to  provide  favorable  con- 
ditions of  life  for  the  relatively  few  cells  which  preserve  the  repro- 
ductive power.  These  reproductive  cells  alone  are  capable  of 
producing  daughter  cells  which  can  grow  into  new  individuals.  In 
plants  and  animals  which  show  the  distinction  of  sex,  the  repro- 
ductive cells  are  of  two  sorts,  usually  (in  animals)  located  in  sepa- 
rate individuals,  male  and  female.  The  reproductive  cells  for  the 
female  animal  give  rise  to  cells  called  eggs  or  ova;  while  those  of 
the  male  give  rise  to  spermatozoa.  In  some  low  forms  the  ova  are 
capable  of  developing  into  new  individuals  without  aid  from  the 
spermatozoa;  but  in  most  cases,  and  always  in  vertebrates,  the 
ovum  does  hot  develop  far  unless  it  has  fused  with  a  spermato- 
zoon. The  fusion  of  these  two  cells,  the  ovum  from  the  female 
individual  and  the  spermatozoon  from  the  male,  is  called  fertiliza- 
tion; and  the  ovum  which  has  fused  with  a  spermatozoon  is  called 
a  fertilized  ovum.  Leaving  aside  as  unessential  for  our  purpose 

1  This  chapter  is  based  principally  on  the  work  of  His,  Die  Entwickelung  des 
menschlichen  Gehirns,  1904;  and  on  the  treatment  by  O.  Strong,  in  Bailey  and 
Miller's  "Textbook  of  Embryology,"  1909,  pp.  454-572. 

36 


REPRODUCTION  IN  UNICELLULAR  ORGANISMS       37 

the  very  curious  changes  which  occur  in  these  two  cells  in  prepara- 
tion for  their  fusion,  we  will  take  our  start  with  the  fertilized  ovum, 
which  may  properly  be  called  a  new  individual  at  the  very  earliest 
stage  of  its  life,  and  which  promptly  begins  to  develop  itself. 

§  2.  As  has  already  been  said,  the  fertilized  ovum,  itself  a 
single  cell,  proceeds  to  divide  into  two,1  which,  however,  do  not  sep- 
arate, but  remain  in  contact  with  each  other.  Each  of  these  daugh- 
ter cells,  after  growing  in  size,  again  divides  into  two;  and  this  proc- 
ess is  repeated  time  after  time,  and  for  a  while  with  considerable 
regularity,  so  that  the  young  individual  consists  successively  of  one 
cell,  two  cells,  four,  eight,  sixteen,  and  thirty-two.  The  details 
of  this  process  differ,  however,  in  different  orders  of  animals,  and 
in  all  the  regular  doubling  of  the  number  of  cells  is  broken  up  after 
a  few  divisions  by  the  beginning  of  the  differentiation  among  the 
daughter  cells  and  their  unequal  rates  of  growth  and  division. 

A  process  preliminary  to  this  propagation  of  cells  by  division 
may  be  noticed  briefly  at  this  point.  When  a  cell  is  on  the  point  of 
dividing  into  two,  the  nucleus  can  be  seen  to  separate  into  smaller 
bodies,  the  number  of  which  is  constant  for  any  given  sort  of  cell. 
In  preparation  for  their  fusion,  both  ovum  and  spermatozoon  ex- 
trude part  of  their  nuclear  matter,  and  at  the  time  of  fertilization, 
each  has  only  half  of  the  number  of  so-called  "chromosomes"  ap- 
propriate to  the  animal  in  question.  Thus  the  fertilized  ovum, 
which  consists  of  both  ovum  and  spermatozoon,  contains  exactly 
the  full  number  of  chromosomes  appropriate  to  the  species.  When 
the  fertilized  ovum  divides  into  two  cells,  half  of  its  chromosomes  go 
to  each  cell,  and  half  of  each  half  is  from  the  original  ovum,  half 
from  the  spermatozoon.  The  same  thing  is  true  of  the  succeeding 
generations  of  "daughter  cells."  Thus  each  parent  is  equally 
represented,  not  only  in  the  fertilized  ovum,  but  in  each  cell  of  the 
successive  generations  of  cells  that  result  from  repeated  division. 

§  3.  At  first  the  cells  of  the  mass  resulting  from  repeated  cleavage 
of  the  ovum  adhere  compactly  together,  presenting  somewhat  the 
appearance  of  a  mulberry.  Later,  spaces  appear  between  some  of 
the  cells,  and  a  cavity  is  formed  inside  the  mass.  Then  the  cells 

1  The  very  early  stages  in  the  development  of  the  human  embryo  have  not 
been  actually  observed,  but  as  those  mammals  which  have  been  studied  resemble 
each  other  in  essential  points  at  this  stage,  it  is  inferred  that  the  beginnings  of 
human  development  do  not  differ  much  from  the  general  mammalian  condition. 
The  earliest  human  embryos  so  far  available  seem  to  be  of  about  two  weeks' 
growth.  Within  this  period  they  have  increased  in  size  from  0.2  millimetre 
(or  lis"  of  an  inch,  the  diameter  of  the  ovum)  to  a  length  of  some  two  millime- 
tres. They  have  become  elongated  and  cylindrical,  and  show  the  beginnings  of 
head  and  trunk,  and  distinct  rudiments  of  several"  organs,  among  which  are  the 
brain  and  spinal  cord. 


38 


THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 


Am 


lining  the  cavity  become  differentiated  from  those  of  the  exterior 
surface,  and  a  third  group  of  cells  sprouts  in  between  the  two  layers, 
so  that  there  are  formed,  in  all,  no  fewer  than  three  "  germ  layers." 
Of  these,  the  exterior  layer  is  called  the  ectoderm,  the  inner  layer  the 
entoderm,  and  the  intervening  layer  the  mesoderm.  From  this  very 
early  stage  of  development  on,  these  three  layers  retain  their  indi- 
viduality. The  entoderm  forms  the  epithelial  wall  of  the  throat, 

gullet,  stomach,  and  intes- 
tines, and  also  of  the  windpipe 
and  lungs ;  it  gives  rise  also  to 
the  glands  which  connect  with 
these  organs,  including  the 
liver  and  pancreas.  The  meso- 
derm gives  rise  to  the  bones 
and  connective  tissues,  to  the 
muscles,  the  heart  and  blood- 
vessels, and  to  the  kidneys  and 
the  sex  glands.  The  ectoderm 
gives  rise  to  the  outer  layer  of 
the  skin  and  of  the  mucous 
membrane  of  the  mouth  and 
nose,  to  the  appendages  of  the 
skin,  such  as  hair  and  nails,  to 
the  essential  portions  of  the 
sense-organs,  and  to  the  ner- 
vous system.  It  appears 
strange,  at  first  thought,  that 
the  nerve-centres,  which  lie 
so  far  to  the  interior  and  are 

enclosed  in  bone,  should  develop  from  the  superficial  layer  of  the 
embryo,  but  the  early  stages  in  the  development  of  the  nervous 
system  make  this  clear. 

Early  in  the  development,  when  the  embryo  is  less  than  two  weeks 
old — age  being  reckoned  from  time  of  fertilization — there  appears 
a  shallow  groove  (see  Fig.  10)  extending  lengthwise  along  what  may 
be  called  the  back  of  the  embryo.  This  groove  is  formed  by  an  in- 
bending  of  the  ectoderm,  and  is  called  the  neural  groove.  The 
groove  increases  in  depth,  its  edges  arch  together  over  the  top,  and, 
where  they  meet,  grow  together,  thus  transforming  the  groove  into 
a  tube — a  hollow  tube  of  ectoderm  lying  beneath  the  surface  of  the 
embryo,  and  later  separated  widely  from  this  surface  by  the  in- 
growth of  mesoderm  between  it  and  the  superficial  ectoderm.  This 
is  the  so-called  neural  tube,  and  it  is  the  rudiment  of  the  nervous 
system,  which,  in  fact,  always  retains  the  fundamental  character- 


Fio.  10. — Dorsal  View  of  a  Two  Weeks'  Human 
Embryo.  (Von  Spec.)  The  future  head  end 
is  above  in  the  figure.  The  "neurenteric 
canal"  is  an  early  communication  between 
the  neural  tube  and  the  rudimentary  intes- 
tine. Magnified  20  diameters. 


CONSTITUTION  OF  THE  NEURAL  TUBE 


39 


Brain 
(groove  still 
unclosed) 


Amnion 


istic  of  a  tube.  The  lower  portion  of  the  tube  gives  rise  to  the  spinal 
cord,  the  walls  of  which  become  so  thickened  that  the  small  central 
cavity  remaining  in  adult  life  and  called  the  central  canal  is  very  in- 
conspicuous. The  upper  portion  of  the  tube  develops  into  the 
brain ;  and  here  the  form  of  a  tube  is  still  more  concealed,  by  fold- 
ings, swellings,  or  out- 
growths of  the  tube,  and 
great  but  irregular  thick- 
enings of  the  wall.  The 
central  cavity  persists,  how- 
ever, and  appears  in  the 
adult  brain  in  the  form  of 
spaces  of  various  shape, 
called  the  ventricles.  It  is 
of  great  assistance  in  gain- 
ing a  conception  of  the 
complicated  structure  to 
bear  in  mind  the  fact  that 
the  brain  is  primarily  a 
tube  (compare  Fig.  11). 

§  4.  The  nerves  which 
connect  the  brain  and 
cord  with  the  muscles  and 
sense-organs,  arise  partly 
as  outgrowths  of  the  neu- 
ral tube,  and  partly  from 
that  portion  of  the  ecto- 
derm which  formed  the 
lips  of  the  neural  groove 

and    which    Was  left    OUt-    Neur  enterte 

side    when    the    groove 
closed  to  form  the  tube. 

FIG.  11. — Dorsal  View  of  a  Human  Embryo,  Some- 
what More  Advanced  than  the  Preceding.  (Eter- 
nod.)  About  5r.  The  neural  groove  has  closed  to 


Vertebra 


canal 


Neural  tube 


Neural  groove 
(still  unclosed) 


a  tube  in  the  middle  of  its  extent,  but  is  still  open 
at  the  cerebral  and  lumbar  ends. 


This  portion  of  the  ecto- 
derm is  called  the  neural 
crest,  and  it  is  quickly 
overgrown  by  the  adja- 
cent ectoderm,  and  so  comes  to  lie  beneath  the  surface  of  the  em- 
bryo, and  close  to  the  neural  tube,  one  strip  at  the  right  side  of  the 
tube  and  one  at  the  left.  This  part  of  the  ectoderm  gives  rise  to 
the  sensory  nerves  and  to  part,  at  least,  of  the  sympathetic  nervous 
system;  while  the  neural  tube  itself  gives  rise  to  the  motor  nerves 
and  to  the  brain  and  cord. 

The  neural  tube,  when  first  formed,  does  not  as  yet  constitute  a 
nervous  tissue;  its  cells  are  not  nerve-cells,  but  are  still  in  an  un- 


40         THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 

specialized  condition.  The  process  by  which  this  tube  of  undiffer- 
entiated  cells  becomes  a  cord  of  nervous  tissue  is  as  follows.  In 
the  wall  of  the  tube  next  to  the  cavity  or  "lumen"  some  cells  be- 
come reproductively  active,  and  divide,  giving  rise  to  daughter  cells, 
which  move  away  toward  the  outside  of  the  wall,  but  stop  midway, 
and  proceed  to  change  their  form,  sending  out  branches  that 


Germinal  layer 


Nerve-cell 
layer 


Marginal 
layer 


Fia.  12. — Cross  Section  of  the  Neural  Tube  and  Crests  of  a  Human  Embryo  of  about  Five 
Weeks.  (His.)  The  dorsal  side  is  uppermost  in  the  figure.  Many  facts  which  are  men- 
tioned in  the  text  can  be  observed  in  this  figure.  The  three  layers  of  the  tube  are  readily 
distinguished:  the  germinal  layer,  next  to  the  lumen  of  the  tube,  is  dark  with  its  closely 
packed  cells.  The  nerve-cell  layer,  outside  of  this,  is  more  loosely  packed,  while  the  mar- 
ginal' layer  is  free  from  cells.  The  fact  that  the  nerve-cell  layer  develops  to  the  sides, 
and  not  directly  in  the  mid-ventral  and  mid-dorsal  lines,  is  evident.  Commissural  axons, 
crossing  at  the  mid-ventral  line,  can  be  observed ;  also  axons  passing  into  and  out  of  the 
tube  in  the  dorsal  and  ventral  roots.  The  neural  crest  is  clearly  distinguished  from  the 
surrounding  tissues  in  the  left  side  of  the  figure.  A  streaky  appearance  in  the  crest  indi- 
cates the  direction  of  the  growing  axons  toward  the  dorsal  root.  Within  the  tube,  close 
to  the  entrance  of  the  dorsal  root,  is  seen  the  rudiment  of  the  dorsal  column  which  is 
composed  of  the  axons  which  enter  by  the  root. 

unite  with  branches  of  other  similar  cells.  In  this  way  a  network  of 
minute  fibres  is  formed.  These  cells,  again,  are  not  nerve-cells, 
nor  is  this  tissue  of  fibres  true  nervous  tissue;  it  is  a  framework  or 
scaffolding;  it  is  the  rudiment  of  the  supporting  tissue  of  the  nerve- 
centres,  which  in  its  fully  developed  form  is  called  neuroglia.  Al- 
ways the  growth  of  this  supporting  framework  precedes  the  appear- 
ance of  nerve-cells  in  any  region  of  the  centres.  The  neuroglia 
strengthens  the  texture  of  the  nerve-centres.  In  later  life,  it  "pro- 
liferates "  or  multiplies  wherever  injury  or  degeneration  of  the  ner- 
vous tissue  tends  to  produce  vacant  spaces.  The  neuroglia,  grow- 


CONSTITUTION  OF  THE  NEURAL  TUBE 


41 


ing  into  the  spaces,  fills  them.  When  the  true  nerve-cells  begin  to 
grow,  the  network  of  neuroglia  serves  to  guide  them  into  their 
places. 

The  nerve-cells  originate  in  the  same  manner  as  the  neuroglia 
cells.  At  the  same  portion  of  the  tube,  namely  the  layer  next  to 
the  lumen,  cells  divide,  giving  rise  to  daughter  cells  which  migrate 
outward  to  the  middle  of  the  wall.  Here  they  stop,  perhaps  be- 
cause checked  by  a  dense  layer  of  neuroglia  fibres,  and  here  they 
proceed  to  develop  peculiarities  which  mark  them  as  nerve-cells. 


FIG.  13.— The  Growing  Axon.  (Cajal.)  From  the  embryo 
of  a  duck.  A  is  the  neural  tube,  the  letter  being  placed 
about  at  the  junction  of  the  nerve-cell  and  the  marginal 
layers;  B,  the  space  immediately  surrounding  the  tube. 

The  wall  of  the  neural  tube  thus  comes  to  consist  of  three  layers, 
the  inner  or  germinal  layer,  where  the  process  of  cell-division  is 
active,  and  where  the  young  nerve-cells  originate;  the  outermost  or 
marginal  layer,  which  is  free  from  nerve-cells ;  and  a  middle  or  nerve- 
cell  layer,  in  which  the  nerve-cells  are  developing  (see  Fig.  12). 

§  5.  The  first  visible  step  in  the  specialization  of  the  nerve-cell 
is  the  appearance  of  a  few  fine  fibrils,  the  beginning  of  the  "  neuro- 
fibrils,"  which  later  develop  into  an  extensive  system  within  the 
cell  and  its  outgrowths.  The  next  step  of  importance  is  the  ap- 
pearance of  a  branch,  which  grows  out  from  the  cell  toward  the 
marginal  layer.  The  growing  end  of  this  branch  then  becomes  en- 
larged into  a  cone  which  seems  to  press  its  way  between  other  cells 


42         THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 

and  into  the  meshes  of  the  neuroglia  net,  and  so  the  branch  elongates. 
The  name  given  to  this  branch  of  the  cell  is  the  "axis  cylinder 
process,"  or,  more  briefly,  the  axon.  The  axon  itself  branches, 
but  not  very  richly;  its  branches  are  usually  short  and  at  right  angles 
to  its  own  course.  The  branches  of  the  axon  are  known  as  collat- 
erals. While  the  axon  is  pushing  its  way  farther  and  farther  from 
the  cell  body,  the  latter  is  developing  its  internal  system  of  fibrils; 
and  it  presently  puts  forth,  on  the  side  opposite  to  the  axon,  other 
branches,  quite  different  from  the  axon,  more  richly  branched, 
branched  in  many  cases  much  like  a  tree,  and  hence  called  den- 
drites  (compare  Fig.  13). 

Leaving  further  description  of  the  cell  body  and  dendrites  to  a 
later  page,  let  us  return  to  the  axon,  and  endeavor  to  follow  its  de- 
velopment. In  most  parts  of  the  neural  tube,  the  axon  emerges 
from  the  layer  of  nerve-cells  into  the  marginal  layer,  which  thus 
becomes  a  layer  of  axons.  Its  further  course  differs  much  in  differ- 
ent cases.  In  the  case  of  a  very  important  group  of  cells,  which 
lie  on  the  ventral 1  side  of  the  neural  tube,  the  axon  passes  straight 
out  through  the  marginal  layer  and  into  the  surrounding  tissues; 
there  it  grows  into  a  developing  muscle,  and,  as  with  the  general 
growth  of  the  body  the  muscle  comes  to  lie  further  and  further  from 
the  neural  tube,  the  axon  continues  to  elongate.  In  this  manner 
it  comes  to  connect  some  small  portion  of  the  muscle  with  the  spinal 
cord.  This  axon  is  a  motor  nerve-fibre,  and  its  cell,  which  lies 
back  in  the  ventral  portion  of  the  neural  tube,  is  a  motor  nerve-cell. 
Later  this  cell  will  send  "nerve  impulses"  down  along  the  axon, 
arousing  a  portion  of  the  muscle  to  action.  As  the  motor  nerve-cells 
are  numerous  and  near  together,  their  axons  emerge  from  the  tube 
in  bundles  or  "rootlets,"  which  come  together  to  form  "roots." 
These  roots,  emerging  from  the  ventral  side  of  the  tube,  are  the 
ventral  or  motor  roots  of  the  spinal  nerves  (see  Fig.  14). 

There  are  also  dorsal  or  sensory  roots.  These  are  formed,  not 
by  outgrowth  of  axons  from  cells  in  the  tube,  but  by  the  growing  into 
the  tube  of  axons  from  outside  cells  that  lie  in  the  two  strips  which 
are  formed  from  the  neural  crest,  on  the  right  and  the  left  of  the  tube 
and  toward  its  dorsal  side.  Cells  within  these  strips  develop  in 
much  the  same  way  as  the  nerve-cells  within  the  tube;  except  that 
each  of  the  former  sends  out  two  axons,  one  of  which  grows  toward 
the  periphery  of  the  body  and  connects  with  some  sense-organ,  while 
the  other  grows  toward  and  penetrates  the  neural  tube.  The  axon 
which  grows  toward  the  periphery,  with  others  from  near  by,  forms 
a  dorsal  root;  this  dorsal  root  soon  joins  the  ventral  root  from  the 

1  "Ventral"  means  on  the  side  toward  the  animal's  ventrum  or  belly;  and 
"dorsal,"  the  corresponding  word,  means  on  the  side  toward  the  dorsum  or  back. 


GROWTH  OF  NERVE  AXONS  AND  FIBRES 


43 


adjacent  part  of  the  tube,  and  the  two  together  form  one  of  the  spinal 
or  cranial  nerves. 

The  nerves  come  off  in  pairs,  on  the  right  and  left  sides  of  the 
tube.  The  two  strips  which  gave  rise  to  the  sensory  cells  and  to 
their  axons  become  compacted  into  two  rows  of  "ganglia,"  or 
bunches  of  nerve-cells,  which  lie  on  the  dorsal  roots,  and  are  called 


FIG.  14. — Axons  Growing  in  Different  Directions.  (Cajal.)  From 
the  spinal  cord  of  an  embryonic  chick.  A,  ventral  root;  B, 
spinal  ganglion  and  dorsal  root;  R,  bifurcation  of  dorsal  root 
axons  on  entering  the  cord ;  d,  cell  giving  rise  to  ventral  root 
axon;  c,  c,  commissural  axons. 


spinal  ganglia.  The  cells  within  these  ganglia,  which  at  first  sent 
out  an  axon  in  each  direction,  later  take  a  place  to  one  side  of  their 
axons,  and  then  withdraw  to  some  distance  from  them,  retaining 
connection  by  a  single  strand;  in  their  fully  developed  form  they  ap- 
pear to  have  but  a  single  axon,  which,  however,  speedily  divides 
into  two.  »These  ganglion  cells  put  out  no  dendrites;  but  that  one 
of  their  axons  which  penetrates  the  neural  tube  splits  in  the  marginal 


44 


THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 


layer  into  an  ascending  and  a  descending  branch;  it  also  sends  off 
collaterals  to  the  neighboring  motor  nerve-cells. 

Not  all  of  the  nerve-cells  in  the  tube  send  axons  out  to  form  motor 

fibres.  The  axons  of  many  of 
them,  on  reaching  the  marginal 
layer,  simply  turn  upward  or 
downward  within  it,  while  still 
others  pass  horizontally  around 
through  the  marginal  zone,  and 
cross  in  its  ventral  portion  to 
the  other  side  of  the  tube.  By 
this  contrivance  the  right  and 
left  halves  of  the  tube  are  con- 
nected. These  connections  are, 
therefore,  called  "commissural 
fibres,"  and  their  crossing-place 
at  the  mid-ventral  line  is  called 
the  "ventral  commissure." 

§  6.  The  axons  thus  far  de- 
scribed may  now  be  grouped 
into  three  classes:  (1)  the  "ven- 
tral root  axons,"  which  arise 
from  cells  in  the  ventral  part  of 
the  tube  and  pass  out,  as  mo- 
tor fibres,  connecting  the  tube 
with  muscles;  (2)  the  "dorsal 
root  axons,"  which  arise  from 
cells  in  the  spinal  ganglia,  and 
by  their  two  axons,  connect  the 
tube  with  sense-organs;  and  (3) 
the  "central  axons, 
wholly  within  the 
connect  one  part  of  it  with 
another  (compare  Fig.  15). 
There  is  still  a  fourth  class  of 
axons  to  be  added.  Some  of  the 
cells  in  the  neural  crest,  instead 
of  remaining  near  the  dorsal  part 
of  the  tube  and  sending  axons 
into  it,  migrate  in  a  ventral  di- 
rection and  develop  into  nerve-cells;  but  these  send  out  only  a  single 
axon,  which  passes  to  some  of  the  organs  of  the  interior  of  the  body — 
the  stomach,  intestines,  pancreas,  salivary  glands,  heart,  and  blood- 
vessels— and  enters  into  connection  with  muscular  or  glandular  tissue 


FIG.  15. — Development  of  the  Nerve-Cells  of 
the  Spinal  Ganglia.  (Cajal.)  The  drawing 
is  from  an  embryo  chick.  A  is  the  spinal 
cord,  containing  d,  a  motor  nerve-cell,  from 
which  issues  an  axon;  this  with  other  simi- 
lar axons  emerges  from  the  cord  into  B,  the 
ventral  root  (the  connection  between  this 
root  and  the  cord  has  been  accidentally 
broken  in  this  section).  C  is  the  dorsal  root, 
consisting  of  axons  which  issue  from  the 
spinal  ganglion  D.  Axons  issuing  from  this 
ganglion  in  the  opposite  direction  join  those 
of  the  ventral  root,  to  form  E,  a  spinal  nerve. 
Within  the  spinal  ganglion  are  seen  cells 
which  are  still  bipolar,  h;  others,  i,  which  are 
becoming  transformed  into  unipolar  cells; 
and  others,  as  ;,  which  are  already  distinctly 
unipolar.  F  is  a  sympathetic  ganglion, 
showing  cells  with  axons,  a,  which  join  the 
spinal  nerve,  and  others  with  axons,  e,  which 
do  not  join  this  nerve,  but  which,  as  a  mat- 
ter of  fact,  pass  up  or  down  in  the  "  sym- 
pathetic chain." 


"  which  lie 
tube,    and 


FORMATION  OF  THE  SPINAL  CORD  45 

there.  Such  are  the  visceral  or  "sympathetic"  nerve-fibres;  and 
their  cell  bodies  are  gathered  in  the  ganglia  of  the  "sympathetic 
system,"  some  of  which  lie  in  two  rows  close  to  the  backbone  on  its 
ventral  side,  while  others  are  scattered  among  the  internal  organs. 
These  axons  and  their  cell  bodies,  it  will  be  noted,  are  entirely  out- 
side of  the  neural  tube.  They  do  not,  however,  remain  uncon- 
nected with  it;  for  certain  cells  exist  in  the  lateral  portions  of  the 
tube,  which  send  out  axons  through  the  ventral  roots  to  the  sympa- 
thetic ganglia. 

§  7.  We  are  now  in  a  position  to  understand  the  process  by  which 
the  lower  part  of  the  neural  tube  becomes  the  spinal  cord.  We  have 
first  the  three  layers  of  the  wall,  the  germinal  layer  next  the  lumen, 
the  nerve-cell  layer  in  the  middle,  and  on  the  outside  the  marginal 
layer,  which  becomes  the  axon  layer.  Later,  the  axons  receive 
sheaths  of  a  white  "medullary"  substance  called  "myelin";  and 
thus  the  axon  layer  becomes  "white  matter";  the  white  matter  of 
the  cord  lies  in  the  marginal  layer.  The  nerve-cell  layer,  on  the 
contrary,  does  not  acquire  much  of  this  medullary  substance,  but 
retains  its  light-gray  or  watery  color,  and  is  therefore  called  the 
"gray  matter."  Thus  the  gray  matter  of  the  cord  is  surrounded 
by  white  matter.  Meanwhile  the  germinal  layer  decreases  in  thick- 
ness, since  the  cells  in  it  divide,  one  after  another,  giving  rise  to 
neuroglia  and  nerve-cells,  which  migrate  outward;  in  this  way  the 
germinal  layer  finally  becomes  reduced  to  a  single  thickness  of  cells 
lining  the  cavity  of  the  tube.  The  other  two  layers  on  the  con- 
trary increase  greatly  in  thickness — the  cell  layer  by  receiving 
new  cells  from  the  germinal  layer,  by  growth  in  size  of  the  cells  al- 
ready present  in  it,  by  growth  of  the  dendrites  of  these  cells,  and  by 
the  incoming  of  the  terminations  of  axons  into  the  cell  layer.  The 
axon  layer  increases  by  the  constant  growth  of  new  axons  into  it 
from  the  adjoining  part  of  the  cell  layer,  and  by  the  elongation  of 
axons  from  the  marginal  layer  above  and  below.  The  thickening 
of  the  cell  and  axon  layers  is  not,  however,  uniform  all  around  the 
tube ;  in  particular,  the  mid-ventral  and  mid-dorsal  portions  do  not 
develop  cell  layers  of  much  thickness;  since  their  cells  migrate 
mostly  to  the  right  and  to  the  left.  At  the  mid-ventral  line,  there- 
fore, we  find  in  the  developed  cord,  first,  a  thin  layer  of  gray  matter 
next  to  the  central  canal,  then  a  thin  layer  of  white,  consisting  of 
fibres  crossing  from  one  side  of  the  cord  to  the  other  (the  ventral 
commissure),  and  then  a  deep  groove  or  fissure,  the  "ventral 
fissure,"  which  runs  lengthwise  of  the  cord  between  the  swollen 
walls  of  the  adjacent  parts  of  the  tube.  In  the  dorsal  mid-line,  there 
is  a  thin  layer  of  gray  matter,  and  then  a  thin  sheet  of  neuroglia, 
the  "dorsal  septum,"  which  has  resulted  from  the  narrowing  of 


46         THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 

the   cavity  to  a  slit,  and  the  subsequent  growing  together  of  its 
side  walls. 

The  gray  matter  is  most  abundant  in  the  ventral  half  of  the  cord, 
which  contains  the  cell  bodies  of  the  motor  axons.  These  bodies 
are  especially  numerous  at  the  levels  of  the  shoulders  and  loins, 
where  the  nerves  to  the  arms  and  legs  originate.  The  dorsal  half 
of  the  gray  matter  is  thinner  in  these  parts,  for  the  reason  that  the 
cell-bodies  of  the  sensory  axons,  instead  of  lying  within  the  cord,  are 
in  the  spinal  ganglia  outside  of  the  cord.  From  the  ventral  half  of 


FIG.  16. — Cross  Section  of  a  Child's  Spinal  Cord.  (Marburg.)  Magnified  8  diameters. 
The  stain  employed  to  bring  out  the  features  of  the  cord  has  darkened  the  nerve-fibres 
and  therefore  the  white  matter,  and  left  the  gray  matter  light.  The  ventral  side  of  the 
cord  lies  below  in  the  figure.  The  cracks,  showing  white  in  the  figure,  are  produced  in 
the  process  of  preparing  the  section,  but  doubtless  represent  natural  lines  of  cleavage, 
determined  by  the  course  of  neuroglia  fibres. 

the  gray  matter  emerge  the  motor  roots;  and  into  the  dorsal  half 
enter  the  sensory  roots.  Thus  at  these  two  places,  the  gray  matter 
approaches  nearest  the  surface  of  the  cord,  forming  the  ventral 
and  dorsal  "horns"  of  the  gray  matter.  Between  these  horns,  and 
continuous  with  them,  lies  a  middle  or  lateral  portion  of  the  gray 
matter.  The  cells  in  the  lateral  portion  and  in  the  dorsal  horn  give 
rise  to  central  axons,  which  do  not  emerge  from  the  cord,  but  turn 
upward  or  downward  in  the  marginal  layer.  Many  central  axons 
come  also  from  the  ventral  horn  (see  Fig.  16). 

The  main  principle  to  be  borne  in  mind  in  studying  the  marginal 
or  axon  layer — later  the  white  matter  of  the  cord — is  that  an  axon 
entering  it  from  the  gray  matter  usually  turns  up  or  down  very 
promptly,  and  thus  starts  its  longitudinal  course  near  to  the  gray 
matter.  As  it  elongates  itself,  whether  upward  or  downward,  it 


FORMATION  OF  THE  SPINAL  CORD 


47 


is  crowded  outward  by  axons  emerging  at  the  level  which  it  has 
reached.  The  further  it  goes,  the  more  is  it  crowded  toward  the 
outer  surface  of  the  cord.  There  is,  however,  one  striking  excep- 
tion to  this  general  rule.  Most  of 
the  axons  from  the  "  motor  area " 
of  the  brain,  which  grow  down  into 
the  cord  late  in  its  development 
(during  the  fifth  month  of  foetal 
life),  instead  of  lying  at  the  very 
outside  of  the  cord,  bore  their  way 
through  the  midst  of  the  lateral 
strand  of  white  matter.  With  this 
and  a  few  other  exceptions,  the 
axons  which  lie  near  the  gray  mat- 
ter are  near  their 
cell  bodies,  while 
those  which  lie  to- 
ward the  outside 
of  the  cord  are  far 
from  their  place  of 
origin.  For  this 
reason,  that  por- 
tion of  the  white 
matter  which  lies 
next  the  gray  is 
called  the  "  ground 
bundle,"  while  the 
portions  further 
out  are  composed 
in  large  measure 
of  definite  groups  or  "tracts"  of 
axons  connecting  distant  portions 
of  the  cord  and  brain.  Many  of 
the  axons  in  the  ground  bundle 
never  extend  far  from  their  cells, 
but  turn  back  into  the  gray  matter 
and  so  connect  neighboring  levels 
of  the  cord.  From  comparative 
study  of  the  nervous  systems  of 
various  orders  of  animals,  it  is 
believed  that  these  short,  axons  of  the  ground  bundle  represent 
a  primitive  and  universal  system  of  connections  within  the  nerve- 
centres;  while  the  long  axons  of  the  tracts  represent  a  later  and 
higher  development. 


FIG.  17. —  Bifurcation  of  the  Sensory 
Root  Fibres,  at  Their  Entrance  to  the 
Cord.  (Cajal.) 


48         THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 

The  sensory  axons  entering  by  the  dorsal  roots  bifurcate  into 
ascending  and  descending  branches,  and  these,  like  other  axons, 
entering  the  marginal  layer,  lie  at  first  near  to  their  point  of  entry 
(compare  Fig.  17).  As  they  grow  upward  (the  downward  branches 
remain  short)  they  are  subsequently  crowded  away  from  the  region  of 
the  dorsal  roots  by  other  similar  axons.  They  therefore  turn  into  the 
marginal  layer  on  the  dorsal  side  of  their  roots,  i.  e.,  toward  the  mid- 
dorsal  line;  and  they  become  still  further  crowded  toward  this  line 
by  the  fresh  fibres  entering  higher  up  the  cord.  In  this  manner  a 
thick  column  of  sensory  axons  is  formed  between  the  dorsal  roots  and 
the  mi'd-dorsal  line;  and  at  each  level  of  the  cord,  those  axons  which 
lie  nearest  the  mid-dorsal  line  have  come  from  farthest  down  the  cord, 
while  those  which  lie  nearest  the  roots  have  come  from  near  by. 

The  dorsal  column  of  white  matter  is  clearly  marked  off  from  the 
rest  of  the  white  matter  by  the  intervention  of  the  dorsal  roots  and 
the  dorsal  horn  of  the  gray  matter.  The  remainder  of  the  white 
matter  is  less  completely  divided  by  the  ventral  horn  and  roots  into 
a  lateral  and  a  ventral  column. 

§  8.  The  brain  is  formed  from  the  forward  or  headward  end  of 
the  neural  tube.  In  the  human  embryo  of  two  weeks'  growth  the 
tube  shows  at  this  end  a  considerable  degree  of  complication.  A 
sharp  bend  appears  in  it,  the  front  end  being  bent  in  the  ventral 
direction.  In  front  of  this  "cephalic  flexure"  the  tube  expands 
somewhat  into  a  pouch  or  vesicle,  and  behind  into  another  vesicle 
which  tapers  downward  into  the  spinal  part  of  the  tube.  Just  be- 
fore and  behind  the  flexure  are  two  shallow  transverse  grooves  which 
mark  off  a  third  vesicle  between  the  other  two.  These  three  primary 
brain  vesicles,  appearing  thus  early  in  the  individual's  life,  are  con- 
stant throughout  all  the  vertebrates,  and  represent  the  fundamental 
divisions  of  the  brain.  Already,  however,  a  division  of  the  fore- 
most vesicle  by  another  shallow  transverse  groove  is  visible  in  the 
human  embryo;  and  a  little  later  a  backward  bend  within  the  hind- 
most vesicle  makes  it  convenient  to  regard  this  vesicle,  also,  as  sub- 
divided. Thus  there  are  five  vesicles  out  of  which  the  various  parts 
of  the  brain  are  developed.  They  are  called,  from  before  backward, 
the  end-brain,  the  inter-brain,  the  mid-brain,  the  hind-brain,  and  the 
after-brain  or  medulla;  or,  in  Greek  derivatives,  the  telencephalon 
("encephalon"  meaning  brain),  the  diencephalon,  the  mesenceph- 
alon,  the  metencephalon,  and  the  myelencephalon.1  Of  these  five 

1  "Myel"  is  the  Greek  derivative  equivalent  to  the  Latin  "medulla,"  which 
means  marrow.  The  spinal  cord  was  called  the  "spinal  marrow,"  or  medulla 
spinalis;  and  the  enlarged  extension  of  the  cord  at  the  base  of  the  cranial  cavity 
was  known  as  the  medulla  oblongata.  Recently,  the  name  "medulla  spinalis" 
has  been  little  used,  and  "medulla"  has  come  to  mean  the  medulla  oblongata. 
The  latter  is  also,  most  briefly,  designated  as  the  "bulb." 


GROWTH  OF  THE  FIVE  BRAIN  VESICLES 


the  after-brain  develops  into  the  medulla;  the  hind-brain  develops 
into  the  pons  and  cerebellum;  the  mid-brain  consists  largely  of  the 
corpora  quadrigemina,  and  the  inter-brain  of  the  optic  thalamus; 
while  the  end-brain  develops  into  the  cerebrum.  The  eye,  which  is 
embryologically  a  part  of  the  brain,  first  appears  as  a  pouch  bulging 
outw.ard  at  about  the  junction  of  the  inter-brain  and  end-brain; 
its  permanent  attachment  is  to  the  thalamus  (compare  Fig.  18). 

In  the  lowest  part  of  the  medulla  the  conditions  which  affect  the 
external  form  of  the  parts  do  not  differ  essentially  from  those  present 
in  the  cord;  the  walls  of  the 
neural  tube  thicken  greatly,  leav- 
ing a  small  cavity  in  the  centre. 
A  little  further  up,  however,  the 
dorsal  wall,  without  growing  in 
thickness,  increases  greatly  in 
width,  and  lies  as  a  mere  skin, 
containing  no  nerve-cells,  over 
the  expanded  cavity,  which  here 
is  called  the  "  fourth  ventricle  of 
the  brain." 

Forward  of  this,  the  dorsal 
wall  becomes  enormously  thick- 
ened, forming  the  cerebellum. 
Still  further  forward,  in  the 
mid-brain,  the  dorsal  wall  main- 
tains a  moderate  thickness,  and 
swells  into  two  hills  on  each  side, 
the  corpora  quadrigemina.  The 
cavity  of  the  neural  tube  here 

remains  small,  and  forms  the  "aqueduct"  connecting  the  fourth 
with  the  third  ventricle.  It  is  in  the  inter-brain  that  the  aqueduct 
expands  into  the  third  ventricle;  and  here  again  the  dorsal  wall  of 
the  tube  becomes  a  broad,  thin  membrane,  folded  into  the  ventricle 
and  carrying  blood-vessels  with  it.  As  the  cerebral  hemispheres 
undergo  their  enormous  growth,  the  cavity  expands  with  them,  and 
remains  connected  with  the  third  ventricle  by  two  small  openings, 
the  foramina  of  Munro.  Within  the  hemispheres,  the  cavities  are 
known  as  the  lateral  ventricles;  in  the  numbering  they  count  as  the 
first  and  second.  Thus,  in  spite  of  all  its  bendings  and  thicken- 
ings, the  neural  tube  remains  a  tube,  and  its  lumen  is  continuous 
from  the  lateral  ventricles,  through  the  foramina  of  Munro,  the 
third  ventricle,  the  aqueduct,  and  the  fourth  ventricle,  with  the 
central  canal  of  the  cord.  This  cavity  is  filled  with  a  lymph  known 
as  the  cerebro-spinal  fluid. 


FIG.  18. — Brain  of  a  Developing  Chick. 
(Mihalkovics.)  The  numbers  indicate: 
I,  the  end-brain;  II,  the  inter-brain;  ///, 
the  mid-brain;  IV,  the  hind-brain;  V,  the 
after-brain  or  bulb,  tapering  downward 
into  the  cord. 


50 


THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 


§  9.  In  the  end-brain  of  the  two-weeks-old  embryo,  certain  parts 
can  be  distinguished  which  retain  their  identity  throughout  the  later 
development  (compare  Fig.  19).  The  ventral  wall,  on  each  side  of 
the  middle  line,  shows  a  slight  swelling,  which  becomes  pronounced 
at  the  age  of  four  weeks,  and  later  grows  forward  and  is  entered 


Ganglion  of  fifth  nerve        Ear 


Liver 


Arm 


Leg 


FIG.  19.— Human  Embryo  of  About  Four  Weeks.  (Streeter.)  1,  2,  3,  4,  5,  the  brain 
vesicles;  continuous  with  5,  the  spinal  cord  can  be  followed  down,  close  beneath  the 
ectoderm,  to  the  posterior  extremity.  The  spinal  ganglia  show  as  round,  dark  masses, 
connected  by  (light)  rootlets  with  the  cord,  and  on  the  other  side  connected  with  the 
nerves,  which  also  show  light.  The  rudiments  of  arm  and  leg  show  as  transparent  masses, 
and  a  group  of  nerves  can  be  seen  entering  each. 

by  axons  of  cells  lying  in  the  upper  part  of  the  nasal  cavity.  This 
is  the  olfactory  lobe.  Its  forward  end  swells  into  the  olfactory  bulb, 
which  receives  the  axons  from  the  nose  and  contains  nerve-cells 
whose  axons  in  turn  pass  back  through  the  hinder  part  of  the  lobe 
(the  olfactory  tract)  to  other  parts  of  the  brain.  Just  lateral  to  the 
ventral  portion  of  the  end-brain  out  of  which  the  olfactory  lobe  is 
formed,  there  appears,  at  four  weeks,  a  shallow  furrow,  which  indi- 


GROWTH  OF  THE  FIVE  BRAIN  VESICLES  51 

cates  the  future  position  of  the  mass  of  gray  matter  in  the  base  of 
the  hemispheres  known  as  the  corpus  striatum.  The  dorsal  half 
of  the  end-brain  shows,  even  at  two  weeks,  a  slight  swelling,  which 
grows  rapidly.  This  is  called  the  "brain-mantle,"  or  "pallium," 
and  it  gives  rise  to  the  great  bulk  of  the  cerebral  hemispheres.  The 
pallium  is  the  part  of  the  nervous  system  whose  complex  develop- 
ment characterizes  the  higher  mammals,  and  especially  man. 

In  comprehending  the  general  shape  of  the  cerebral  hemispheres, 
there  is  one  fact  of  prime  importance.  The  great  increase  in  size 
of  this  part  of  the  neural  tube  does  not  proceed  by  further  extension 
of  its  length,  from  its  end  forward.  The  middle  of  the  forward 
end  does  not  itself  grow,  but  remains  as  a  fixed  limit,  or  so-called 
"lamina  terminalis,"  which  is  a  membrane  without  nervous  func- 
tions. The  growth  of  the  end-brain"  may,  therefore,  be  described 
as  primarily  lateral;  and  then,  within  each  lateral  half,  forward, 
backward,  and  upward,  as  well  as  to  the  side.  One  result  of 
this  method  of  growth  is  the  division  of  the  swelling  pallium  into 
right  and  left  hemispheres — a  division  which  appears  very  early 
in  the  growth  of  the  embryo.  Accompanying  this  is  the  enlarge- 
ment of  the  cavity  which  forms  the  lateral  ventricles,  and  which 
extends  forward  and  backward  within  each  hemisphere.  The  back- 
ward growth  of  the  hemispheres  causes  them  to  overlap  the  thala- 
mus  and  mid-brain,  and  finally,  in  the  human  species,  the  cere- 
bellum as  well.  The  corpora  striata  grow  forward  and  backward 
with  the  pallium,  and  overlap  the  thalamus  so  as  to  appear,  in  the 
adult  brain,  rather  to  the  side  of  the  thalamus  than  in  front  of  it. 
A  vertical  or  "frontal"  section  through  the  midst  of  the  brain  shows, 
at  the  exterior  surface,  the  cortex;  next  this  the  white  matter  of  the 
hemispheres;  then  the  corpus  striatum;  and  in  the  centre  the  thal- 
amus. 

§  10.  As  the  hemispheres  grow  forward,  backward,  and  upward 
from  their  place  of  attachment  to  the  inter-brain,  each  of  them  comes 
to  present  a  two-lobed  appearance,  somewhat  like  that  of  a  bean, 
the  notch  corresponding  to  the  point  of  attachment.  The  front  and 
rear  lobes  grow  at  first  downward,  and  then  toward  each  other, 
making  the  notch  deeper;  the  wall  above  the  notch  grows  out  and 
overhangs  it.  Thus  is  formed  a  fissure,  the  "fissure  of  Sylvius," 
the  first  and  principal  landmark  on  the  lateral  surface  of  the  hemi- 
sphere. The  part  in  front  of  this  fissure  is  the  frontal  lobe,  and  that 
behind  and  below  it  is  the  temporal  lobe.  The  surface  within  the 
fissure,  which  is  considerable,  is  clearly  visible  at  the  fourth  month 
of  foetal  life,  but  becomes  partly  concealed  at  the  seventh  month, 
by  the  overgrowth  of  the  frontal  and  temporal  lobes;  in  the  fully 
developed  brain  it  is  wholly  concealed,  and  from  its  secluded  posi- 


52         THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 

tion  is  called  the  "  island."  Both  the  Sylvian  fissure  and  the  longi- 
tudinal fissure  which  separates  the  hemispheres  are  visible  during 
the  second  month;  in  the  succeeding  months  many  other  fissures 
appear,  till  the  surface  becomes  thickly  seamed  with  them. 


Lateral 
ventricle 


Thalamus 


Lenticular. 


FIG.  20. — Frontal  Section  of  the  Fore-brain  of  an  Eleven  Weeks'  Embryo.  (His.)  The  skull 
as  well  as  the  brain  is  shown,  and,  on  the  left  side  of  the  figure,  the  eye.  Gray  matter 
appears  dark,  and  white  matter  light;  the  skull  also  appears  light,  and  the  ventricular 
spaces,  or  cavity  of  the  neural  tube,  appear  white.  There  is  also  a  clear  space  between 
the  skull  and  the  brain.  The  two  largest  and  nearly  equal  spaces  are  the  lateral  ven- 
tricles; each  is  surrounded  by  the  pallium,  except  below,  where  it  is  bounded  by  the 
dark  rounded  mass  of  the  caudate  nucleus,  a  part  of  the  striatum.  On  the  left  side  of 
the  figure,  the  caudate  nucleus  is  wholly  separated  by  a  white  mass  of  fibres  from  another, 
less  dark  nucleus,  the  lenticular,  which  also  is  part  of  the  striatum.  The  mass  of  fibres 
separating  these  two  parts  of  the  striatum  is  the  internal  capsule,  and  the  fibres  connect 
the  pallium  with  the  thalamus.  The  thalamus  appears  as  two  nearly  semicircular  masses 
of  gray  matter,  separated  by  a  median  cleft,  which  is  the  third  ventricle,  part  of  the  cavity 
of  the  tube.  On  the  right  side,  thalamus  and  striatum  are  not  yet  joined  by  the  crossing 
fibres.  The  pallium  is  thickest  at  its  junction  with  the  striatum;  it  shows  three  layers, 
the  germinal  layer  (dark)  next  the  lateral  ventricle,  a  light  fibre-layer  next,  and  on  the 
outside  another  dark  layer  of  cells,  the  cortex.  The  lowermost  part  of  the  brain,  in  the 
figure,  is  olfactory.  Into  each  lateral  ventricle  extends  a  membrane,  which  is  a  part  of 
the  wall  of  the  tube  doubled  inward  and  carrying  blood-vessels. 

The  development  of  neuroglia,  nerve-cells,  and  axons  in  the  brain 
proceeds  in  essentially  the  same  manner  as  in  the  cord.  The  germi- 
nal cells,  which  by  division  give  birth  to  cells  destined  to  grow  into 
neuroglia  and  nerve-cells,  in  the  brain  as  in  the  cord,  lie  next  to  the 
cavity  of  the  neural  tube.  The  primitive  neuroglia  framework  is 
first  formed;  after  which  the  cells  that  migrate  from  the  germinal 


DEVELOPMENT  OF  THE  CORTEX 


53 


layer  into  the  middle  or  cell  layer  begin  to  take  on  the  character- 
istics of  nerve-cells.  Thus  there  appear  in  the  developing  cortex  of 
the  cerebrum  the  same  three  layers  as  in  the  cord:  the  germinal 
layer  inside,  then  the  nerve-cell  layer,  and,  outermost,  the  marginal 
layer.  But  there  is  one  marked  difference  between  the  cord  and  the 
cortex.  The  axons  of  the  nerve-cells,  instead  of  issuing  from  the 
cell  layer  into  the  marginal  layer,  go  from  their  cells  in  the  opposite  di- 
rection, toward  the 
germinal  layer. 
Before  reaching  this, 
they  turn  to  the  side; 
and  so  form  a  layer 
of  axons  between  the 
cell  layer  and  the  in- 
ner cavity  or  ventri- 
cle. Thus  it  is  that 
the  white  matter  of 
the  hemispheres  lies 
inside  the  gray,  in- 
stead of  outside  as 
in  the  cord.  From 
its  position  on  the 
outside,  the  gray  mat- 
ter of  the  hemispheres 
has  been  named  the 
"bark "or  "cortex." 
The  first  nerve-fibres 
to  appear  in  the 
white  matter  of  the 
cerebrum  do  not 
arise  from  the  cor- 
tex, but  come  into 
the  hemispheres 
from  below.  They 
arise  from  cells  in 
the  thalamus,  and  break  through  the  corpus  striatum,  dividing  its 
gray  matter  into  two  principal  masses,  and  forming  the  "internal 
capsule"  between  them.  From  here  they  pass  into  the  adjoining 
wall  of  the  hemisphere,  and  run  along  in  it,  between  the  germinal 
and  nerve-cell  layers.  The  axons  from  the  thalamus  are  sensory: 
they  come  from  groups  of  nerve-cells,  which  are  connected  with  the 
nervous  apparatus  of  sight,  hearing,  and  touch;  and  they  termi- 
nate in  areas  of  the  cortex  which  come  thus  to  be  chiefly  concerned 
with  the  corresponding  senses. 


FIG.  21. — Brain  of  an  Embryo  of  About  Three  Months. 
(His.)  Inter-brain  and  mid-brain  are  already  concealed 
by  the  pallium;  the  cerebellum  and  bulb  are  shown;  also, 
through  the  fissure  of  Sylvius,  some  of  the  olfactory  re- 
gion can  be  seen. 


54         THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 

§  11.  The  cortex  develops  late  in  comparison  with  other  parts  of 
the  nervous  system.  Nerve-cells  begin  to  appear  here  at  about  the 
beginning  of  the  third  month  of  foetal  life,  or  more  than  a  month  later 
than  in  the  cord  (compare  Fig.  20).  The  appearance  of  the  axons 
and  dendrites  of  the  cortical  cells  is  also  late,  and  indeed  the  cor- 
tex is  by  no  means  fully  developed  at  birth.  The  specialization  of 
the  pallial  wall  into  a  nervous  structure,  with  gray  cortex  and  under- 
lying white  matter,  begins  at  a  definite  part  of  the  wall;  namely, 
where  it  joins  the  corpus  striatum,  or  at  the  junction  of  the  pallial 


FIG.  22.— Frontal  Section  of  the  Brain  at  the  End  of  the  Fourth 
Month.  (His.)  The  callosum  appears  clearly.  At  each  side  of 
the  figure  a  depression  in  the  wall  of  the  hemisphere  indicates 
the  fissure  of  Sylvius. 

outgrowth  with  the  main  stem  of  the  neural  tube.  From  here,  the 
development  of  a  cortex  spreads  up  over  the  lateral  surface,  and 
finally  over  the  top  down  into  the  median  fissure  between  the  hemi- 
spheres. Near  the  bottom  of  this  fissure  the  cortex  ends,  leaving  a 
narrow  strip  of  the  wall  free  from  the  usual  covering  of  gray  matter. 
The  exposed  strip  of  white  matter  of  the  right  hemisphere  is  in  close 
proximity  to  that  of  the  left,  and  at  a  later  stage  the  two  strips  grow 
together  in  the  central  portion  of  their  extent,  and  thus  the  white  mat- 
ter of  the  two  hemispheres  becomes  continuous.  Axons,  in  vast 
numbers,  here  cross  over  from  one  hemisphere  to  the  other,  bring- 
ing the  two  into  functional  connection.  This  bridge  between  the 
hemispheres  is  the  corpus  callosum  (see  Fig.  22). 

§  12.  In  the  cerebellum,  the  process  of  inner  growth  is  essen- 
tially the  same  as  in  the  cerebrum.  Here,  too,  the  axons  of  the  cells 
which  have  reached  the  nerve-cell  layer  pass  inward,  and  not 
toward  the  marginal  layer,  so  that  the  gray  matter  lies  on  the  sur- 


THE  TWELVE  PAIRS  OF  CRANIAL  NERVES  55 

face  and  the  white  matter  inside.  Possibly  the  axons  take  the  inner 
course  because,  in  outgrowths  of  the  tube  such  as  the  cerebrum  and 
cerebellum,  this  is  the  shortest  path  to  the  fundamental  centres  of 
the  tube  proper.  The  external  location  of  the  gray  matter,  further, 
favors  the  spreading  out  of  its  great  mass  into  a  thin  layer  readily 
accessible  at  all  points  to  axons  coming  from  without.  In  the  cere- 
bellum also,  as  in  the  cerebrum,  there  are,  besides  the  cortex,  other 
collections  of  nerve-cells  lying  in  the  basal  part  near  to  its  attach- 
ment. The  cerebellum  resembles  the  cerebrum  also  in  its  late  de- 
velopment; in  man  its  nervous  structures  are  far  from  complete  at 
birth. 

§  13.  The  brain-stem  cannot  be  properly  understood  without 
reference  to  the  nerves  which  issue  from  it.  These  are  called  the 
cranial  nerves,  and  there  are  twelve  of  them  on  each  side,  desig- 
nated by  numbers  and  also  by  names  more  or  less  expressive  of 
their  distribution  and  function.  The  list  comprises: 

I.  The  olfactory. 

II.  The  optic. 

III.  The  oculomotor.     This  is  the  principal  motor  nerve  to  the 
eye;  it  innervates  the  muscle  of  accommodation,  and  also  four  of  the 
six  muscles  which  move  the  eyeball. 

IV.  The  trochlear.     This  is  the  motor  nerve  to  the  "trochlear" 
or  superior  oblique  muscle  of  the  eyeball. 

V.  The  trigeminal,  so  called  because  it  promptly  splits  into  three 
branches,  which,  between  them,  supply  the  skin  of  the  face,  and  the 
sensitive  surfaces  of  the  eyeball,  nose,  and  front  of  the  mouth,  with 
sensory  axons.     The  fifth  pair  is  thus  the  nerve  of  "touch"  for  the 
whole  face.     It  also  contains  motor  axons  in  smaller  quantity;  these 
innervate  the  muscles  of  mastication. 

VI.  The  abducens,  the  motor  nerve  to  the  external  rectus  muscle 
of  the  eyeball,  which  rolls  the  eye  to  the  side. 

VII.  The  facial.     This  is  the  main  motor  nerve  of  the  face,  and 
controls  the  muscles  of  facial  expression.     It  also  includes  a  small 
and  distinct  bundle  of  sensory  fibres,  which  come  from  the  taste- 
buds  of  the  front  portion  of  the  tongue. 

VIII.  The  auditory.     This  again  consists  of  two  distinct  por- 
tions, both  sensory,  one  of  which  is  connected  with  the  cochlea  of 
the  inner  ear,  the  organ  of  hearing,  and  the  other  with  the  remainder 
of  the  inner  ear,  which  apparently  has  to  do,  not  with  hearing,  but 
with  sensations  of  movement  and  posture. 

IX.  The  glosso-pharyngeal,  or  sensory  nerve  to  the  back  of  the 
tongue  and  to  the  throat,  including  the  taste-buds  in  this  region. 

X.  The  so-called  vagus,  in  allusion  to  its  wide  or  wandering  dis- 
tribution.    It  contains  both  sensory  and  motor  fibres  between  the 


56         THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 

central  organs  and  the  larynx,  windpipe,  lungs,  gullet,  stomach, 
pancreas,  upper  part  of  intestine,  and  heart.  The  outgoing  fibres 
to  the  heart  can  be  called  "motor"  only  in  a  loose  sense,  since  their 
action  is  confined  to  the  slowing  or  stoppage  of  the  heart-beat. 

XL  The  spinal  accessory,  a  motor  nerve  to  some  of  the  muscles 
at  the  back  of  the  neck  which  participate  in  head  movements. 

XII.     The  hypoglossal,  the  motor  nerve  of  the  tongue. 

Of  these  twelve  pairs  of  nerves  the  first  consists  of  axons  arising 
from  cells  in  the  olfactory  mucous  membrane  of  the  upper  part  of 
the  nose,  these  cells  being  the  peripheral  organs  of  smell,  and  being 
derived  from  the  ectoderm.  They  thus  represent  a  primitive  con- 
dition of  the  sensory  nerves  (compare  p.  26),  in  which  the  sensitive 
cells  located  in  the  periphery  have  sent  axons  to  the  centre,  instead 
of  being  supplied  by  axons  growing  out  from  the  centre  or  from 
ganglia  located  near  the  centre.  The  axons  of  the  olfactory  nerve 
enter  the  brain  at  the  olfactory  bulb,  as  previously  described. 

§  14.  The  optic  nerve  also  is  peculiar  in  its  origin,  but  in  a  differ- 
ent way.  Both  this  nerve  and  the  retina  of  the  eye  arise  as  an  out- 
growth of  the  neural  tube;  they  are,  therefore,  properly  to  be  reck- 
oned as  a  part  of  the  brain.  Besides  the  rods  and  cones,  which  are 
specialized  nerve-cells  sensitive  to  light,  the  retina  contains  many 
nerve-cells  similar  to  those  found  in  the  nervous  centres;  and  some 
of  these  send  their  axons  back  through  the  optic  nerve.  The  nerves 
from  the  right  and  left  eyes  meet  at  the  base  of  the  brain  in  the  optic 
"chiasm"  or  crossing.  Here  many  of  the  axons  cross  to  the  other 
half  of  the  body;  and  after  crossing,  the  nerves  proceed  and  enter 
the  thalamus. 

The  nerve  of  the  inner  ear  is  somewhat  peculiar  in  its  mode  of 
development,  in  that  its  ganglion  lies  within  the  ear  itself;  and 
besides,  the  cells  in  this  ganglion  retain  their  "bipolar"  form. 

The  remaining  cranial  nerves  develop  in  the  same  manner  as  the 
spinal  nerves.  Their  sensory  axons  arise  from  ganglia,  analogous 
to  the  spinal  ganglia,  which  lie  on  the  roots  of  the  nerves;  and  their 
motor  axons  grow  out  from  cells  in  the  tube 

§  15.  The  process  of  development,  as  well  as  the  final  structure, 
of  the  brain-stem  is  highly  complicated,  and  it  will  be  sufficient  for 
our  purposes  to  mention  one  general  principle  which  is  of  consid- 
erable aid  in  understanding  the  whole  matter.  We  make  a  distinc- 
tion between  the  more  and  the  less  primitive  structures  contained 
in  any  part.  The  more  primitive  include  (1)  the  nerves  entering 
or  leaving  the  part,  (2)  the  groups  of  cells  connected  with  these 
nerves,  and  (3)  the  fibres  which  link  together  these  groups  of  cells. 
These,  taken  together,  form  the  local  mechanism  of  the  part.  Less 
primitive,  in  relation  to  any  particular  region,  are  fibres  which  grow 


FACTORS  IN  GROWTH  OF  THE  NERVOUS  SYSTEM    57 

into  it  from  other  parts  of  the  neural  tube,  and  also  any  cells  de- 
veloped locally  but  having  connection  with  fibres  from  other  parts. 
Now  since  the  local  mechanisms  of  any  part  develop  in  advance  of 
the  less  primitive  structures,  these  accordingly  grow  over  the  primi- 
tive structures,  here  as  in  the  spinal  cord.  Thus  the  order  of  de- 
velopment helps  toward  an  understanding  of  the  position  of  the 
numerous  structures  which  are  found  within  the  brain-stem.  All 
along  the  neural  tube,  it  is  the  ventral  side  which  develops  first; 
and  accordingly  we  find,  in  the  adult  condition  of  the  brain-stem, 
that  the  most  primitive  structures  lie  on  the  ventral  side  of  the  ventri- 
cles, and  close  to  them.  In  the  bulb,  as  was  previously  said,  the 
dorsal  side  of  the  neural  tube  merely  expands  into  a  membrane. 
The  lateral  wall  of  the  tube,  however,  gives  rise  to  many  nerve-cells, 
and  these,  developing  later  than  the  ventral  portion,  are  less  primi- 
tive and  more  related  to  distant  parts  of  the  system.  Many  of  these 
lateral  cells  migrate  in  a  ventral  direction,  and  settle  in  positions 
ventral  to  the  original  ventral  cells;  and  the  fibres  which  in  immense 
numbers  grow  into  the  bulb  from  above  and  below,  pass  on  either 
the  ventral  or  the  lateral  side  of  the  primitive  structures.  Thus  it 
finally  comes  about  that  the  bulb  contains,  close  beneath  the  ven- 
tricle, a  rather  small  central  mass  devoted  to  the  local  mechanisms, 
and  giving  rise  to  the  local  nerves;  while  enclosing  this  mass  on  the 
sides  and  below  is  the  large  mass  of  cells  and  fibres  which  are  less 
local  in  character.  The  development  of  the  pons  and  mid-brain 
proceeds  along  the  same  lines  as  that  of  the  bulb,  except  that  in  the 
mid-brain  there  is  an  important  development  of  the  dorsal  wall  of 
the  tube,  which  gives  rise  to  the  corpora  quadrigemina. 

§  16.  The  factors  in  the  growth  of  the  nervous  system,  as  thus 
far  mentioned  and  applied,  are  (1)  the  production  of  young  nerve- 
cells  from  the  germinal  cells  at  the  lumen  of  the  tube;  (2)  the  migra- 
tion of  these  young  nerve-cells  away  from  the  lumen;  and  (3)  the 
outgrowth  and  prolongation  of  the  axon.  The  first  of  these  factors 
is  only  temporary  in  its  action;  the  reproductive  activity  in  the  ger- 
minal layer  comes  to  an  end  at  different  times  in  different  parts  of 
the  brain  and  cord,  but  in  the  cortex  of  the  cerebrum,  where  it 
starts  latest  and  is  longest  continued,  it  ceases  during  the  fourth 
month  of  fcetal  life.  It  is  probable  that  all  the  nerve-cells  which  will 
finally  belong  to  the  individual  adult  are  formed  at  this  early  period 
in  embryonic  development.  The  migration  of  young  nerve-cells 
is  also  in  the  main  a  temporary  phenomenon.  The  lengthening 
of  the  axon,  on  the  contrary,  is  continued  for  a  long  time,  and  the 
growth  of  the  dendrites  of  the  cell  is  also  a  slow  and  long-continued 
process.  Many  of  the  young  nerve-cells  remain  throughout  foetal 
life  and  even  much  longer  in  an  undeveloped  state.  Many  of  them 


58         THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 

never  develop.  It  appears  that  undeveloped  nerve-cells  in  the  cor- 
tex can  be  made  to  grow  by  stimuli  affecting  the  region  where  they 
lie  and  arousing  neighboring  cells  to  activity — in  other  words,  by 
experience  and  education.  For  example,  Donaldson,1  on  examin- 
ing the  brain  of  Laura  Bridgman,  who  became  blind  and  deaf  when 
a  baby  of  less  than  two  years,  found  those  parts  of  the  cortex  which 
are  normally  connected  with  the  eye  and  ear  to  be  under-developed, 
and  to  contain  an  unusual  number  of  undeveloped  nerve-cells.  The 
growth  of  the  gray  matter,  after  the  production  of  new  nerve-cells 
has  ceased,  results  from  the  growth  of  the  cells  and  especially  of 
their  dendrites;  from  the  coming-in  of  axons  from  the  white  matter 
to  branch  and  terminate  in  the  gray  matter;  and  from  the  continued 
growth  of  the  neuroglia  framework. 

The  increase  of  the  white  matter  in  the  nervous  system  is  due  in 
part  to  the  lengthening  of  axons  and  the  advent  of  new  axons  from 
the  developing  cells  in  the  gray.  It  is  also  partly  due  to  a  growth 
in  thickness  of  the  individual  axons.  But  it  is  very  largely  due  to 
another  factor  which  has  barely  been  mentioned  thus  far.  The 
axon  becomes  invested  with  a  sheath  of  "  myelin,"  a  mixture  of  fat- 
like  substances;  and  the  volume  of  this  sheath,  at  first  small,  in- 
creases gradually  till  it  often  becomes  much  greater  than  the  vol- 
ume of  the  axon  which  it  invests.  Whether  this  myelin  is  secreted 
by  the  axon,  or  by  neuroglia  cells,  is  not  yet  determined.  The 
myelinization  of  an  axon  does  not  occur  as  soon  as  the  axon  has 
made  its  appearance,  but  there  is  often  a  considerable  interval 
during  which  the  axon  remains  without  any  sheath.  In  general, 
those  bundles  of  axons  which  develop  early  also  acquire  myelin 
early,  and  those  bundles  which  appear  late  become  myelinated  late. 
Thus,  for  example,  the  ventral  and  dorsal  root  axons  and  the  short 
axons  of  the  ground  bundle  become  myelinated  early;  while  the  latest 
of  all,  in  the  spinal  cord,  are  the  long  descending  axons  which  come 
from  cells  in  the  cerebral  cortex.  Within  the  cerebrum,  the  first 
axons  to  appear  are  those  from  the  thalamus,  and  these  also,  after 
an  interval  of  several  months,  are  the  first  to  become  myelinated. 
The  inward-growing  or  sensory  axons  of  the  cerebrum  develop 
before  the  outward-growing  or  motor;  and  these  in  turn  before  the 
numerous  "association"  axons  which  extend  from  one  part  of  the 
cortex  to  another. 

§  17.  The  function  of  the  myelin  sheath,  to  which  we  shall  re- 
turn in  a  later  chapter,  is  not  definitely  known;  but  it  seems  rea- 
sonable to  suppose  that  it  has  some  function,  and  that  therefore  the 
axon  itself  does  not  become  fully  functional  till  it  has  become  envel- 

1  American  Journal  of  Psychology,  1890,  III,  293.  Growth  of  the  Brain,  1898, 
p.  240. 


FACTORS  IN  GROWTH  OF  THE  NERVOUS  SYSTEM    59 

oped  by  its  sheath.  It  was  an  easy  inference  that  a  given  bundle  of 
axons,  and  the  connected  portions  of  gray  matter,  did  not  become 
functional  at  all  till  these  axons  were  myelinated;  and  on  this  basis 
it  seemed  possible  to  assert  (so  Flechsig1)  that,  within  the  cortex, 
the  sensory  portions  first  became  functional,  then  the  motor,  and 
later  still  the  remaining  and  presumably  more  intellectual  parts. 
It  has,  however,  been  shown  by  Watson2  that  in  the  white  rat,  co- 
ordinated reflex  action,  requiring  the  use  of  both  sensory  and  motor 
axons  and  also  their  central  connections,  occurs  promptly  at  birth, 
though  no  myelinization  whatever  has  so  far  taken  place;  and  further 
that  the  rat  is  capable  of  forming  and  retaining  definite  associations 
at  an  age  when  its  cortex,  which  is  in  all  probability  concerned  in 
such  learning,  is  entirely  unprovided  with  myelinated  connections. 
The  case  of  the  rat,  therefore,  seems  conclusive  against  the  view 
that  orderly  function  is  impossible  before  the  advent  of  myeliniza- 
tion. On  the  other  hand,  von  Bechterew  reports3  that  sharply 
localized  response  to  electrical  stimulation  of  small  portions  of  the 
nerve-centres  occurs  only  after  myelinization  has  set  in ;  and  Held 4 
found  that  opening  one  eye  of  a  new-born  animal  whose  eyes  do 
not  naturally  open  for  some  time  after  birth  causes  the  central 
connections  of  this  eye  to  become  myelinated  earlier  than  those  of 
the  other  eye.  Wre  may,  perhaps,  conclude  that,  in  some  cases  at 
least,  function  is  possible  without  myelin;  but  that  actual  function- 
ing stimulates  the  deposition  of  the  myelin,  which  in  turn  in  some 
way  assists  function.  There  is  also  evidence  to  show,  in  the  case  of 
the  human  species,  that  control  of  the  muscular  system  which  we 
call  "voluntary"  requires  to  make  use  of  myelinated  nerve- 
tracts. 

§  18.  The  weight  of  the  human  brain  at  birth  averages  about 
380  grams,  or  13  ounces.  This  is  about  one-eighth  of  the  weight 
of  the  entire  body.  During  childhood  and  youth,  other  organs 
rapidly  outstrip  the  brain,  so  that  at  maturity  the  brain  weighs  but 
one-fiftieth  of  the  entire  body.  The  spinal  cord  at  birth  weighs 
one  one-hundredth  as  much  as  the  brain,  and  at  maturity  one- 
fiftieth.  In  other  words,  the  growth  of  the  brain  is  carried  further  in 
foetal  life  than  the  growth  of  the  other  organs.  This  is  true  as  re- 
gards size,  but  not,  perhaps,  as  regards  the  minute  details  of  struct- 
ure which  count  for  so  much  in  the  functions  of  the  nerve-centres. 

1  Flechsig,  Gehirn  und  Seele,  1896. 

2  J.  B.  Watson,  Animal  Education,  1903,  pp.  115  ff. 

3  W.  v.  Bechterew,  Die  Leitungsbahnen  im  Gehirn  und    Riickenmark,    1899, 
p.  104. 

4  Cited  from  Edinger,  Vorlesungen  uber  den  Ban  der  nervosen  Zentralorgane, 
1904,  p.  32. 


60         THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 

The  increase  of  the  brain's  weight  occurs  mostly  in  childhood,  as 
is  seen  from  the  following  figures: 

At  birth,  the  weight  of  the  brain  averages  .     .     .       380  grams 

At  one  year,        "         "       "     "       "  "  ...       945      " 

At  two  years,      "         "       "     "       "  "  ...  1,025      " 

At  three  years,    "         "       "     "       "  "  ...  1,100      " 

At  four  years,      "         "       "     "       "  "  ...  1,300      " 

After  the  age  of  four,  the  human  brain  continues  to  increase  more 
slowly,  reaching  1,400  grams  at  from  eight  to  ten  years;  after  which 
it  gains  in  weight  scarcely  at  all.  According  to  some  series  of  meas- 
urements, however,  the  maximum  weight  is  reached  at  fifteen,  after 
which  age  a  very  gradual  decrease  sets  in.  There  is  no  doubt  that 
there  is  uniformly  some  decrease  after  the  age  of  fifty-five ;  and  in  old 
age,  there  is  often  a  marked  shrinking  in  its  weight.  The  above 
figures  apply  to  the  brain  of  males.  In  the  female,  the  weight  at 
birth  is  nearly  the  same  as  in  the  male;  the  increase  during  child- 
hood is  somewhat  less,  and  the  adult  weight  averages  about  150 
grams  less  than  for  the  male.  In  proportion  to  the  total  weight 
of  the  body,  however,  the  female  brain  is  as  heavy  as  that  of  the  male; 
or  it  may  be  slightly  heavier.  Brain-weight  also  depends  to  some 
extent  on  stature;  distinctly  tall  individuals  averaging  about  2  per 
cent,  more  than  distinctly  short  individuals. 

It  should  be  noted,  however,  that  the  figures  quoted  above  are 
i  gross  averages,  and  that  there  is  much  variation.  The  extreme 
range,  if  idiots  are  excluded",  is  from  1,000  to  perhaps  1,900  grams; 
90  per  cent,  of  males  lying  between  the  limits  1,200  and  1,600 
grams,  and  60  per  cent,  between  1,300  and  1,500.  Most  of  the 
individuals  whose  brains  have  been  weighed  on  autopsy,  and  have 
thus  formed  the  basis  for  scientific  study,1  have  been  inmates  of 
workhouses  and  similar  public  institutions;  they  therefore  repre- 
sent the  poorer  and  less  successful  portion  of  the  population.  There 
is  some  evidence  that  the  more  prosperous  and  successful  portion 
have,  on  the  average,  somewhat  heavier  brains,  and  that  the  growth 
of  their  brains  is  longer  continued.  This  evidence  is  derived  from 
the  post-mortem  weight  of  the  brains,  sometimes  of  men  who  were 
so  eminent  for  mental  ability  that  the  examination  was  desired  in 
the  interests  of  science,  and  sometimes  of  men  who  realized  the 

1  The  figures  given  have  been  taken  from  Donaldson,  who,  in  his  work  on  The 
Growth  of  the  Brain,  1898,  has  brought  together  the  results  of  the  most  reliable 
series  of  measurements  by  various  authorities,  and  subjected  them  to  careful 
analysis.  The  brain-weights  given  for  the  ages  from  six  to  sixteen  are  derived 
from  the  measurement  of  so  few  individuals  that  it  has  not  seemed  worth  while 
to  attempt  to  trace  the  full  curve  of  growth.  The  main  outlines,  however,  as 
given  in  the  text  seem  fairly  reliable. 


VARIATIONS  IN  WEIGHT  OF  HUMAN  BRAIN          61 


value  to  science  of  such  an  examination,  and  therefore  directed 
that  their  brains  should  be  made  available  for  study.  As  a  result, 
there  are  now  on  record  the  brain-weights  of  about  one  hundred 
men  of  more  or  less  eminence.  The  average  of  these  is  about 
1,470  grams,  which  is  two  to  four  per  cent,  above  the  average  of 
workhouse  inmates.  They  range  from  1,200  to  2,000  grams, 
overlapping  the  range  of  ordinary  men  to  such  an  extent  that  it 
would  clearly  be  impossible  to  draw  any  conclusion,  from  the  mental 
achievements  of  an  individual,  as  to  his  brain-weight,  or  vice  versa. 
A  few  well-known  names  may  be  cited  from  the  list:1 

Cuvier,  naturalist 1830  grams 

Thackeray,  novelist 1658 

Siemens,  physicist 1600 

Daniel  Webster,  statesman 1518 

Chalmers,  theologian 1503 

Agassiz,  naturalist 1495 

De  Morgan,  mathematician 1494 

Gauss,  mathematician 1492 

Broca,  anthropologist 1484 

Grote,  historian 1410 

Bertillon,  anthropologist 1398      " 

Liebig,  chemist V.    .    .    ...    .    .'  .     .  1352      " 

The  average  of  this  group  of  certainly  eminent  minds  is  above 
that  of  the  whole  number  of  more  or  less  eminent  men.  But  the 
correlation  between  eminence  and  brain-weight  is,  at  the  best,  far 
from  close. 

§  19.  The  question  of  decrease  of  brain-weight  with  advanced  age 
can  be  examined  in  the  light  of  the  evidence  from  the  lists  of  "  emi- 
nent" men,  who  may  be  taken  as  at  least  representing  the  more 
intellectual  part  of  the  population.  As  few  young  men  are  included 
in  the  lists,  it  will  be  necessary  to  group  all  not  older  than  fifty-five 
together.  The  averages  for  the  different  ages  come  out  as  follows: 


No.  of 
Individuals 

Ages 

Av.  Brain- 
Weight 

32 
33 
24 
15 

25-55 
56-65 
66-75 

76-89 

1482 
1492 
1448 
1389 

In  view  of  the  wide  variation  of  the  individual  cases,  so  small  a 
difference  as  that  between  the  first  and  second  age-groups  must  not 

1  These  data  are  quoted  from  E.  A.  Spitzka,  "A  Study  of  the  Brains  of  Six 
Eminent  Scientists,"  etc.,  in  Transactions  of  the  American  Philosophical  Society, 
Philadelphia,  1907.  In  this  paper  he  has  brought  together  all  previous  records 
of  the  brain- weight  of  eminent  men. 


62        THE  NERVOUS  SYSTEM  IN  THE  INDIVIDUAL 

be  taken  to  mean  that  the  brain-weight  of  this  class  of  men  increases 
beyond  the  age  of  fifty-five;  it  is  conceivable  that  it  should  do  so, 
but  the  data  are  not  sufficient  to  show  it.  There  is,  however,  some 
indication  that  the  brain-weight  does  not  start  to  decrease  much 
before  sixty-five.  The  measurements  of  the  less  favored  and  in- 
tellectual classes  show1  a  loss  of  about  50  grams  from  the  age  of 
fifty-five  to  that  of  sixty-five,  and  an  equal  loss  in  the  succeeding 
decade  of  life.  It  is  probable,  therefore,  that  the  brain  reaches  a 
greater  size  in  the  more  intellectual  classes,  and  maintains  its  weight 
longer.  The  differences  in  size  are,  however,  too  small  to  serve  as  a 
measure  of  the  differences  between  men  in  intellectual  ability.  We 
must  conceive  the  growth  of  the  brain  which  takes  place  after  the 
age  of  eight  to  consist  for  the  most  part  in  the  development  of  very 
minute  structures,  such  as  the  dendrites  and  the  fine  terminations 
of  axons.  These  structures  are  individually  so  small  that  a  vast 
increase  in  their  number  would  make  but  a  small  impression  on  the 
total  weight  of  the  brain. 

§  20.  It  is  the  cerebrum  which  we  should  expect  to  show  most  de- 
velopment after  early  childhood,  because  it  is  probably  this  part  of 
the  nervous  system  which  is  chiefly  concerned  with  learning,  educa- 
tion, and  all  individual  acquisitions.  More  definitely,  the  develop- 
ment to  be  expected  would  occur  in  the  cortex,  and  in  the  axons 
connecting  its  various  parts.  That  such  development  does  take 
place  is  not  merely  to  be  expected;  but  it  seems  to  be  demonstrated 
by  microscopical  study.  Already  mentioned  are  the  observations 
tending  to  show  an  increase  in  the  number  of  developed  nerve-cells 
(with  their  dendrites)  as  the  result  of  use.  There  are  other  obser- 
vations showing  an  increase  of  the  myelinated  axons  and  branches 
of  axons  traversing  the  cortex.  The  axons  which  enter  the  cortex 
from  the  white  matter  below  are  found  to  penetrate  further  and 
further  into  the  cortex,  or  at  least  to  become  myelinated  further  and 
further,  as  the  individual  advances  toward  maturity.  Axons  and 
their  collaterals  which  traverse  the  cortex  in  directions  parallel  to 
its  surface,  both  in  the  outer  or  marginal  layer  and  in  two  or  more 
layers  in  the  midst  of  the  nerve-cells,  also  increase  greatly  in  myeliniz- 
ation  as  late  as  the  thirtieth  year  of  life,  and  probably  later  still.  This 
growth  in  complexity  of  organization  of  the  cortex  appears  sufficient, 
so  far  as  the  evidence  goes,  to  be  paralleled  with  the  increasing  com- 
plexity of  mental  capabilities  which  occurs  during  early  life.2 

1  See  Donaldson,  The  Growth  of  the  Brain,  1898,  p.  325. 

2  See  Watson,  op.  cit.,  p.  103;    Kaes,  Archiv  fur  Psychiatric  und  Nervenkrank- 
heiten,  1894,  XXV,  p.  695;  Donaldson,  Growth  of  the  Brain,  1898,  p.  241;  Edinger, 
Vorlesungen  uber  den  Ban  der  nervosen  Zentralorgane,  7th  ed.,  1904,  vol.  I,  p.  335. 


CHAPTER  III 

GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM1 

§  1.  Regarded  as  isolated,  and  as  possessed  only  of  those  proper- 
ties which  belong  to  all  living  matter  of  the  peculiar  chemical  con- 
stitution and  structural  form  which  are  described  by  the  word 
"nervous,"  the  nerve-fibres  and  nerve-cells  are  of  great  interest  to 
physiological  and  psycho-physical  researches.  But  in  their  normal 
position  and  activity  they  are  always  combined  into  organs,  which 
are  then  arranged  in  a  symmetrical  whole.  Thus  combined  they 
are  dependent  upon  each  other  for  the  parts  which  they  play  in  the 
entire  system.  The  condition  and  function  of  each  element  are 
thus  determined  by  the  condition  and  function  of  the  rest.  One 
part  of  this  system  excites  another,  or  modifies  the  excitation  re- 
ceived from  another.  We  are,  therefore,  unable  to  isolate  perfectly 
any  one  of  these  elements,  and  so  study  its  normal  functions  apart. 
It  is,  indeed,  possible  to  dissect  out  a  nerve  with  a  muscle  attached, 
to  keep  it  alive  for  a  time,  and  thus  to  inquire  what  an  isolated  nerve 
will  do.  In  this  way  many  of  the  most  important  discoveries  in  the 
general  physiology  of  the  nerves  have  been  made.  But  every  nerve 
is  itself  a  compound  of  nervous  elements  which  have  been  placed 
for  purposes  of  experiment  under  abnormal  conditions.  The  action 
of  the  nerve-cells,  even  when  gathered  into  small  masses  called 
ganglia,  is  not  easily  open  to  direct  inspection.  Moreover,  when 
different  tracts  of  nerves,  or  different  regions  in  the  central  organs 
where  ganglion-cells  abound,  are  partially  isolated  by  being  laid 
bare  for  the  direct  application  of  stimulus,  just  so  far  as  they  are 
separated  from  the  system  they  are  in  abnormal  condition  and  show 
abnormal  results;  and  just  so  far  as  they  are  normal  in  condition  and 
function  they  are  still  connected  with  the  system.  It  is  the  mutual 
condition  and  reciprocal  action  of  the  elements,  when  combined 
into  this  totality,  which  constitute  the  nervous  mechanism.  To 
describe  in  brief  outline  the  gross  structure  of  this  mechanism  is 
the  purpose  of  this  chapter. 

1  Among  the  very  numerous  and  excellent  treatments  of  the  anatomy  of  the 
nervous  centres,  reference  may  be  made  to  Edinger,  Vorlesungen  uber  den  Ban 
der  nervosen  Zentralorgane,  7th  ed.,  Leipzig,  1904;  Schafer  and  Symington, 
"  Neurology,"  in  Quain's  Anatomy,  llth  ed.,  London,  1909;  O.  S.  Strong,  in 
Bailey's  Textbook  of  Histology,  New  York,  1910. 

63 


64       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

§  2.  It  will  be  of  great  service  toward  understanding  such  a  de- 
scription if  it  is  begun  under  the  guidance  of  some  appropriate  idea. 
Nerve-fibres  and  nerve-cells  exist  in  enormous  numbers  within  the 
human  nervous  system,  and  are  combined  in  different  proportions 
to  make  the  different  organs  of  this  system.  The  significance  of 
the  combination  appears  only  in  the  light  of  reflection  upon  the 
amount  and  kind  of  work  which  is  to  be  done.  The  most  general 
office  of  the  nervous  mechanism  may  be  said  to  be  that  of  "  concate- 
nating" all  the  functions  of  the  living  body  in  accordance  with  the 
complex  internal  and  external  conditions  to  which  it  is  subject. 
But  in  the  case  of  any  of  the  higher  animals,  and  especially  in  the 
case  of  man,  this  one  office  requires  the  doing  of  a  quantity  and 
variety  of  work  that  are  proportionate  to  the  complexity  of  these 
conditions.  How  shall  such  a  quantity  and  variety  of  work  be  done  ? 
The  actual  arrangement  of  the  elements  of  this  system,  in  the  exer- 
cise of  their  reciprocally  conditioned  activities,  is  the  solution  of 
the  problem.  As  in  all  very  complex  questions  of  this  sort,  so  this 
particular  problem  is  solved  by  a  wise  division  of  labor. 
I  The  manner  in  which  the  human  nervous  mechanism  is  developed 
as  a  response  to  the  before-mentioned  problem  has  already  been 
made  clear  in  its  general  outlines,  especially  as  applied  to  the  lower 
forms  of  animal  life.  Even  the  simple  protoplasmic  speck  called 
an  amceba  may  be  considered  as  a  living  molecular  mechanism. 
Minute  and  almost  structureless  as  it  appears,  the  amoeba  is  really 
composed  of  a  great  number  of  molecules  that  are  undergoing  con- 
stant change;  and,  as  we  have  seen,  it  is  capable  of  exercising  several 
wonderful  functions  that  do  not  belong  to  any  non-living  collection 
of  molecules.  Most  important  for  our  present  purpose  is  the  fact 
that  the  amceba  is  irritable  and  automatic.  With  these  properties 
the  molecular  mechanism  of  this  small  bit  of  protoplasm,  under  the 
stimulus  of  changes  in  the  pressure  and  temperature  of  its  medium, 
and  in  accordance  with  the  unknown  laws  of  its  internal  self-orig- 
inating changes,  solves  the  problem  of  constant  readjustment  which 
its  environment  presents  to  it. 

§  3.  Let  it  be  supposed  that  the  same  problem  becomes  more 
complicated,  and  the  animal  structure  which  is  to  solve  it  corre- 
spondingly complex.  The  metabolic  function  of  the  animal  may 
then  be  assigned  to  a  separate  system  of  structures;  and  the  closely 
related  secretory  and  excretory  functions  as  well.  The  repro- 
ductive function  may  then  also  acquire  its  own  peculiar  organs. 
The  muscles  perform  movements  in  masses  because  they  retain  in 
an  eminent  degree  the  "amoeboid"  contractility.  But  the  property 
of  being  irritable  and  automatic  becomes  the  special  endowment  of 
the  nervous  system.  All  these  different  systems,  in  order  that  they 


DIFFERENTIATION  OF  NERVOUS  FUNCTION         65 

may  be  moved  in  united  masses,  are  then  adjusted  to  a  mechanical 
framework  (of  indifferent  value  so  far  as  really  vital  changes  are 
concerned)  of  cartilage,  bone,  etc. 

But  the  eminently  irritable  and  automatic  system  of  molecules 
called  nervous  must  also  undergo  a  further  differentiation  of  func- 
tion. In  the  structureless  protoplasm  of  the  amoeba,  the  external 
molecules  are,  of  course,  the  ones  primarily  to  be  affected  by  the 
external  stimuli.  It  is  with  the  internal  molecules,  on  the  other 
hand,  that  the  changes  called  "automatic"  begin.  But  the  con- 
tinual flux  of  its  protoplasmic  substance  indicates  that,  in  its  sim- 
plest form,  any  of  the  molecules  of  the  animalcule  may  in  turn  act 
either  as  irritable  or  as  automatic.  The  primary  differentiation  of 
this  substance  by  the  so-called  "surface  of  separation"  (see  p.  14) 
points,  however,  to  a  division  of  labor. 

We  have  already  seen  that,  in  the  somewhat  higher  forms  of  ani- 
mal life,  we  come  upon  an  increased  differentiation  of  parts  into 
"receptors,"  "conductors,"  and  "effectors"  (see  p.  16),  and  a  cor- 
responding further  division  of  labor.  When  a  somewhat  compli- 
cated nervous  mechanism  appears  in  the  ascending  scale  of  animal 
development,  the  idea  which  lies  at  the  base  of  this  rudimentary 
differentiation  of  the  system  calls  for  these  three  kinds  of  nervous 
substance:  (1)  superficial  cells  susceptible  to  external  stimuli;  (2) 
central  and  eminently  automatic  cells,  also  susceptible  to  internal 
stimuli;  (3)  a  strand  of  irritable  protoplasm  connecting  the  two. 
In  order  that  the  more  highly  organized  animal  may  exercise  "a 
will  of  its  own,"  certain  of  its  muscle-fibres  must  be  placed  under 
the  control  of  the  central  and  automatic  cells.  In  order,  also,  that 
the  entire  muscular  system  may  feel  the  reflex  influence  of  external 
stimuli,  and  so,  by  co-ordinated  contractions,  adapt  the  organs  of 
the  body  to  the  changes  of  its  environment,  the  muscle-fibres  must 
be  indirectly  connected  with  such  superficial  cells  as  are  sensitive 
to  these  stimuli.  The  nervous  system,  therefore,  in  its  most  funda- 
mental form  consists  of  these  three  sets  of  contrivances  with  their 
respective  functions:  (^L)  sensitive  cells  upon  the  surface  of  the  body; 
(B)  central  cells  that  are  both  automatic  and  modifiers  and  dis- 
tributers of  sensory  impulses;  (0)  connecting  tracts,  or  strands,  that 
can  convey  the  nervous  impulses  either  centripetally  from  A  to  B, 
or  centrifugally  from  B  to  the  contractile  muscular  tissues  of  the 
body. 

Higher  developments  of  this  triple-formed  fundamental  type  of 
a  nervous  system  are  reached  by  further  differentiations  of  A,  B, 
and  C.  If  various  kinds  of  stimuli  are  to  act  upon  this  system,  then 
the  sensitive  cells  upon  the  surface  (^4)  must  be  modified  into  vari- 
ous external  organs  of  sense.  The  terminations  of  the  centrifugal 


FIG.  23.— View  of  the  Cerebro-spinal  Axis. 
(After  Bourgery.)  £.  The  right  half  of  the 
cranium  and  trunk  has  been  removed,  and  the 
roots  of  the  spinal  nerves  dissected  out  and 
laid  on  their  several  vertebrae.  F,  T,  O,  cere- 
brum; C,  cerebellum;  P,  pons^  Varolii;  mo, 
medulla  oblongata;  ms,ms,  upper  and  lower 
extremities  of  the  spinal  marrow.  Cl  to 
CVIII  are  cervical  nerves;  DI  to  DXII, 
dorsal;  LI  to  LV,  lumbar;  SI  to  SV,  sacral; 
Col,  coccygeal. 


SYMPATHETIC  AND  CEREBROSPINAL  67 

or  motor  nervous  strands  may  also  be  variously  modified  so  as  to 
connect  with  and  control  the  contractile  tissue  of  many  sets  of  mus- 
cles. The  central  cells  may  be  variously  grouped  and  arranged, 
with  functions  more  or  less  localized,  so  as  to  receive,  modify,  and 
distribute,  in  manifold  ways,  the  different  sensory  impulses.  Other 
such  central  cells  may  become  more  particularly  related  to  the  phe- 
nomena of  conscious  sensation  and  volition. 

Such  a  highly  developed  nervous  system  will  then  consist  of  the 
following  parts:  (^4)  End-organs  of  Sense,  like  the  skin,  the  eye,  and 
the  ear;  (A1)  End-organs  of  Motion,  like  the  so-called  motor  end- 
plates;  (B)  Central  Organs,  like  the  various  peripheral  and  sporadic 
ganglia,  the  spinal  cord,  and  brain,  in  which  may  come  to  exist 
(6)  certain  portions  more  distinctively  automatic,  (61)  certain  others 
more  concerned  in  receiving  and  distributing  reflexly  the  sensory 
impulses,  and  (fr11)  still  others  more  particularly  connected  with  the 
phenomena  of  consciousness;  and  (C)  Conducting  Nerves,  which  will 
be  either  (c)  centripetal,  afferent,  and  sensory,  or  (c1)  centrifugal, 
efferent,  and  motor,  designed  to  connect  the  central  organs  and  the 
end-organs. 

§  4.  The  nerves  and  ganglionic  masses  of  nervous  matter  in  the 
human  body  are  arranged  in  two  great  systems,  the  Sympathetic 
and  the  Cerebro-spinal.  The  Sympathetic  Nervous  System  con- 
sists of  a  pair  of  nervous  cords,  situated  one  on  each  side  of  the  spinal 
column;  of  three  main  plexuses,  situated  in  the  cavities  of  the 
thorax  and  abdomen;  of  a  great  number  of  smaller  ganglia,  lying 
in  relation  to  the  viscera  of  the  same  cavities,  and  widely  distrib- 
uted over  the  body,  especially  in  connection  with  the  vascular  sys- 
tem; and  of  a  great  multitude  of  fine  distributory  nerves.  Each 
of  the  two  cords  comprises  a  number  of  ganglia  united  by  interme- 
diate nerves.  In  the  other  regions  of  the  spinal  column  the  num- 
ber of  these  ganglia  equals  that  of  the  vertebrae  (sacral  5,  lumbar 
5,  thoracic  or  dorsal  12),  but  in  the  neck  (cervical)  there  are  only 
3.  From  this  gangliated  cord  a  communicating  and  a  distribu- 
tory series  of  nerve-branches  are  derived.  By  the  communicating 
branches  the  two  systems  are  brought  into  close  anatomical  and 
physiological  relation,  and  a  kind  of  double  interchange  takes  place 
between  them.  The  distributory  branches  of  nerves  in  the  sympa- 
thetic system  bring  the  gangliated  cord  into  connection  with  the 
blood-vessels  and  viscera  of  the  body.  The  involuntary  muscles 
in  the  coats  of  these  vessels  and  in  the  walls  of  the  viscera  are  thus 
related,  and  through  the  sympathetic  fibres  brought  into  connec- 
tion with  the  cerebro-spinal  axis.  The  three  main  plexuses  re- 
ferred to  are  collections  of  nerve-cells  and  a  dense  plexiform  ar- 
rangement of  nerve-fibres.  One  of  them  is  situated  at  the  base  of 


68       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

the  heart,  to  which  it  gives  off  branches  that  wind  around  that 
organ  and  penetrate  its  muscular  substance;  another  is  placed  at 
the  upper  part  of  the  abdominal  cavity,  and  gives  origin  to  numer- 
ous plexiform  branches  that  supply  the  viscera  of  the  abdomen; 
the  third  is  in  front  of  the  last  lumbar  vertebra,  and  supplies  the 
vaso-motor  nerves  and  nerves  of  the  muscular  coats  and  mucous 
membranes  of  the  various  organs  in  that  region  of  the  body.  Fur- 
ther details  in  the  anatomy  of  the  sympathetic  nervous  system  are 
of  little  interest  to  psycho-physical  studies.  To  such  studies  it  is 


Fia.  24. — The  Cranium  Opened  to  Show  the  Falx  Cerebri  and  Tentorium  Cerebelli,  and  the 
Places  of  Exit  for  the  Cranial  Blood-vessels.  (Schwalbe.)  a,  a,  Falx;  6,  &,  the 
tentorium;  3,  3,  Sinus  transversus,  and  2  to  3,  Sinus  rectus,  receiving  from  in  front  the 
Vena  magna  Galena;  4,  internal  jugular  vein;  5,  superficial  temporal  vein;  and  6,  mid- 
dle temporal  vein. 

of  great  interest,  however,  to  know  that  this  system  forms  a  bond 
between  the  sensations,  emotions,  and  ideas  which  have  their 
physical  basis  in  the  molecular  condition  of  the  cerebro-spinal 
centres,  and  those  various  organs  in  the  thoracic  and  abdominal 
regions  whose  condition  is  so  closely  related  to  such  psychical 
states. 

§  5.  The  Brain  and  Spinal  Cord  are  the  great  centres  of  the  cere- 
bro-spinal system.  These  bodies  are  situated  in  the  bony  cavity  of 
the  skull  and  spinal  column.  They  have  three  Coverings  or  Mem- 
branes, the  innermost  one  of  which  is  directly  united  with  the  sur- 
face of  the  nervous  substance,  and  sends  numerous  processes  into 


B 


FIG.  25.— A,  Ventral,  and  B,  Dorsal,  View  of  the 
Spinal  Cord  and  Medulla  Oblongata.  B1  the  Filum 
terminate,  which  has  been  cut  off  from  A  and  B. 
1,  Pyramids  of  the  medulla,  and  I1,  their  decussa- 
tion;  2,  olives;  3,  lateral  strands  of  the  medulla; 
41,  calamus  scriptorius;  5,  the  funiculus  gracilis; 
and  6,  the  funiculus  cuneatus;  7,  the  ventral  and 
9,  the  dorsal,  fissures;  8,  the  ventro-lateral  im- 
pression; 10,  dorso-lateral  groove.  C,  the  cervical, 
and  L,  the  lumbar,  enlargements  of  the  cord. 


JO- 


•7 

Pb 


70       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

its  interior.  (1)  The  Dura  Mater,  which  is  the  membrane  lying 
next  to  the  wall  of  the  bony  cavity,  is  tough,  white,  fibrous,  and  of 
structure  somewhat  different  in  the  cranial  from  the  spinal  cavity. 
Three  processes  of  the  dura  mater  divide — only  incompletely — 
the  cavity  of  the  skull  into  two  symmetrical  halves  and  into  an 
upper  and  lower  space :  (a)  the  falx  cerebri,  a  sickle-shaped  process 
between  the  two  hemispheres  of  the  large  brain ;  (6)  the  falx  cere- 
belli,  a  similar  process  between  the  two  lateral  lobes  of  the  cerebel- 
lum, or  small  brain;  and  (c)  the  tentorium  cerebelli,  an  arched  process 
over  the  cerebellum  separating  it  from  the  back  portions  of  the  large 
brain.  The  membrane  lying  next  inward  is  called  (2)  Arachnoid; 
this  membrane  is  transparent  and  of  delicate  connective  tissue. 
The  space  below  this  surface  is  called  subarachnoid;  the  subarach- 
noid  or  cerebro-spinal  fluid,  which  fills  the  intercommunicating 
compartments  into  which  this  space  is  divided  by  bundles  of  deli- 
cate areolar  tissue,  is  alkaline  and  poor  in  albumen.  (3)  The  Pia 
Mater  is  a  vascular  membrane,  a  minute  network  of  fine  branches 
of  arteries  and  veins  held  together  by  delicate  connective  tissue. 
These  ramifications  of  the  blood-vessels  in  the  pia  mater  are  on  their 
way  to  or  from  the  nervous  substance  of  the  spinal  cord  and  brain. 
The  membrane,  therefore,  closely  invests  this  substance,  being, 
however,  more  intimately  attached  to  the  cord  than  to  the  brain. 
The  pia  mater  is  well  supplied  with  nerves. 

By  these  three  membranes  the  nervous  masses  of  the  cerebro- 
spinal  system  are  protected,  held  together  and  in  place  with  a  soft 
and  yielding  but  sufficiently  firm  pressure,  and  nourished  by  the 

§  6.  The  Spinal  Cord,  or  Medulla  Spinalis,  extends  in  the  verte- 
bral canal  from  the  aperture  in  the  cranial  cavity  (foramen  magnum), 
above  which  it  is  continuous  with  the  medulla  oblongata,  down- 
ward to  opposite  the  body  of  the  first  lumbar  vertebra,  where,  after 
tapering  off,  it  is  spun  out  into  a  slender  thread  of  gray  nervous 
substance  (filum  terminate)  that  lies  in  the  axis  of  the  sacral  canal. 
Its  length  is  from  fifteen  to  eighteen  inches;  its  weight,  when  di- 
vested of  membranes  and  nerves,  about  an  ounce,  or  not  far  from 
one-fiftieth  of  that  of  the  brain.  It  is  nearly  cylindrical  in  shape, 
its  front  and  back  surfaces  being  somewhat  flattened;  it  has  two 
considerable  enlargements  of  its  girth — an  upper  (cervical),  from 
which  arise  the  nerves  that  supply  the  upper  limbs;  and  a  lower 
(lumbar),  which  supplies  the  lower  limbs  with  nerves. 

§  7.  The  external  structure  of  the  spinal  cord  requires  us  to 
notice  the  fissures  which  almost  completely  divide  it  for  its  whole 
length  into  right  and  left  (lateral)  halves,  and  are,  therefore,  fitly 
called  "median";  of  these  fissures  (a)  the  one  in  front  (anterior 


COLUMNS  AND  COMMISSURES  OF  THE  CORD       71 


or  ventral1  median)  is  somewhat  broader  than  (b)  the  one  behind 
(posterior  or  dorsal  median).  The  ventral  fissure  is  penetrated  by 
the  pia  mater,  carrying  blood-vessels;  the  dorsal  fissure  is  not  a 
genuine  fissure,  but  a  wall  or  septum  of  neuroglia. 

Each  of  these  symmetrical  and  nearly  half-cylindrical  halves  of 
the  cord  is  subdivided  by  the  lines  of  entrance  of  the  dorsal  and 
ventral  nerve-roots  into  three  columns:  (a)  the  ventral,  which  lies 
between  the  ventral  fis- 
sure and  the  ventral  root; 
(6)  the  dorsal,  which  lies 
between  the  dorsal  fissure 
and  the  dorsal  root;  and 
(c)  the  lateral  column, 
which  lies  at  the  side  of 
the  cord  between  the  other 
two  columns.  The  line 
of  division  between  the 
lateral  and  ventral  col- 
umns is  not  perfectly 
sharp,  because  the  fibres 
of  the  ventral  roots  emerge 
over  a  considerable  width 
of  the  surface. 

The  Commissures  of 
the  spinal  cord  are  two 
bands  of  nervous  matter 
which  unite  its  halves, 
thus  preventing  it  from 
being  completely  sepa- 
rated by  the  fissures.  The 
one  in  front,  at  the  bot- 
tom of  the  ventral  median 

fissure,  is  composed  of  transverse  nerve-fibres  and  is  called  (a) 
the  ventral  white  commissure;  the  one  behind,  at  the  bottom  of  the 
dorsal  fissure,  is  (6)  the  dorsal  gray  commissure.  The  gray  commis- 
sure is  nearly  twice  as  large  as  the  white,  except  at  the  cervical 
and  lumbar  enlargements  of  the  cord,  where  the  white  is  larger.2 
Along  its  whole  length  the  gray  commissure  encloses  *a  circular  or 

1  There  is  a  certain  advantage  in  the  use  of  "  ventral "  and  "  dorsal "  in  place 
of  the  common  words  "anterior"  and  "posterior."    The  advantage  lies  in  the 
fact  that  ventral  and  dorsal  apply  with  equal  fitness  to  all  animals,  whether  they 
have  the  erect  position  or  not.     It  is  convenient  also,  in  some  parts  of  the  brain, 
to  speak  of  anterior  in  the  sense  of  "rostral"  or  "toward  the  mouth." 

2  See  Henle,  Anaiomie  des  Menschen,  text,  p.  309. 


FIG.  26.—  A,  Ventral,  and  B,  Lateral,  View  of  a 
Portion  of  the  Cord  from  the  Cervical  Region,  f . 
(Schwalbe.)  1,  ventral  median,  and  2,  dorsal  medi- 
an, fissures.  At  3  is  the  ventro-lateral  impression, 
over  which  spread  the  ventral  roots  (5).  The  dorsal 
roots  (6),  with  their  ganglion  (61),  arise  from  the 
dorso-lateral  groove,  and  uniting  with  the  ventral 
roots  form  the  compound  nerve  (7). 


72       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

elliptical  canal  (central  canal),  the  vestige  of  the  cavity  of  the 
embryonic  neural  tube.  The  gray  commissure  consists  for  the 
most  part  of  extremely  fine  nerve-fibres  devoid  of  medullary  sheath; 
while  the  white  commissure  is  composed  of  medullated  fibres. 
The  thickness  of  the  commissures  is,  as  a  rule,  proportional  to  the 


FIG.  27. — Transverse  Section  Through  the  Spinal  Cord  in  the  Upper  Thoracic  Region. 
(From  Starr's  Atlas  of  Nerve  Cells,  by  permission  of  the  Columbia  University  Press.) 
The  section  was  stained  with  the  "Weigert  stain,"  which  leaves  the  gray  matter  light, 
while  darkening  the  white  matter.  The  dorsal  columns  and  horns  are  above  in  the  fig- 
ure, the  ventral  columns  and  horns  below.  Magnified  10  diameters. 

size  of  the  corresponding  nerve-roots;  their  form,  as  they  pass 
into  the  lateral  parts  of  the  cord,  varies  in  different  sections  of  its 
length. 

§  8.  Transverse  sections  of  the  spinal  cord  show  us  that,  as  its 
external  appearance  would  indicate,  the  substance  of  which  it  is 
composed  is  arranged  in  two  symmetrical  halves,  almost,  but  not 
quite,  separated  by  the  median  fissures.  This  substance,  like  that 
of  all  the  nervous  centres,  consists  of  both  white  and  gray  nervous 
matter.  The  former  is  external  and  composes  the  columns  of  the 
cord;  while  the  latter  is  internal  and  is  surrounded  by  the  white. 
The  relative  amount  of  the  two  kinds  of  nervous  matter  varies  in 
the  different  parts  of  the  cord.  At  its  beginning  from  the  filum 
terminale  scarcely  any  white  matter  appears;  the  amount  of  such 


WHITE  AND  GRAY  MATTER  OF  THE  CORD   73 


matter,  however,  increases  from  below  up- 
ward, and  is  largest  in  the  cervical  part  of  the 
cord.  The  amount  of  gray  matter  is  greatest 
in  the  upper  and  lower  enlargements  of  the 
cord. 

The  gray  columns  on  either  side  of  the 
cord,  together  with  the  commissures  which 
unite  them,  form  a  figure  somewhat  like  a 
large  Roman  H,  with  diverging  sides;  but 
the  lateral  masses  of  these  crescent-shaped 
bodies  are  narrower  in  the  thoracic  (or  dorsal) 
region,  and  broader  in  the  cervical  and  lum- 
bar enlargements.  Sometimes  the  figure  is 
rather  like  that  of  a  pair  of  butterflies'  wings. 
The  two  limbs  of  each  side  of  the  figure  into 
which  the  gray  columns  are  thuci  formed  are 
called  Horns ;  (a)  the  ventral  horn  is  rounded, 
(b)  the  dorsal  long  and  narrow  (compare 
Fig.  27). 

§  9.  The  gray  matter  of  the  cord  contains 
nerve-cells  and  their  dendrites,  and  short 
lengths  of  axons  which  pass  from  the  white 
columns  into  the  gray  to  terminate  there  in  a 
tuft  of  fine  branches;  collaterals  of  axons 
(compare  p.  42)  similarly  enter  the  gray 
matter  and  split  up  into  fine  branches;  in 
addition,  the  gray  matter  contains  neuroglia. 
From  cells  in  the  ventral  horns  of  gray  matter 
issue  axons  which  make  their  way  through  the 
white  matter  to  the  surface  of  the  cord,  and 
pass  outside,  to  form  the  ventral  roots  (com- 
pare p.  42).  These  are  the  fibres  which  then 
pass  to  muscles  and  other  effectors;  and, 
therefore,  the  cells  from  which  they  arise  are 
called  motor  nerve-cells.  Accordingly,  the 
ventral  horn  is  largely  motor  in  function.  The 
dorsal  horns,  on  the  contrary,  do  not  contain 
the  cells  of  origin  for  the  fibres  in  the  dorsal 
or  sensory  roots;  but  these  fibres,  as  has  al- 
ready been  stated  (p.  42),  arise  from  cells  in 
the  ganglia  situated  on  the  dorsal  roots  (the 
spinal  ganglia)  and  grow  into  the  cord  by 
the  dorsal  roots.  Since  the  cells  of  the  sen- 
sory root  fibres  lie  outside  the  cord,  while 


FIG.  28.  —  Transverse  Sec- 
tions of  the  Cord  at  Dif- 
ferent Levels.  (Erb.)  The 
ventral  side  of  the  cord 
lies  above  in  each  cross- 
section.  The  shaded  areas 
are  the  pyramidal  or  cor- 
tico-spinal  tracts,  which 
have  degenerated  in  this 
individual  because  of  in- 
jury to  the  motor  area  of 
the  left  hemisphere.  The 
shaded  area  in  the  lateral 
column  is  the  "crossed" 
pyramidal  tract,  and  that 
in  the  ventral  column  the 
"direct"  pyramidal. 


74       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

those  of  the  motor  root  fibres  lie  in  the  ventral  horns,  it  is  natural 
that  the  latter  should  be  thicker  than  the  dorsal  horns.  The  thick- 
ness of  the  ventral  horns,  however,  varies  greatly  in  different 
levels  of  the  cord  (see  Fig.  28). 

While  in  the  cervical  and  lumbar  enlargements  of  the  cord,  from 
which  issue  the  nerves  to  the  extremities,  the  ventral  horns  are 
large,  in  the  mid-thoracic  region,  the  nerves  from  which  supply  the 
less  mobile  trunk,  the  motor  fibres  are  relatively  few,  and  the  ventral 
horns  are  correspondingly  slender.  More  careful  study  of  the  shape 
of  the  ventral  horns  in  the  enlargements  shows  that  they  increase 
by  the  addition  of  gray  matter  at  their  sides;  and  the  evidence  is 
that  these  lateral  portions  of  the  ventral  horns  are  the  seat  of  the 
cells  whose  fibres  supply  the  muscles  of  the  extremities.  The  cells 
of  origin  of  the  fibres  which  pass  to  the  sympathetic,  seem  to  lie  in 
the  dorsal  part  of  the  ventral  horn.  Thus  a  certain  amount  of 
localization  of  function  can  be  made  out  in  the  ventral  horn  of 
the  cord. 

Well-defined  groups  of  cells  appear  elsewhere  in  the  gray  matter 
of  the  cord.  For  example,  one  column  of  large  cells  at  the  base  of 
the  dorsal  horn  ("Clarke's  column")  gives  rise  to  fibres  which  pass 
up  to  the  cerebellum. 

The  spinal  cord  shows  no  clear  division  into  segments,  such 
as  appear,  for  instance,  in  the  ganglion  chain  of  the  earthworm 
(p.  22);  and  the  spinal  "centres,"  or  groups  of  nerve-cells  which 
control  particular  muscles,  do  not  exist  as  compact  nuclei,  but 
rather  as  slender  columns  of  cells,  within  the  ventral  horn,  extend- 
ing for  a  distance  of  two  or  three  vertebrae  up  and  down  the  cord. 
The  fibres  destined  for  a  particular  muscle  issue  from  the  cord  by 
two  or  three  adjacent  ventral  roots;  and  each  root  carries  fibres 
destined  for  several  muscles.  The  spinal  centres  of  neighboring 
muscles  overlap. 

The  spinal  ganglia,  which  supply  the  sensory  fibres  for  the  skin 
and  other  tissues,  are  clearly  segmental;  and  the  fibres  from  each 
ganglion  have  a  definite  field  of  distribution.  But  the  distribu- 
tions of  adjacent  ganglia  overlap  in  the  skin,  so  that  destruction 
of  a  single  ganglion  or  dorsal  root  does  not  entirely  abolish  sensa- 
tion in  any  cutaneous  area. 

The  White  Substance  of  the  spinal  cord,  besides  connective  tis- 
sue and  lymph-  and  blood-vessels,  is  composed  of  nerve-fibres  of 
comparatively  large  or  of  medium  size.  The  essential  constituent 
of  these  fibres  is  the  axon,  the  diameter  of  which  is  generally  one- 
third  or  one-fourth  of  their  breadth.  When  fully  developed,  they 
are  rarely  or  never  without  a  medullary  sheath,  but  probably  have  no 
neurilemma.  Their  diameter  is  not  constant;  the  thickest  fibres 


NERVE-FIBRES  AND  NERVE-CELLS  IN  THE  CORD   75 


Cbl 


(TWO  to  TOTO-  of  an  inch)  are  found  in  the  outer  portions  of  the 
ventral  columns,  where  their  size  is  tolerably  uniform.  In  the 
lateral  columns  the  nerve-fibres  vary  greatly  in  size,  the  finer  ones 
lying  inward  near  the  gray  matter.  In  the  dorsal  columns  they  in- 
crease in  thickness  as  they  approach  the  posterior  gray  commissure. 
In  the  upper  thoracic,  and  through  the  whole  of  the  cervical,  region, 
there  is  found  a  wedge-shaped  bundle  of  fine  fibres  that  is  separated 
off  from  the  dorsal  columns  toward  the  middle  line  of  the  cord  by  a 
strong  septum;  this  is 
called  fasciculus  gra- 
cilis,  or  "column  of 
Goll." 

§  10.  Some  idea  of 
the  complexity  of  the 
cord  may  perhaps  be 
gained  from  the 
counts  of  fibres  which 
have  been  made  by 
several  authors.  In  so 
small  an  animal  as 
the  frog,  Hardesty1 
found,  in  general 
agreement  with  the 

parliW  work  nf  Riro-P      FlG'  29--View  of  the  Brain  in  Profile.     *.     (Henle.)     Cb, 
r  WOrK  01  Dirge,         cerebrum  ;    Cbl,  cerebellum;  Mo,  medulla  oblongata;  P. 
that  the  total  number        P°ns  Varolii;  *,  nssure  of  Sylvius. 

of    fibres   in   all   the 

dorsal  and  ventral  spinal  roots  combined  would  be  some  such 
number  as  20,000  to  30,000.  The  number  varied  with  the  size 
of  the  frog,  increasing  with  the  body-weight.  The  fibres  of  the 
dorsal  roots  were  more  numerous  than  those  of  the  ventral  roots; 
in  one  specimen  there  were  8,572  dorsal  and  6,211  ventral  in  the 
roots  of  one  side.  In  man,  Ingbert2  determined  the  number  of 
fibres  in  the  dorsal  roots  entering  one  side  of  the  cord;  the  total 
count  was  about  650,000.  The  ventral  roots  must  contain  at  least 
half  as  many  more,  so  that  the  total  number  of  root  fibres  entering 
or  leaving  the  human  cord  cannot  be  less  than  two  million. 

§  11.  The  same  elements  of  nerve-fibres  and  nerve-cells,  in  con- 
junction with  connective  tissue  and  neuroglia,  and  enveloped  in  the 
three  enclosing  membranes  (dura  mater,  arachnoid,  and  pia  mater) 

1  Irving  Hardesty,  "The  Number  and  Arrangement  of  the  Fibres  Forming  the 
Spinal  Nerves  of  the  Frog,"  Journal  of  Comparative  Neurology,  IX,  64-112,  1899. 

2  Chas.  Ingbert,  "An  Enumeration  of  the  Medullated  Nerve  Fibres  in  the 
Dorsal  Roots  of  the  Spinal  Nerves  of  Man,"  Journal  of  Comparative  Neurology, 
XIII,  53-120,  1903. 


76       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

already  described,  are  combined  with  an  increased  variety  and  com- 
plexity of  arrangement  to  form  those  intercranial  central  organs 
with  which  the  upper  end  of  the  spinal  cord  is  continuous.  Uni- 
formity of  elementary  parts,  together  with  the  greatest  intricacy  of 


FIG.  30. — Under  Surface  of  the  Brain.  (Van  Gehuchten.)  The  Roman  numerals  at  the 
left  margin  of  the  figure  indicate  the  12  cranial  nerves;  hyp,  hypophysis;  ch,  optic 
chiasm;  c.  mam,  mammillary  body;  pc,  peduncle  of  the  cerebrum;  pr,  pons;  o,  olive; 
PV,  pyramids;  CI,  first  spinal  nerve. 


arrangement,  prevails,  above  all  other  regions  of  the  body,  in  the 
structure  of  the  brain.  The  significance  of  the  elements  and  ele- 
mentary parts  can,  therefore,  only  be  understood  when  they  are 
considered  in  the  localities  and  relations  to  other  parts  which  are 
assigned  them  by  this  so  intricate  arrangement. 


DIFFERENT  ASPECTS  OF  THE  ENCEPHALON        77 

§  12.  The  Encephalon,  or  Brain,  in  the  most  extended  sense  of 
the  word,  includes  all  that  portion  of  the  central  nervous  system 
which  is  contained  within  the  cavity  of  the  skull.  Its  division  into 
five  principal  parts,  and  the  subdivisions  of  some  of  these,  have  been 
mentioned  in  the  chapter  on  the  development  of  the  individual 
nervous  system  (see  p.  48).  On  removing  the  entire  human  brain 
from  the  skull,  and  viewing  it  from  above,  one  sees  only  the  cere- 
bral hemispheres,  which  have  grown  back  and  covered  the  other 
parts.  A  view  from  the  side  shows  the  cerebellum  lying  beneath 

Cb 


FIG.  31. — Mesial  Section  of  the  Brain.  ^.  (After  a  photograph  by  Retzius.)  Cb,  the 
mesial  surface  of  the  right  cerebral  hemisphere;  Cbl,  the  middle  lobe  of  the  cerebellum 
in  section;  Te,  roof  of  the  mid-brain;  Pe,  peduncle  of  the  cerebrum;  v4,  fourth  ventricle; 
Aq,  aqueduct;  Th,  thalamus;  Mam,  mammillary  body;  //,  optic  nerve. 

the  back  part  of  the  cerebrum,  and  the  bulb  lying  beneath  the  cere- 
bellum (compare  Fig.  29);  the  transverse  bundles  of  the  pons  can 
also  be  seen.  From  the  under  side  can  be  seen,  in  addition,  the 
peduncles  or  crura  of  the  cerebrum,  which  form  the  ventral  part  of 
the  mid-brain;  the  mammillary  bodies  and  the  hypophysis,  which 
are  parts  of  the  inter-brain;  and  also  the  cranial  nerves.  In  the 
bulb  can  be  distinguished  on  this  surface,  two  central  hillocks 
called  the  "pyramids";  and  two  somewhat  similar  at  the  sides, 
called  the  "  olives  "  (see  Fig.  30).  If  a  section  is  made  in  the  median 
plane,  separating  the  cerebrum  into  its  hemispheres,  and  dividing 
the  rest  of  the  brain  into  right  and  left  halves,  the  cerebrum,  cere- 
bellum, and  brain  stem  are  easily  distinguished;  and  the  pons  is 
known  by  the  swelling  on  the  ventral  side  made  by  its  transverse 
fibres.  Next  forward  of  the  pons  and  cerebellum  lies  the  mid- 


78       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 


brain;  the  dorsal  part  of  this,  close  to  the  cerebellum,  consists  of 
the  quadrigemina,  four  rounded  eminences,  of  which  two,  the  right 
anterior  and  posterior,  show  in  the  section.  The  broad  cavity  show- 
ing in  this  section  beneath  the  cerebellum  is  the  fourth  ventricle, 
and  the  slender  prolongation  of  this  under  the  quadrigemina  is  the 
"aqueduct."  Forward  of  this  the  cavity  swells  into  the  third  ven- 
tricle, the  walls  of  which  constitute  the  inter-brain.  To  the  inter- 
brain  belong  the  pineal  gland  above  and  the  hypophysis  and 
mammillary  body  beneath.  The  main  portion  of  the  inter-brain, 

Callosum        Fornix 

/        /         /  Caudate  nucleus 


Habenul 
Pineal  gla 


Anterior  quadrigeminu 

Posterior  quadrigeminum' 
VI 


Cerebellu 


Third  ventricle 
Thalamus 

External  geniculate  body 
Internal  geniculate  body 

Superior  cerebellar 
peduncle 

Fourth  ventricle 
Bulb 


FIG.  32. — Dorsal  Surface  of  the  Brain-Stem.     (Sobotta  and  McMurrich.) 

the  thalamus,  lies  to  the  side  of  the  third  ventricle,  and  its  inner 
surface  can  be  seen  in  the  figure.  Of  the  cerebrum,  the  median 
surface  of  the  frontal,  parietal,  and  occipital  lobes,  and  of  the  tip 
of  the  temporal  lobe,  is  shown ;  and  the  callosum  and  the  f ornix  are 
seen  in  section  (compare  Fig.  31). 

To  obtain  a  view  of  the  dorsal  surface  of  the  brain-stem,  it  is 
necessary  to  trim  away  the  cerebellum  and  most  of  the  cerebrum. 
We  then  see  the  bulb  swelling  out  from  the  cord;  and  the  fourth 
ventricle  beginning  below  in  an  acute  angle.  The  quadrigemina 
are  fully  exposed;  slightly  further  forward  is  the  pineal  gland;  and, 
to  its  side,  the  habenula.  The  large  masses  to  the  front  and  side  of 
the  quadrigemina  are  the  thalami  (one  on  each  side);  and  still 
further  in  the  same  direction  is  seen  the  caudate  nucleus,  a  part 
of  the  striatum.  Appended  to  the  thalamus  are  two  small  eminences, 


TWELVE  PAIRS  OF  CRANIAL  NERVES 


79 


called  the  geniculata,  median  and  lateral,  which  are  of  interest  as 
portions  respectively  of  the  auditory  and  visual  apparatus  (see 
Fig.  32). 

§  13.  It  is  important  again  to  note  in  this  connection  the  nerves 
which  belong  with  each  part  of  the  brain,  and  which  issue  from  the 
brain-stem.  A  list  of  the  twelve  pairs  of  cranial  nerves  has  already 
been  given  (see  p.  55).  To  emphasize  their  relations  to  the  brain 
more  clearly,  we  note  that  the  first  or  olfactory  nerve  enters  the 
olfactory  bulb,  the  end-station  or  terminal  nucleus  of  this  nerve; 
and  that  fibres  passing  back  from  this  through  the  olfactory  tracts 


ec 


FIG.  33. — Section  of  the  Bulb  at  the  Decussation  of  the  Pyramids.  6/1.  (Schwalbe.) 
f.l.a.,  ventral  fissure;  s.l.p.,  dorsal  fissure;  py,  py,  bundles  of  pyramidal  fibres,  crossing 
at  d;  V,  ventral  column;  S,  lateral  column;  C.a,  ventral  horn  with  groups  of  ganglion 
cells,  a  and  &;  cc.  central  canal ;  f.r.,  reticular  formation;  ce  and  g,  dorsal  horns;  Hl  and 
H2,  dorsal  columns;  n.c.  and  n.g.,  their  nuclei;  x,  central  gray  matter. 

serve  to  connect  this  sense-organ  with  other  parts  of  the  system. 
The  second,  or  optic,  pair  of  nerves  are  seen  to  enter  the  inter-brain. 
The  third  and  fourth  nerves,  both  motor  to  the  eye,  issue  from  the 
mid-brain.  The  fifth  or  trigeminal,  the  great  sensory  nerve  of  the 
face,  including  also  a  motor  root  to  the  muscles  of  mastication, 
issues  from  the  pons.  The  remaining  seven  are  connected  with  the 
bulb;  the  sixth,  seventh,  and  eighth  with  its  forward  end,  the  others 
further  back. 

§  14.  The  brain-stem  may  properly  be  considered  as  a  continua- 
tion of  the  cord.     As  compared  with  the  cord,  it  is  in  the  first  place 


80       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 


thicker,  owing  (1)  to  the  large  body  of  nerves  which  it  receives;  (2) 
to  the  passage  through  it  of  all  the  fibres  which  connect  the  cere- 
brum and  cerebellum  with  the  cord;  and  (3)  to  the  appearance  of 
special  groups  of  cells  within  it.  In  shape,  the  difference  of  the 
brain  stem  from  the  cord  is  largely  due  to  the  circumstance  that  the 

central  cavity  of  the  neural  tube 
sometimes  opens  out  into  ventri- 
cles, and  at  all  times  lies  near 
the  dorsal  side.  Passing  upward 
from  the  cord,  we  find  the  central 
canal,  in  the  lower  region  of  the 
bulb,  first  verging  toward  the 
dorsal  surface,  and  then  opening 
out  into  the  fourth  ventricle. 
The  dorsal  wall  here  becomes  a 
wide  membrane,  overlying  the 
ventricle,  and  itself  overlain  by 
the  cerebellum.  The  fourth  ven- 
tricle continues  upward  through 
the  pons,  narrows  in  the  mid- 
brain  to  the  aqueduct,  which 
broadens  again  in  the  inter-brain 
to  the  third  ventricle.  This  last 
is  continuous  through  a  narrow 
opening  with  the  "  lateral  ventri- 
cle" of  each  hemisphere. 

§  15.  A  series  of  sections  across 
the  brain-stem  will  give  some  idea 
of  its  internal  structure  (compare 
Fig.  33). 

If  we  make  our  first  section 
through  the  bulb,  shortly  above 
the  imaginary  line  which  sepa- 
rates it  from  the  spinal  cord,  we 
find  an  arrangement  of  parts 
much  like  that  of  the  cord.  The 
ventral,  lateral,  and  dorsal  columns  of  white  matter  can  be  identified; 
also  the  ventral  and  dorsal  horns  of  gray  matter.  The  most  striking 
feature  of  this  section  is  the  appearance  of  large  numbers  of  fibres, 
crossing  from  the  ventral  column  of  each  side  to  the  lateral  column 
of  the  other.  TWs  is  the  crossing  or  "  decussation  of  the  pyramids  " ; 
here  motor  fibres  from  the  left  hemisphere  cross  to  the  right  side  of 
the  cord,  and  vice  versa,  so  that  the  left  hemisphere  of  the  brain  con- 
trols the  right  half  of  the  body.  This  section  shows  another  pe- 


fla. 


FIG.  34.— Section  of  the  Bulb  at  the  Level 
of  the  Sensory  Decussation.  (Schwalbe.) 
s.l.p.,  dorsal,  and  f.l.a.,  ventral  fissures; 
cc.,  central  canal,  surrounded  by  n.  XI 
and  n.  XII,  the  nuclei  of  the  llth  and 
12th  nerves;  H  and  Hz,  the  dorsal  col- 
umns, with  n.g.  and  n.c.  (also  n.c1.), their 
nuclei;  a.  V.,  spinal  root  of  the  fifth  nerve, 
the  fibres  of  which  terminate  successively 
in  the  adjoining  gray  matter,  g,  which  may 
be  regarded  as  a  continuation  of  the  dor- 
sal horn  of  the  cord;  F.r.,  the  "reticular 
formation,"  containing  fibres  which  issue 
from  the  nuclei  of  the  dorsal  columns  and 
of  the  fifth  nerve,  and  which  cross  the 
middle  line  in  the  sensory  decussation, 
d.a.;  /.a.,  /.a.1,  f.a2.,  arciform  fibres,  passing 
toward  the  cerebellum;  n.L,  ol,  o,  n.ar., 
different  nuclei  of  gray  matter,  of  which 
o  is  the  lower  end  of  the  olivary  nucleus; 
py,  pyramid. 


INTERNAL  STRUCTURE  OF  BRAIN-STEM 


81 


culiarity,  as  compared  with  the  cord;  there  is  a  small  mass  of  gray 
matter  in  the  midst  of  the  dorsal  column. 

A  little  higher  up  the  bulb,  as  the  next  section  shows,  the  dorsal 
columns  are  chiefly  filled  with  gray  matter,  and  their  white  matter 
has  nearly  disappeared;  for  the  fibres  which  have  ascended  the  cord 
in  the  dorsal  columns  end  here,  in  this  gray  matter,  and  their  places 
are  taken  by  new  fibres  arising  from  this  gray  matter.  These  new 
fibres,  instead  of  continuing  upward  in  the  dorsal  region,  promptly 


Solitary  bundle 
Vestibule-spinal  tract  Nucleus  X 


Vestibular  nucleus 
Nucleus  IX 


Dorsal  spino- 
cerebellar  tract 


Quinto- spinal 
tract 


Ventral  spino- 
cerebellar  tract 


Thalamo-olivary 


Accessory  olivary 
nucleus 

Olivary  nucleus 
Accessory  olivary 
nucleus 


Ext.-arcuate  fibres 


Pyramid 


FIG.  35.— Section  of  the  Bulb  Through  the  Olive.     (Magnified  4  diameters.) 
Kopsch,  Lehrbuch  der  Anatomie.) 


(Rauber- 


cross  to  the  other  half  of  the  medulla,  and  pass  upward  near  the 
middle  line.  The  gray  matter  in  the  dorsal  columns  is  named  the 
"nuclei  of  the  dorsal  columns";  the  crossing  of  the  sensory  fibres 
is  the  "sensory  decussation,"  and  the  bundle  of  these  fibres  which 
runs  up  near  the  middle  line,  and  which  can  be  traced  to  the  thala- 
mus,  is  the  "fillet,"  or  " bulbo-thalamic  tract."  This  is  one  of 
the  chief  sensory  pathways  toward  the  cerebral  cortex. 

§  16.  A  section  near  the  upper  limit  of  the  bulb  shows  that  the 
central  canal  has  now  opened  out  into  the  fourth  ventricle.  On 
the  floor  of  the  ventricle  are  seen  the  nuclei  of  the  local  nerves. 
Near  the  nucleus  of  the  tenth  or  vagus  nerve  is  a  spot  which  is  es- 
sential for  life,  for  if  it  is  punctured,  breathing  ceases.  This  is  the 
"respiratory  centre."  The  most  striking  feature  of  the  bulb  at  this 


82      GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

level  is  the  olivary  nucleus  showing  in  section  as  a  wavy  line  (see 
Fig.  35).  Of  the  numerous  crossing  fibres  which  appear  in  this 
section,  some  are  from  the  olives,  and  some  belong  to  the  sensory 
decussation,  which  is  not  yet  complete.  At  the  side  is  seen  the  in- 
ferior peduncle  of  the  cerebellum,  not  as  yet  quite  separate  from  the 
bulb;  a  little  higher  up  the  separation  is  complete. 

At  the  upper  limit  of  the  bulb,  where  it  passes  over  into  the  pons, 
there  enters  the  eighth  nerve  (see  Fig.  36),  its  two  branches,  the 


it  JET* 


FIG.  36.— Section  of  the  Bulb  at  the  Entrance  of  the  Eighth  Nerve.  (Schwalbe.)  Note 
particularly  the  division  of  the  eighth  nerve,  VIII,  into  a,  its  cochlear  portion,  and  6,  its 
vestibular  portion.  The  portion  left  clear  in  the  drawing  is  to  be  understood  as  filled 
largely  with  decussating  fibres.  Py,  pyramid;  o,  olive;  V,  fibres  from  the  fifth  nerve; 
c.r.,  inferior  peduncle  of  the  cerebellum. 

cochlear  and  the  vestibular,  separating  as  they  enter,  and  having 
entirely  different  central  terminations.  At  this  level  are  large  masses 
of  gray  matter  connected  with  these  two  nerves.  The  central  con- 
tinuation of  the  cochlear  nerve  is  called  the  "lateral  fillet." 

§  17.  A  section  through  the  pons,  at  the  level  of  entrance  of  the 
fifth  nerve  (compare  Fig.  37),  shows  the  peculiar  arrangement  which 
gives  the  name  of  "bridge"  to  this  part  of  the  brain-stem.  Numer- 
ous transverse  fibres  extend  across  the  middle  line,  and  appear, 
indeed,  to  run  from  one  side  of  the  cerebellum  to  the  other;  but  this 
appearance  is  illusory,  as  these  fibres  all  originate  from  cells  in  the 
pons  itself;  they  cross  from  one  side  to  the  other,  and  enter  the  cere- 


INTERNAL  STRUCTURE  OF  THE  BRAIN-STEM       83 

bellum,  forming  its  middle  peduncle.  Interwoven  with  these 
transverse  pontine  fibres  are  the  longitudinal  fibres  of  the  pyramids. 
Small  collections  of  nerve-cells  (the  "pontine  nuclei")  lie  imbedded 
in  the  meshes  of  this  network  of  fibres,  and  give  origin  to  the  pontine 
fibres.  Above  this  bridge-like  portion  of  the  pons  is  another  re- 


FIG.  37.— Cross  Section  of  the  Pons  at  the  Level  of  the  Fifth  Nerve.  (Marburg.)  V  (at  the 
left)  is  the  fifth  nerve;  parts  of  the  sixth,  seventh,  and  eighth  nerves  also  are  indicated  by 
Roman  numerals;  FPo.,  bundles  of  pontine  fibres,  which  pass  to  BPo,  the  middle  cere- 
bellar  peduncle.  The  cerebellum  is  cut  away,  but  is  to  be  thought  of  as  lying  to  each 
side  and  on  top,  above  the  fourth  ventricle  which  shows  at  the  top  of  the  figure.  Py 
is  the  pyramidal  tract;  Lm,  the  fillet;  Flm,  close  beneath  the  ventricle,  is  the  median 
longitudinal  bundle;  ND  is  the  nucleus  of  Deiters;  Crst,  a  remnant  of  the  inferior  cere- 
bellar  peduncle;  Vs,  the  descending  bundle  of  fibres  from  the  fifth  nerve,  which  has  been 
shown  in  sections  lower  down.  The  "hood"  includes  everything  between  the  ventricle 
and  the  uppermost  pontine  fibres. 

sembling  the  bulb,  the  bundles  of  which  are  indeed  continued  up- 
ward here.  This  dorsal  part  of  the  pons  is  called  the  "hood"  or 
tegmentum;  and  the  ventral  part  is  called  the  foot. 

§  18.  A  section  somewhat  farther  forward  cuts  the  mid-brain  at 
the  level  of  the  posterior  quadrigemina  (compare  Fig.  38).  The 
fillets,  medial  and  lateral,  are  visible  above  the  foot;  and  the  lateral 
fillet  is  seen  to  have  moved  dorsally,  its  fibres  passing  to  the  posterior 
quadrigeminum,  which  is  a  centre  for  hearing.  The  abundant 
decussating  fibres  above  the  fillet  are  from  the  superior  peduncles 
of  the  cerebellum.  The  third  ventricle  has  now  narrowed  to  the 
aqueduct,  which  is  surrounded  by  gray  matter,  containing  the 
nuclei  of  the  third  and  fourth  nerves,  which  are  motor  to  the  eye 
muscles. 

§  19.  A  section  through  the  anterior  quadrigemina  (Fig.  39)  may 
strike  also  the  hind  part  of  the  inter-brain,  which  overlaps  the  mid- 


84       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 


FIG.  38.— Cross-section  of  the  Mid-Brain  at  the  Level  of  the  Posterior  Quadrigemina.  (Mar- 
burg.) In  place  of  the  fourth  ventricle  we  have  here  the  narrow  Aqueduct,  Aq.  Above 
this,  on  each  side,  lie  the  posterior  quadrigemina,  Qp.  The  aqueduct  is  immediately 
surrounded  by  gray  matter,  below  which  is  Flm,  the  median  longitudinal  bundle.  Below 
this  is  a  large  mass  of  decussating  fibres,  D,  from  the  superior  cerebellar  peduncles;  below 
and  to  the  side  of  this  is  the  fillet,  Lm;  and  above  this,  near  the  quadrigeminum,  the 
lateral  fillet,  LI.  So  much  for  the  tegmentum;  the  foot  still  shows  the  interweaving  of 
pontine  and  pyramidal  fibres. 


Gray  matter  of  the  Aqueduct 
Aqueduct 


Red  nucleus 


FIG.  39. — Section  Through  the  Anterior  Quadrigemina.  Magnified  about  It  diameters. 
(Dejerine.)  II,  7/7,the  second  and  third  nerves;  Th,  thalamus;  Ge  and  Gi,  the  external 
and  internal  geniculate  bodies;  P,  peduncle  of  the  cerebrum,  containing  the  pyramidal 
tracts;  F,  fillet;  Flm,  median  longitudinal  bundle;  Niil,  nucleus  of  the  third  nerve. 


STRUCTURE  AND  RELATIONS  OF  INTER-BRAIN  85 

brain.  In  the  mid-brain  can  still  be  recognized  the  foot  and  the 
hood,  but  the  foot  has  bifurcated  into  the  peduncles  of  the  right  and 
left  cerebral  hemispheres.  The  tegmentum  contains  the  red 
nucleus,  in  which  terminate  most  of  those  fibres  from  the  cerebel- 
lum, that  were  seen  decussating  in  the  preceding  section.  At  the 


FIG.  40.— Horizontal  Section  Through  the  Brain.    (Edinger.)  $.   White  matter  shows  white, 
gray  matter  gray,  and  ventricular  spaces  black. 


side  appear  the  median  and  lateral  geniculate  bodies.  The  cut-off 
stump  of  the  second  or  optic  nerve  is  seen  entering  the  lateral  genicu- 
late body. 

§  20.  The  relations  of  the  inter-brain  can  perhaps  be  better  seen 
in  a  section  (see  Fig.  97,  p.  223),  which  is  made  vertically  through 
the  cerebrum,  and  further  to  the  front  than  the  preceding  section; 
but  which  includes  the  thalamus,  because  of  that  overlapping  of 


86       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

some  of  the  end-brain  and  inter-brain  of  which  mention  has  al- 
ready been  made  (see  p.  51). 

Of  the  pallium,  the  temporal  lobe  is  seen  below,  and  the  frontal 
lobe  above,  separated  by  the  fissure  of  Sylvius,  which  spreads  out 
at  its  bottom  into  the  "  Island."  Passing  inward  from  the  island, 
we  encounter  first  a  strip  of  white  matter,  then  a  strip  of  gray 
^(the  "claustrum"),  next  the  lenticular  nucleus,  showing  three  parts; 
after  that,  the  white  matter  of  the  internal  capsule,  and  finally  the 
thalamus  toward  the  centre  and  the  caudate  nucleus  above.  The 
caudate  and  lenticular  nuclei  are  parts  of  the  striatum,  and  were 
separated,  in  the  early  development  of  the  brain  (p.  53),  by  fibres 
growing  through  from  the  thalamus  and  from  the  cortex. 

§  21.  A  horizontal  section  through  the  cerebrum  gives  another 
view  of  these  same  structures.  The  full  extent  of  the  internal 
capsule  is  better  seen  here  than  in  the  vertical  section.  It  will  be 
noted  that  the  lateral  ventricles,  the  caudate  nucleus,  and  the  cal- 
losum  are  each  cut  twice  in  the  section.  This  results  from  the  curved 
growth  of  the  cerebrum  about  a  fixed  point  (compare  p.  51),  which 
gives  to  each  of  these  structures  an  arched  shape.  The  arch  of  the 
callosum  can  be  seen  in  Fig.  31,  which  also  shows  another  arched 
band,  the  fornix,  lying  beneath  the  callosum.  The  fornix  appears 
in  section  in  Fig.  40.  It  consists  of  fibres  which  arise  in  the  temporal 
lobe,  and  which  arch  forward  over  the  thalamus,  and  then  descend 
to  the  mammillary  body  in  the  base  of  the  inter-brain.  It  belongs 
to  the  archipallium,  and  some  of  its  fibres  decussate,  forming  the 
"  psalterium,"  which  is  the  commissure  of  the  archipallium,  as  the 
callosum  is  the  commissure  of  the  neopallium. 

§  22.  The  cerebellum  consists  of  a  middle  lobe  called  the  vermis, 
and  two  lateral  hemispheres.  As  in  the  cerebrum,  the  greater  part 
of  the  gray  matter  lies  outside,  in  a  cortex,  whose  surface  is  folded 
into  numerous  fissures.  Other  nuclei  lie  in  the  base  of  the  vermis. 
The  cerebellum  is  attached  to  the  brain-stem  by  three  pairs  of  pe- 
duncles (see  Fig.  41). 

The  inferior  peduncle  consists  of  afferent  fibres  from  the  spinal 
cord,  from  the  nuclei  of  the  sensory  cranial  nerves,  and  from  the 
olive.  The  superior  contains  a  bundle  of  fibres  from  the  cord,  but 
is  largely  composed  of  fibres  which  originate  in  the  cerebellum  and 
pass  forward  to  the  mid-brain  and  thalamus.  The  middle  peduncle 
consists  of  the  pontine  fibres,  which  arise  from  the  pontine  nuclei, 
and  are  connected  with  fibres  descending  from  the  cerebrum.  This 
is  the  path  of  communication  from  the  cerebrum  to  the  cerebellum. 
The  pontine  fibres,  then,  are  afferent  to  the  cerebellum;  they  pass 
to  the  cortex  of  the  cerebellar  hemispheres;  whereas  the  rest  of  the 
afferent  fibres  go  to  the  vermis.  The  development  of  the  cere- 


TRACING  AND  NAMING  OF  NERVE-TRACTS         87 

bellar  hemisphere  varies  with  that  of  the  cerebrum,  being  largest 
in  man. 

(Further  study  of  the  inner  structure  of  the  cortices  and  other 
nuclei  of  the  brain  is  deferred  to  a  chapter  on  the  nerve-cell  and  its 
connections;  and,  in  case  of  the  cerebral  cortex,  to  a  special  chapter 
on  that  cortex.) 

§  23.  As  can  readily  be  imagined  from  the  large  number  of  tracts 
and  nuclei  which  have  been  mentioned  even  in  the  above  hasty 
review,  the  internal  structure  of  the  brain  presents  a  bewildering 
complexity  of  collections  of  nerve-cells  and  of  bundles  of  fibres 


FIG.  41.— Lower  Surface  of  Cerebellum.  #.  (After  Sappey.)  1,  vermis;  3,  hemisphere. 
The  pons,  bulb,  and  various  pairs  of  nerves  are  also  seen,  thus:  12,  13,  the  fifth  nerve; 
14,  the  abducens;  15,  the  facial;  16,  the  "intermediate";  17,  the  auditory;  18,  the 
glosso-pharyngeal;  19,  the  vagus;  20,  the  accessory;  21,  the  hypoglossal. 

coursing  in  various  directions.  In  attempting  to  unravel  this  com- 
plexity, the  most  important  task  is  that  of  tracing  fibres  from  the 
cells  which  give  them  off  to  the  cells  which  receive  them.  If  we 
knew  the  fibres  which  connect  the  various  "centres"  or  collections 
of  cells,  we  should  have  taken  a  long  step  toward  discovering  the 
functions  of  the  centres  thus  connected. 

Many  ingenious  methods  for  tracing  the  nerve-tracts  have  been 
employed,  the  chief  of  which  are  the  embryological  and  the  patho- 
logical or  degeneration  method.  The  principle  of  the  former — the 
myelinization  of  different  tracts  at  different  times  in  the  individu- 
al's development — has  already  been  mentioned  in  the  chapter  on 
embryology  (see  p.  58).  The  principle  of  the  pathological  method 
is  the  general  fact  that  any  part  of  a  living  cell,  when  cut  off  from  the 
cell  nucleus,  dies.  Thus,  a  nerve-fibre  separated  from  its  cell  of 
origin  speedily  begins  to  degenerate,  and  is  then  stained  by  several 
reagents  (especially  by  osmic  acid)  to  a  different  degree  from  normal 


88       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

nerve-fibres  (compare  Fig.  42).  If,  therefore,  a  bundle  of  fibres  has 
been  severed  by  disease  or  injury,  or  by  the  knife  of  an  experimenter, 
the  fibres  of  this  bundle  degenerate,  and  can  thus  be  traced  in  their 
course  amidst  other  fibres.  This  method  has  proved  remarkably 
serviceable.  Many  other  methods  have  been  employed.  Von 
Bechterew  mentions1  no  fewer  than  eleven,  of  which  we  may  cite  the 
comparative  study  of  different  vertebrates,  especially  of  lower  forms 
with  simpler  fibre  systems;  and  the  physiological  method  which  deals 
with  a  living  animal  by  exciting  to  activity  some  limited  region — a 


FIG.  42. — Degeneration  of  Tracts  after  Injury  to  the  Cord.  (Strtimpell.)  Five  sections  of 
the  cord  are  arranged  in  order,  the  uppermost  being  at  the  left.  The  middle  section, 
lying  close  to  an  injury  which  has  completely  severed  the  cord,  shows  degeneration  in 
every  part.  The  amount  of  degeneration  decreases  gradually  in  the  sections  further  from 
the  injury.  The  upper  sections  show  "ascending"  degeneration  in  the  dorsal  and  the 
outside  of  the  lateral  columns.  The  lower  sections  show  descending  degeneration  in  a 
certain  part  of  the  lateral  column. 

sense-organ,  a  tract  of  fibres,  or  a  portion  of  gray  matter — and  then 
observing  either  the  locus  and  character  of  the  end-effect,  or  the 
electrical  change  which  accompanies  the  conduction  of  the  nervous 
impulse  in  the  nerve-fibres. 

The  object  of  such  studies  is  attained  when  we  know:  (1)  the 
origin  of  a  tract,  i.  e.,  the  group  of  nerve-cells  from  which  the  fibres 
proceed;  (2)  the  course  of  the  tract  to  (3)  its  termination  in  some 
other  group  of  nerve-cells;  and  (4)  the  connections  of  the  tract  with 
other  tracts  at  its  origin  and  termination.  The  fact  is  also  to  be 
considered  that  nerve-fibres  often  send  out  collateral  branches  at 
some  part  of  their  course,  and  thus  a  tract  may  have  more  than  one 
terminus.  It  is  part  of  our  object  to  discover  the  function  subserved 
by  each  tract.  If  the  connections  of  the  tract  were  thoroughly 
known,  this  alone  might  be  a  sufficient  indication  of  its  function; 
but  in  the  present  state  of  knowledge,  the  physiological  method  is 
often  the  only  one  which  gives  a  knowledge  of  the  function;  and  in 
many  cases  even  this  fails  us. 

§  24.  A  system  of  naming  the  nerve-tracts,  now  coming  into 
favor,  consists  in  the  use  of  compound  terms,  in  which  the  origin 
and  terminus  of  the  tract  are  indicated.  For  example,  the  "  cortico- 
spinal  tract"  arises  in  the  cortex  and  terminates  in  the  cord;  it  is 

1  Die  Leitungsbahnen  im  Gehirn  und  Ruckenmark,  1899,  pp.  2-9. 


NERVE-TRACTS  AND  THEIR  CONNECTIONS        89 

what  we  have  previously  called  the  pyramidal  tract.  A  list  of  the 
analytical  names  of  the  tracts  will  convey  some  notion  of  the  prog- 
ress thus  far  made  in  the  unravelling  of  the  white  matter;  but  it 
should  be  added  that  very  many  fibres  still  remain  to  be  traced.1 

§  25.  The  most  interesting  of  these  tracts  to  the  student  of 
mental  life  are  probably  those  which  convey  sensory  impulses  to 
the  cerebral  cortex  and  those  which  convey  motor  impulses  away 
from  the  cortex;  and  these  may  now  be  examined  with  some  care. 

The  sensory  pathway  from  the  trunk  and  limbs  enters  the  cord 
by  the  dorsal  roots.  On  reaching  the  cord,  the  sensory  fibres  turn 
in  different  directions.  Some  pass  directly  to  the  ventral  horn  of 
the  gray  matter,  where  they  form  connections  with  the  motor  cells, 
and  so  influence  the  motor  fibres  and  provide  a  direct  reflex  pathway 
back  to  the  muscles.  Others  of  the  sensory  fibres  turn  up  the  dorsal 
columns;  and  some  of  them  continue  in  these  columns  up  through 
the  cord  to  the  bulb,  where  they  terminate  in  the  nuclei  of  the 
dorsal  columns.  The  cells  in  these  nuclei  send  out  axons  which 
continue  the  sensory  pathway  toward  the  cerebrum,  by  first  crossing 
to  the  other  side  of  the  bulb,  and  then  proceeding  upward  in  the 

1  In  understanding  this  list  it  should  be  remembered  that  the  names  of  the 
different  nerve-tracts  are  designed  to  combine  the  place  of  origin  and  the  place 
of  termination;  and  that  tracts  having  the  same  origin  are  grouped  together. 
Those  which  have  hitherto  been  fairly  well  made  out  are  the  following: 

Radiculo-bulbar.  Tecto-bulbar;  Tecto-spinal,  lateral  and 
Spino-cerebellar;  Spino-olivary;  Spino-  medial. 

tectal;  Spino-thalamic.  Thalamo-spinal;  Thalamo-olivary;  Tha- 
Cervico-lumbar.  lamo-habenular;  Thalamo-parietal. 

Nucleo-cerebellar.  Thalamo-temporal;  Thalamo-occipital. 

Bulbo-tectal;  Bulbo-thalamic;  Bulbo-  Habenulo-peduncular. 

mammillary.  Mammillo-thalamic;  Mammillo  -  teg- 
Acustico-tectal;  Acustico-thalamic.  mental. 

Vestibulo-spinal.  Striato-peduncular;  Striato-thalamic. 

Nono-spinal  (or  "  solitary  bundle  ")•  Fronto  -  spinal  (cortico  -  spinal),  direct 
Quinto-spinal;  Quinto-thalamic.  and  crossed;  Fronto-bulbar;  Fronto- 

Retino  -  thalamic    and    Retino  -  tectal          pontine. 

(optic).  Occipito  -  temporo  -  pontine;  Occipito- 
Olfacto-cortical  (or  ammonic) ;  Olfacto-  thalamic. 

habenular;  Olfacto-mammillary.  Cortico-tectal;  Cortico-habenular. 

Olivo-cerebellar.  Ammono-mammillary. 

Ponti-cerebellar.  Fronto-thalamic. 

Rubro-spinal;  Rubro-thalamic.  Temporo -frontal;  Temporo  -  parietal; 
Cerebello-rubral;  Cerebello-thalamic.  Temporo-occipital. 

To  these  should  be  added  the  median  longitudinal  bundle  and  the  numerous 
commissures. 

This  list  will  probably  need  some  revision  from  future  discoveries;  and  it  will 
certainly  require  numerous  additions.  But,  as  it  now  stands,  it  is  a  striking 
monument  to  the  industry  of  neurologists. 


90      GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

"fillet"  to  the  thalamus,  where  they  terminate;  but  the  path  is  fur- 
ther continued  by  fibres  arising  from  the  cells  of  the  thalamus  and 
passing  up  through  the  internal  capsule  to  the  cortex.  Three  sets 
of  fibres,  placed  end  to  end,  are  therefore  required  to  convey  sensory 
impulses  from  the  limbs  and  trunks  to  the  cortex;  three  tracts  are 
linked  to  form  this  sensory  pathway — tracts  which  may  be  named 
the  radiculo-bulbar,  the  bulbo-thalamic,  and  the  thalamo-cortical. 
This  is  the  most  clearly  traced  of  the  sensory  pathways  from  the 
limbs  and  trunk;  but  apparently  it  is  not  the  pathway  for  the  sense 
of  touch,  for  interruption  of  this  pathway  by  injury  does  not  abolish 
conscious  sensations  of  touch,  temperature,  and  pain.  Injury  to 
the  dorsal  columns  interferes  with  the  "  muscle-sense,"  whereas  the 
cutaneous  senses  are  affected  by  injury  to  the  lateral  columns  of 
the  cord. 

Now  some  of  the  sensory  fibres,  on  entering  the  cord,  terminate 
in  the  dorsal  horn  of  gray  matter;  and  cells  located  there  send  out 
fibres  which,  after  crossing  to  the  other  side  of  the  cord,  ascend  in 
the  lateral  columns,  and  can  be  traced  up  to  the  thalamus.  This 
spino-thalamic  tract  is  apparently  employed  by  the  cutaneous  senses, 
though  it  appears  rather  too  slender  to  constitute  the  sole  path  from 
the  skin  to  the  brain. 

Some  of  the  dorsal  root  fibres  pass  to  that  part  of  the  gray  matter 
of  the  cord  which  is  called  Clarke's  column,  and  here  connect  with 
the  cells  which  give  rise  to  a  tract  to  the  cerebellum.  Still  other  con- 
nections are  formed  in  the  spinal  cord  between  the  incoming  sen- 
sory fibres  and  various  tracts  running  to  different  parts  of  the  brain. 
Probably,  however,  these  tracts,  as  well  as  the  cerebellar,  are  not 
concerned  in  conscious  sensation. 

Cutaneous  sensation  from  the  face  is  provided  for  by  the  fifth 
pair  of  cranial  nerves.  From  the  terminal  nuclei  of  these  nerves, 
in  the  pons  and  bulb,  arise  fibres  which  pass  to  the  thalamus — 
the  quinto-thalamic  tract — and  end  there,  as  do  the  sensory  tracts 
from  the  cord. 

CThe  sense  of  taste  is  served  by  fibres  of  the  seventh  and  ninth 
pairs  of  nerves;  these  fibres  end  in  a  common  terminal  nucleus,  but 
the  further  course  of  the  gustatory  pathway  toward  the  cortex  can 
not  yet  be  stated. 

The  fibres  from  the  cochlear  branch  of  the  eighth  nerve,  the  nerve 
of  hearing,  end  in  nuclei  close  to  the  entrance  of  the  nerve.  The 
secondary  fibres,  issuing  from  here,  cross  the  middle  line  of  the  bulb 
and  end  in  another  mass  of  gray  matter,  called  the  superior  olive. 
From  these,  tertiary  fibres  arise  which  conduct  the  auditory  impres- 
sions forward  to  the  mid-brain  and  thalamus — more  precisely,  to 
that  part  of  the  mid-brain  known  as  the  posterior  quadrigeminal 


NERVE-TRACTS  AND  THEIR  CONNECTIONS 


91 


body,  and  to  that  part  or  appendage  of  the  thalamus  known  as  the 
internal  geniculate  body. 

§  26.  The  central  connections  of  the  second  or  optic  nerve — the 
nerve  of  sight — are  particularly  worth  noting.  The  right  and  left 
optic  nerves,  as  they  pass  backward,  approach  each  other,  meet, 
and  appear  to  cross  in  much  the  shape  of  an  X  or  of  the  Greek  letter 


FIELD   OF   VIEW 


Left 


Right 


Left  hemisphere 


Right  hemisphere 


BRAIN 

FIG.  43. — Diagram  of  the  Semi-Decussation  of  Optic  Fibres  in  the  Chiasm. 

X,  from  which  resemblance  the  crossing  is  called  the  "  optic  chiasm." 
The  nerves  which  lead  back  from  the  crossing  and  into  the  brain 
are  called  the  "optic  tracts."  Just  how  much  crossing  of  fibres 
occurs  in  the  chiasm  depends  on  the  species  of  animal,  and  on  the 
position  of  the  eyes  in  the  head.  Those  animals  which  have  eyes 
on  the  sides  of  their  heads,  and  directed  to  right  and  left  so  as  to 
give  almost  totally  different  fields  of  view,  show  at  the  chiasm  a 
nearly  complete  crossing  of  fibres.  In  such  animals,  therefore,  the 
right  eye  is  connected  with  the  left  half  of  the  brain,  and  the  left 


92       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

eye  with  the  right  half  of  the  brain.  This  crossed  relation  holds 
for  the  other  sense-organs.  But  in  animals  whose  eyes  are  placed 
somewhat  forward,  with  fields  of  view  more  or  less  overlapping, 
the  decussation  at  the  chiasm  is  less  complete;  and  in  animals  such 
as  man,  whose  eyes  are  directed  straight  forward  and  have  almost 
identical  fields  of  view,  the  crossing  becomes  a  semi-decussation 
(see  Fig.  43).  The  nerve-fibres  from  the  nasal  half  of  each  retina 
cross,  whereas  those  from  the  temporal  half  do  not  cross,  but  bend 
into  the  optic  tract  of  the  same  side,  and  pass  back  to  the  same  side 
of  the  brain.  The  result  is  that  the  right  half  of  the  brain  receives 
the  fibres  from  the  right  half  of  each  eye,  and  the  left  half  similarly. 
Now  since  the  rays  of  light  cross  within  the  eyeball,  the  right  half 
of  each  retina  receives  light  from  the  left  side,  and  therefore  the 
right  half  of  the  brain  receives  the  impressions  that  come  from  the 
left  side.  The  net  result  is  accordingly  the  same  in  animals  with 
eyes  directed  forward  as  in  animals  with  eyes  directed  to  the  side: 
in  each  case  the  impressions  originating  at  one  side  of  the  middle 
line  are  conveyed  to  the  opposite  half  of  the  brain.  The  brain  has, 
therefore,  the  same  crossed  relation  with  the  outer  world  in  the 
case  of  vision  as  in  the  case  of  all  other  receptors. 

The  fibres  of  the  optic  tracts  end  in  the  inter-brain  and  in  the  ad- 
joining mid-brain.  In  the  former,  the  end-stations  are  the  "pul- 
vinar"  and  more  particularly  the  "external  geniculate  body";  in 
the  latter,  their  ending  is  the  anterior  quadrigeminum.  The  quad- 
rigeminum  is  the  principal  ending  in  fishes,  reptiles,  and  birds.  But 
in  mammals  the  inter-brain  endings,  especially  the  geniculatum, 
receive  most  of  the  optic  fibres;  and  in  man  the  mid-brain  receives 
only  a  few  fibres,  which  are  concerned  mostly  with  the  pupillary 
reflex.  The  principal  connections  of  the  optio  nerve,  in  man,  are, 
directly  with  the  thalamus  and  external  geniculate  body,  and  in- 
directly, through  fibres  arising  from  these  bodies,  with  the  cortex 
of  the  occipital  lobe. 

§  27.  The  olfactory  path  begins  with  the  fibres  of  the  olfactory 
nerve,  which  terminate  in  the  olfactory  bulb.  A  secondary  and  then 
a  tertiary  tract  leads  to  the  cortex  of  the  "  archipallium,"  in  the 
pyriform  lobe  and  the  hippocampus. 

The  previous  survey  of  the  sensory  pathways  shows  that,  with 
the  exception  of  the  olfactory  and  possibly  of  the  gustatory,  all  of 
these  pathways  lead  to  the  inter-brain.  The  thalamus,  with  its  ac- 
cessory bodies,  is  an  intermediate  station  in  the  sensory  paths 
toward  the  cortex.  It  is  curious  that  the  thalamus  should  inter- 
vene in  this  manner,  and  no  well-grounded  explanation  presents 
itself.  Why  should  not  the  sensory  fibres  run  right  up  to  the  cortex, 
without  interruptions,  first  in  the  terminal  nuclei  and  then  in  the 


PRINCIPAL  MOTOR  PATHWAY  93 

thalamus  ?  In  general,  we  can  see  that  where  a  fibre  ends  by  split- 
ting into  fine  branches  which  are  mingled  with  the  fine  branches  of 
other  fibres,  opportunities  are  afforded  for  something  analogous  to 
the  switching  that  goes  on  at  a  railway  junction.  Sensory  impulses 
from  various  receptors  may  here  be  collected;  and  those  from  any 
one  receptor  may  be  widely  distributed.  The  terminal  nuclei,  quite 
surely,  provide  for  the  distribution  of  sensory  impulses  to  the  motor 
nerves,  to  the  cerebellum  and  to  the  thalamus.  The  thalamus,  since 
it  possesses  many  short  fibres  connecting  its  own  parts,  is  probably 
something  more  than  a  mere  way-station.  Apparently,  the  sen- 
sory impulses  from  different  receptors  come  together  here  and  join 
in  such  a  way  that  the  impulses  which  pass  from  here  to  the  cortex 
are  already  organized  or  synthesized  to  a  certain  extent.  For  ex- 
ample, the  adult  man  possesses  a  really  remarkable  power  of  lo- 
cating visual  objects  in  reference  to  the  body,  when  the  position  of 
the  head  differs  greatly;  and  although  a  given  visual  appearance  of 
the  object  means  quite  different  locations  in  space  according  to  the 
position  in  which  the  head  happens  to  be  at  the  moment.  There 
must  clearly  be  some  means  of  bringing  together  (or  "synthesizing") 
the  visual  impulses  with  those  other  sensory  impulses  which  indi- 
cate the  position  of  the  head.  The  conjecture  that  this  means  is 
furnished  by  the  thalamus  serves,  at  least,  to  direct  attention  to  the 
kind  of  work  which  is  apparently  done  in  the  switching  stations 
along  the  route  of  sensory  impulses  to  the  cortex. 

§  28.  The  most  interesting  motor  pathway,  from  the  psycholo- 
gist's point  of  view,  is  that  which  leads  from  the  "motor  area"  of 
the  cortex,  and  which  may  be  called  the  pyramidal,  or  the  cortico- 
spinal;  or,  taking  more  precise  account  of  its  origin,  the  fronto- 
spinal.  This  is  the  longest  of  all  the  tracts,  and  is  visible  in  every 
cross  section  of  the  cord  or  brain-stem.  After  passing  down  through 
the  white  matter  underlying  the  cortex,  its  fibres  come  together  in- 
to a  compact  bundle  in  the  internal  capsule  (where  they  are  fre- 
quently injured  by  hemorrhage,  in  apoplexy),  then  emerge  upon  the 
ventral  surface  of  the  mid-brain.  They  retain  this  ventral  position 
down  through  the  pons  and  bulb;  but  in  the  lowest  part  of  the  bulb 
each  pyramidal  tract  (the  right  and  the  left)  splits  into  two,  the 
smaller  of  which  continues  down  the  ventral  column  of  the  cord, 
while  the  larger  part  passes  over  to  the  opposite  side  of  the  cord  and 
descends  in  the  lateral  column.  Those  fibres  which  do  not  cross  in 
the  bulb  do  so,  one  by  one,  at  different  levels  of  the  cord.  The 
cortico-spinal  fibres  terminate  in  the  gray  matter  of  the  cord,  and  the 
motor  pathway  is  continued  by  axons  from  the  motor  cells  of  the 
cord,  which  pass  out  by  the  ventral  roots,  and  finally  reach  the 
muscles. 


94       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

§  29.  Though  the  connections  of  the  cerebrum  with  the  sense- 
organs  and  muscles — together  with  the  paths  of  association  within 
the  cerebrum  itself,  to  which  we  shall  return  in  another  chapter — 
constitute  the  most  interesting  system  of  nervous  connections,  yet 
a  false  impression  would  be  created  if  these  only  were  described; 
for  not  all  the  nuclei  and  fibre-bundles  which  exist  in  the  brain  and 
cord  are  directly  subservient  to  the  cerebrum.  We  may  recognize 
the  existence  of  several  systems  of  tracts  and  nuclei  which  are  more 
or  less  independent  in  their  growth  and  function,  though  connected 
at  various  points.  We  may,  therefore,  recognize  the  following 
systems  of  centres  and  connections:  the  fundamental,  the  mesen- 
cephalic,  the  cerebellar,  the  archipallial,  and  the  neopallial.  A 
brief  description  of  each  follows. 

§  30.  (1)  The  fundamental  system  consists  of  the  sensory  and 
motor  nerves,  with  the  cells  of  origin  of  the  motor  fibres  and  the 
groups  of  cells,  called  terminal  nuclei,  into  which  the  sensory  fibres 
lead  and  in  which  they  terminate.  In  addition,  the  fundamental 
system  includes  the  central  fibres  which  directly  connect  the  motor 
and  sensory  nuclei.  In  the  cord,  this  system  includes  the  gray  mat- 
ter and  the  ground  bundles;  in  the  brain-stem,  it  includes  the  motor 
and  terminal  nuclei  of  the  cranial  nerves,  and  many  short  connect- 
ing fibres.  An  important  part  of  the  fibre  connections  is  represented 
by  the  "median  longitudinal  bundle"  of  the  brain-stem,  a  continu- 
ation of  the  ground  bundles  of  the  cord.  It  extends  the  length  of 
the  brain-stem,  and  consists  mostly  of  short  fibres  connecting  neigh- 
boring nuclei;  it  affords  direct  connection,  for  example,  between  the 
nuclei  of  the  several  motor  nerves  of  the  eye,  and  so  contributes  to 
the  co-ordination  of  the  eye  muscles. 

The  fundamental  system  is  the  oldest  in  the  race,  and  the  earliest 
to  develop  in  the  individual.  In  accordance  with  a  principle  set 
forth  in  the  chapter  on  embryology  (see  p.  57),  the  structures  be- 
longing to  this  system  lie  close  to  the  cavity  of  the  neural  tube;  in 
the  bulb,  they  lie  close  beneath  the  ventricles,  and  in  the  mid-brain, 
close  about  the  aqueduct.  They  are  overlain  by  the  more  volumi- 
nous structures  of  the  later-developing  systems. 

The  function  of  the  fundamental  system  is  to  provide  for  local 
reflexes,  and  also,  through  its  connecting  fibres,  for  reflexes  that  are 
more  wide-spread.  Since  it  includes  the  origins  of  the  motor  nerves, 
it  has  direct  control  of  the  muscles;  and  other  systems  probably  act 
first  on  it,  and  only  through  it  on  the  muscles. 

§  31.  (2)  The  mesencephalic  or,  more  precisely,  the  "tectal" 
system1  is  apparently  the  dominant  system  in  fishes,  amphibia, 

1  From  "tectum,"  the  "roof"  of  the  mid-brain,  comprising  the  corpora  quad- 
rigemina. 


SYSTEMS  OF  CENTRES  AND  CONNECTIONS         95 

reptiles,  and  birds;  but  in  mammals  and  especially  in  man,  its  im- 
portance has  decreased.  Its  nuclei  are  those  of  the  quadrigemina;  its 
incoming  fibres  are  partly  from  the  optic  nerve,  and  partly  from  the 
terminal  nuclei  of  the  cochlear  nerve;  it  also  receives  slender  tracts 
from  the  cord.  Its  outgoing  fibres  lead  to  the  nuclei  of  the  motor 
nerves  of  the  eye,  and  also,  by  slender  tracts,  to  the  bulb  and  cord. 

§  32.  (3)  The  cerebellar  system  is,  in  man,  very  extensive.  To 
its  gray  matter  must  be  reckoned  the  cortex  of  the  cerebellum,  the 
nuclei  which  lie  imbedded  in  the  base  of  the  cerebellum,  and  four 
other  nuclei,  which  lie  outside  the  cerebellum,  but  near  it  in  the 
brain-stem,  and  closely  connected  with  it.  These  are  the  olivary 
nucleus,  the  nucleus  of  Deiters,  the  pontine  nuclei,  and  the  red 
nucleus  of  the  mid-brain.  The  olivary  and  pontine  nuclei  send 
numerous  fibres  into  the  cerebellum;  the  red  nucleus  receives  many 
fibres  from  the  cerebellum;  and  the  nucleus  of  Deiters  both  sends 
and  receives.  The  cerebellum  also  receives  many  fibres  directly 
from  the  terminal  nuclei  of  the  sensory  nerves;  the  "direct  cerebel- 
lar" tract  from  the  cord  is  an  example,  and  there  are  other  bundles 
from  the  cranial  nerves.  The  connection  of  the  vestibular  nerve 
with  the  cerebellum  seems  especially  close.  To  the  cerebellar  sys- 
tem should  also  be  reckoned  those  tracts  which  lead  into  the  olives 
(from  the  cord  below,  and  from  the  thalamus  above),  and  into  the 
pontine  nuclei  (from  the  cerebral  cortex).  This  connection  be- 
tween the  cerebrum  and  the  cerebellum,  by  way  of  the  pontine 
nuclei,  may  be  reckoned  either  with  the  cerebral  or  with  the  cere- 
bellar system,  and  is  interesting  as  showing  a  broad  path  of  communi- 
cation between  the  two  great  organs.  The  path  conducts,  appar- 
ently, from  the  cerebrum  to  the  cerebellum. 

All  the  tracts  of  the  cerebellar  system,  thus  far  mentioned,  lead 
toward  the  cerebellum.  The  principal  outgoing  tract  passes  out 
of  the  cerebellum  by  the  superior  peduncle  into  the  mid-brain, 
where  its  fibres  terminate  mostly  in  the  red  nucleus.  From  this 
nucleus  arise  several  tracts,  one  of  which  passes  down  into  the  cord, 
and  must  afford  a  means  by  which  the  influence  of  the  cerebellum 
is  exerted  on  the  cord  and  so  on  the  muscles.  From  Deiters'  nu- 
cleus, also,  a  tract  passes  down  into  the  cord.  This  account  of  the 
cerebellar  system,  though  still  far  from  exhaustive,  is  enough  to 
awaken  respect  for  this  organ  and  its  probable  importance. 

§  33.  (4)  The  "  archipallial"  system,  closely  related  in  function 
to  the  sense  of  smell,  includes  in  its  gray  matter  certain  portions  of 
the  cortex  ("pyriform  lobe,"  "hippocampus"),  and  certain  nuclei 
in  the  inter-brain  ("mammillary  body,"  "habenula").  Its  fibre- 
bundles  include  the  olfactory  tracts,  the  "fornix"  (connecting  the 
archipallial  cortex  with  the  inter-brain),  and  several  other  tracts. 


96       GROSS  STRUCTURE  OF  THE  NERVOUS  SYSTEM 

§  34.  (5)  The  neopallial  system  includes  much  the  greatest  part 
of  the  human  cortex,  and  so  the  greatest  part  of  all  the  gray  mat- 
ter of  the  nervous  system.  Besides  this,  it  includes  other  nuclei, 
the  chief  of  which  lie  in  the  inter-brain.  Most  of  the  thalamus  be- 
longs here,  and  also  the  external  and  internal  geniculate  bodies. 
The  interposition  of  these  nuclei  in  the  path  of  sensory  impulses  to 
the  cortex  has  already  been  discussed.  The  tracts  of  the  neopallial 
system  include  the  various  sensory  tracts  which  lead  into  the  inter- 
brain,  the  fibres  leading  from  the  inter-brain  to  the  cortex,  the  out- 
going fibres  from  the  cortex  (cortico-spinal,  cortico-bulbar,  cortico- 
pontine,  cortico-thalamic,  etc.),  and,  most  numerous  of  all,  the  fibres 
joining  one  part  of  the  cortex  with  another. 

§  35.  Regarding  the  relations  of  these  several  systems,  in  some 
instances  (as  from  the  neopallium  to  the  cerebellum)  broad  paths 
of  communication  are  anatomically  visible;  in  other  cases,  such  con- 
nections have  yet  to  be  discovered.  Since  all  systems  make  use  of 
the  same  muscles,  they  must  all  converge  upon  them,  and  therefore, 
as  was  said  before,  on  the  fundamental  system  which  alone  has  a 
direct  connection  with  the  muscles.  In  this  sense,  the  fundamental 
system  might  be  called  the  centre  of  the  whole  mechanism.  On 
the  other  hand  it  is  clear  that,  in  intelligent  human  behavior,  the 
neopallial  is  the  dominant  system.  Such  a  survey  as  has  preceded 
serves  to  correct  the  tendency  to  formulate  a  too  simple  and  dia- 
grammatic scheme  of  the  inner  relations  of  the  nervous  system. 
To  regard  the  whole  system  as  a  connected  mechanism  is  eminently 
proper,  but  it  is  distinctly  a  biological  mechanism,  and  its  plan  shows 
the  marks  of  growth  and  adaptation,  and  of  possible  effects  from  use 
and  from  learning,  being  quite  different  from  such  a  design  as  is 
drawn  up  by  an  engineer  for  a  machine  or  an  electric  system. 

The  previous  description  makes  the  significant  fact  quite  clear 
that  the  white  matter  of  the  brain  and  cord,  instead  of  being,  as  it 
appears,  a  homogeneous  mass,  is  a  vast  and  intricate  network  of 
fibres  and  bundles  of  fibres,  which  have  the  office  of  forming  me- 
chanical connections  between  definite  parts  of  the  gray  matter. 
These  connections  are  manifold,  but  are  not  indiscriminate.  Ap- 
parently, there  is  no  one  centre  to  which  all  paths  lead;  all  parts  of 
the  nervous  mechanism  are  interconnected,  but  some  much  more 
directly  than  others.  In  this  way,  both  long  and  short  paths,  both 
converging  and  radiating  paths,  between  both  the  nearer  and  the 
more  remote  parts,  seem  to  be  characteristic  features  of  the  entire 
system. 


CHAPTER  IV 
ELEMENTS  OF  THE  NERVOUS  STRUCTURE 

§  1.  In  considering  the  nervous  systems  of  invertebrates,  and 
the  embryology  of  the  human  system,  we  have  already  acquired 
some  knowledge  of  the  elementary  structures  of  which  the  nerves 
and  nerve-centres  are  composed.  In  the  development  of  the  nervous 
system,  the  germinal  cells  lining  the  cavity  of  the  neural  tube  gen- 
erate daughter  cells,  the  earlier  of  which  become  neuroglia  cells, 
and  the  later  form  the  nerve-cells.  From  the  latter  the  so-called 
axons  are  branches,  which  often  run  to  great  distances;  some  of 
them  pass  out  of  the  neural  tube,  and  others  come  from  the  cells  of 
the  spinal  ganglia,  to  contribute  to  the  formation  of  the  nerves. 
Besides  the  axons,  the  cells  in  the  brain  and  cord  give  out  other 
branches,  which  are  shorter  and  less  cylindrical.  Because  they 
often  branch  like  the  limbs  of  a  tree,  they  are  called  "dendrites." 
Nerve-cells,  axons,  and  dendrites,  along  with  neuroglia  cells,  are, 
therefore,  the  best-known  elements  of  the  nervous  system. 

The  different  kinds  of  branches  which  the  nerve-cells  give  out 
were  first  fully  described  by  Deiters.1  He  based  his  work  on  that  of 
R.  Wagner2  and  Remak.3  The  term  "dendrites"  was  suggested 
by  His.  In  general,  as  has  already  been  indicated,  the  dendrites 
are  relatively  thick  and  short,  and  have  a  knotted  appearance. 
That  these  knots,  or  knobs,  are  not  the  result  of  treatment,  but  are 
the  natural  characteristic  of  these  elements,  would  seem  to  be  proved 
by  the  facts  that  all  kinds  of  stainings  show  them,  and  that  their 
distribution  is  characteristic  of  different  kinds  of  cells.  And  since 
they  are  more  marked  in  the  embryo,  they  may  be  looked  upon  as 
points  of  growth.  The  axons  are  more  uniform  in  calibre,  smoother; 
and  their  diameter  differs  in  some  correspondence  to  their  lengths. 
In  some  cells  of  the  so-called  "  Golgi  type,"  the  branches  of  the  axon 
soon  end  in  a  fine  network.  In  certain  cases  where  a  branch  leads 
off  from  the  main  axon,  before  the  latter  emerges  from  the  gray 

1  Untersuchungen  uber  Gehirn  u.  Rilckenmark  d.  Menschen  u.  d.  Saugetiere, 
pp.  55  ff.  (Braunschweig,  1865). 

2  Handworterb.,  Ill,  i,  pp.  377  ff. 

3  "Ueber  multipolar  Ganglienzellen,"  in  the  Berichte  uber  d.  Verhandl.  d.  Kgl. 
preuss.  Academie,  pp.  29  ff.  (Berlin,  1854). 

-97 


98 


ELEMENTS  OF  THE  NERVOUS  STRUCTURE 


FIG.  44.— Part  of  the    Cross  Section  of  a 
Nerve.     (Schafer).     The  myelin  sheath  is 


matter,  it  may  be  regarded  as  of  importance  enough  to  be  called 
a  "  paraxon."  Sometimes,  also,  the  axon  divides  into  two  branches, 
each  with  its  own  medullary  sheath.  In  the  spinal  cord,  some  axons 
even  have  three  branches. 

In  general,  the  axon  is  developed  before  the  dendrites  appear. 
The  first  dendrite  usually  appears  just  opposite  the  axon  and  travels 

toward  the  centre.  The  axon 
itself  starts  from  the  neuroblast 
and, "  guided  by  some  mysterious 
power,"  grows  through  the  em- 
bryonic body  to  its  appropriate 
muscle.  This,  and  similar  facts, 
have  led  some  observers  to  affirm 
that "  all  nerve-fibres  of  the  body 
are  extraordinarily  long  out- 
growths from  either  central  or 
peripheral  ganglion  cells."  Every 
nerve-fibre  is  thus  "  to  be  recog- 
nized as  being,  from  beginning  to 
end,  a  product  or,  more  correctly, 

iNerve.     t»cnarer;.     Tne  myenn  sneatn  is  .       »  n     •      i  11  » l 

stained  black  while  the  axon  remains  white.       a  Part>  ot  a  Single  nerve  cell. 

§  2.  The    peripheral    nerves, 

and  the  white  matter  of  the  centres,  contain  no  nerve-cells  or 
dendrites;  and  the  nerves  contain  no  neuroglia.  A  cross  section 
of  a  nerve  (see  Fig.  44)  shows  great  numbers  of  nerve-fibres, 
which  then  appear  as  little  circles.  Teasing  of  a  nerve  divides 

1  Lenhosse'k,  Der  feinere  Ban  des  Nervensy stems  (Berlin,  1895),  p.  89.  There 
are  few  more  obscure  and  uncertain  problems  than  that  proposed  by  the  question: 
"What  controls  the  direction  of  the  fibres  in  their  outward  growth?"  But  this 
problem  may  be  considered  as  only  a  special  case  of  the  general  biological  prob- 
lem as  to  the  causes  which  control  the  direction  of  all  growths  in  living  bodies. 
And  biology  is  not  yet  able  satisfactorily  to  solve  this  problem,  even  when,  as  is 
highly  probable,  influences  from  a  variety  of  controlling  forces  must  be  admitted 
to  share  in  the  result.  In  the  special  case  of  the  nerve-fibre,  some  have  held 
that  the  growth  of  its  protoplasmic  node  as  it  moves  forward  through  the  tissues, 
taking  up  nutrition  from  the  surroundings,  has  its  direction  controlled  by  the 
mechanical  ease  of  passing  through  certain  tissues  rather  than  others  (so  His: 
"Die  Entwickelung  d.  ersten  Nervenbahnen  beim  menschlichen  Embryo," 
Archiv  f.  Anat.  u.  Phys.,  Anat.  Abt.,  1887,  pp.  376  ff.;  and  "Die  Entwickelung 
d.  Nervensystems  bei  Wirbelthieren,"  Abhandlungen  d.  math-phys.  Klasse  d.  kgl. 
sacks.  Ges.  d.  Wiss.,  Bd.  XVIII,  189).  Others  account  for  the  phenomenon  as 
due  to  the  chemical  attractions  of  secretions  given  off  from  the  muscles  (so 
Cajal).  Still  others  consider  that  the  cells  are  all  the  time  functioning,  and 
this  primitive  functioning  accounts  for  their  growth;  and  also,  perhaps,  that  their 
growth  is  dependent  on  electrical  stimuli.  All  these  explanations,  however  inter- 
esting and  helpful  they  may  be,  leave  the  matter  still  an  unexplained  mystery, 
of  a  character  similar  to  that  of  all  the  performances  of  living  tissues. 


STRUCTURE  OF  THE  NERVE-FIBRE 


99 


it  into  fibres  which  run  along  parallel  to  each  other.  They  are 
bound  together  by  sheets  of  connective  tissue,  which  give  the 
necessary  tensile  strength  to  the  nerve.  The  whole  nerve  is  en- 
closed by  connective  tissue  (called  the  epineurium),  and  within  it 
can  be  distinguished  larger  and  smaller  bundles 
of  fibres — each  bundle  being  enclosed  by  con- 
nective tissue  (the  perineurium);  while  more 
delicate  sheets  of  the  same  tissue  penetrate  the 
bundles  between  the  individual  fibres  (the 
endoneuriurri).  The  branching  of  a  nerve 
often  consists  simply  in  the  separation  of  one 
of  its  bundles  from  the  rest;  in  the  same  way 
the  minuter  subdivision  of  the  branches  con- 
sists of  the  separation  of  the  fibres  from  one 
another.  The  fibres,  however,  maintain  their 
individuality  throughout  the  peripheral  nerve 
and  its  branches.  Only  at  the  very  end  of 
the  fibre,  in  the  receptor  or  effector  organ,  does 
the  single  nerve-fibre  split  up  into  fine  branches. 
§  3.  The  nerve-fibre  is  thus  the  element  or 
unit  of  which  the  nerve  is  essentially  composed. 
The  fibre  itself  consists  of  the  axon,  surrounded 
by  one  or  more  sheaths  (compare  Fig.  45). 
Most  of  the  axons  in  the  peripheral  nerves 
have  two  sheaths,  the  "primitive  sheath"  on 
the  outside,  and  the  "  medullary "  or  "myelin 
sheath,"  inside  of  the  primitive  sheath  and  next 
to  the  axon.  The  primitive  sheath  is  a  thin 
membrane,  while  the  medullary  sheath  is  often 
comparatively  thick,  and  is  composed  of  a 
white,  fatlike  substance  called  "myelin."  A 
considerable  proportion  of  the  fibres  in  the 
peripheral  nerves  do  not  possess  a  myelin 
sheath,  and  are  called  non-medullated  fibres; 
in  general,  the  fibres  originating  in  the  sympa- 
thetic ganglia  belong  to  this  class.  On  the  other 
hand,  the  fibres  in  the  white  matter  of  the  brain 
and  cord  possess  the  myelin  sheath  but  not 
the  primitive  sheath,  and  there  is  a  short  length 
of  the  axon,  just  after  it  emerges  from  its  cell,  which  is  not  provided 
with  either  of  the  sheaths.  It  seems  clear,  accordingly,  that  the 
axon  is  the  most  essential  part  of  the  nerve-fibre,  since  it  is  the 
only  part  which  is  present  in  all  fibres  and  in  all  parts  of  the 
nervous  system. 


FIG.  45. —  Short  Pieces 
of  Two  Nerve-Fibres. 
(Schafer.)  The  axon  ap- 
pears gray,  and  shows 
its  fibrils;  the  myelin 
sheath  is  stained  black; 
the  primitive  sheath 
appears  white.  R,  node 
of  Ranvier;  c,  the  nu- 
cleus of  one  of  the 
sheath-cells. 


100 


ELEMENTS  OF  THE  NERVOUS  STRUCTURE 


FIG.  46.— Motor  Cell  of  the  Ventral  Horn  of  the 
Cord,  with  Scheme  of  the  Course  of  its  Axon. 
(Barker.)  n,  the  nucleus,  with  nl,  nucleolus; 
d,  dendrites,  only  the  stumps  of  which  are 
shown;  a.h.,  hillock  from  which  the  axon 
arises;  at  ra,  the  axon  becomes  invested  with 
the  myelin  sheath;  n.R.,  a  node,  with  branch- 
ing of  the  axon;  ml,  a  muscle,  in  which  the 
axon  terminates  in  td.,  the  motor  end-plate. 


A  nerve-fibre  has  the  gen- 
eral form  of  a  thread  or  long 
narrow  cylinder.  It  is  not, 
however,  a  perfect  cylinder, 
but  shows  at  short  intervals 
slight  constrictions  or  "  nodes." 
The  space  between  two  nodes 
may  be  called  a  segment;  it 
is  primarily  a  segment  of  the 
primitive  sheath,  correspond- 
ing to  one  cell  of  the  sheath- 
forming  substance.  The  mye- 
lin sheath  is  also  interrupted 
at  each  node,  and  thus  ap- 
pears in  segments.  The  axon, 
however,  is  continuous  through 
the  node,  though  it  is  nar- 
rowed at  this  point. 

The  inner  structure  of  the 
axon  is  so  minute  that  even 
the  higher  pjowers  of  the  mi- 
croscope afford  barely  enough 
magnification  to  enable  the 
histologist  to  see  it. 

§  4.  The  size  of  the  differ- 
ent nerve-fibres  in  the  human  f 
body  varies  greatly,  according  J 
to  their  kind,  position,  and, 
perhaps,  function.  As  a  rule 
the  non-medullated  fibres  are 
smaller  than  the  medullated, 
the  former  being  from  iniW  to 
•jrAnr  °f  an  mcn  m  diameter, 
and  the  latter  (in  the  trunk 
and  branches  of  the  nerve) 
from  i-gVir  to  ^TT  of  an  inch. 
But  this  rule  is  not  always 
followed.  In  the  white  mat- 
ter of  the  cord  the  medullated 
fibres  range  in  size  from  -rhrv 
to  -a  0*0  o  of  an  inch,  in  parts 
of  the  anterior  columns,  and 
about  TffVfr  of  an  inch  in  those 
regions  of  the  lateral  and  pos- 


CONSTITUTION  OF  THE  GRAY  MATTER 


101 


terior  columns  which  are  nearest  the  gray  matter  of  the  cord.  In  the 
gray  matter  of  the  cord  and  brain  the  fibres  are  much  finer — being 
from  TroiF  to  14000  of  an  inch  in  diameter,  or  even  of  an  almost 


immeasurable  fineness;  they  are  finest  of  all  in  the  superficial  layers 
of  the  brain  and  in  the  nerves  of  special  sense.  In  some  instances  the 
axis-cylinder  may  be  not  more  than  1 0  0*0  o  o  of  an  inch  in  diameter. 

The  number  of  fibres  which  enter  into  the  composition  of  indi- 
vidual nerves  also  varies  greatly.  In  the  common  motor  nerve  of 
the  tongue  it  has  been  estimated  at  about  five  thousand,  in  that  of 
the  eyes  at  fifteen  thou- 
sand, in  the  optic  nerve 
at  one  hundred  thou- 
sand at  least. 

Success  in  analysis  of 
the  nerve  depends,  in 
large  measure,  on  the 
discovery  of  stains 
which  color  some  por- 
tions more  deeply  than 
others,  and  so  bring  out 
their  differences.  The 
most  important  fact  re- 
garding the  inner  struct- 
ure of  the  axon  is  that 
it  consists  of  plasma  in 
which  run  very  minute 

fibrils.     These  fibrils  ex-     FlG-  47.— Purkinje  Cell.     (Starr,  Strong  and  Learning.) 

tend  lengthwise  of  the 

axon,  and  are  continuous  for  long  distances;  it  is  inferred  by  many 
authorities  that  they  are  the  conducting  part  of  the  axon.  In 
short,  just  as  a  moderate  magnification  of  a  nerve  shows  it  to  be 
a  bundle  of  nerve-fibres,  so  higher  magnification  applied  to  the  single 
fibre  seems  to  show  that  it  too  is  but  a  bundle,  and  that  the  real  unit 
of  the  nerve  is  the  minute  fibril.  It  should  be  mentioned,  however, 
that,  in  our  lack  of  exact  knowledge  of  the  physical  or  chemical 
process  which  is  conducted  along  the  nerve  we  cannot  make  an 
inference,  with  any  assurance,  from  the  structure  of  the  axon  to 
the  function  of  its  separate  parts. 

§  5.  The  white  matter  of  the  brain  and  cord  is  made  up  much 
as  are  the  nerves,  except  that  the  sheets  of  connective  tissue  are 
absent,  and  also  the  primitive  sheath  of  the  fibre.  The  nervous 
substance  is  protected  by  the  enveloping  bone  and  membranes, 
and  strengthened  by  the  fibres  of  the  neuroglia  which  pass  between 
the  nerve-fibres. 


102 


ELEMENTS  OF  THE  NERVOUS  STRUCTURE 


The  gray  matter  contains  neuroglia,  nerve-cells  and  their  den- 
drites,  the  terminations  of  axons  which  enter  from  the  adjoining 
white  matter,  and  blood-vessels,  which  are  present  also,  though  in 
less  abundance,  in  the  white  matter  and  in  the  nerves.  That  this 
list  exhausts  the  contents  of  the  gray  matter  is  regarded  by  some 
authorities  as  improbable,  in  view  of  the  small  bulk  of  the  nerve- 


\ 


Fio.  48.— Pyramidal  Cells  from  the  Cerebral    Cortex.      (Kolliker.)    n,  axon;    p,  apical 

dendrite. 

cells.  Donaldson1  estimates  that  the  nerve-cells  occupy  but  1.3 
per  cent,  of  the  bulk  of  the  gray  matter;  while  the  dendrites,  as  he 
believes,  cannot  occupy  more  than  half  the  space  of  the  cells. 
This  estimate  seems  to  leave  much  space  unaccounted  for.  Nissl2 
has  advanced  similar  indirect  evidence  in  favor  of  yet  unknown  com- 
ponents of  the  gray  matter.  It  should  be  said,  however,  that  the 

1  Journal  of  Comparative  Neurology,  1899,  IX,  141. 

2  Die  Neuronenlehre  und  ihre  Anhdnger,  1903,  p.  75. 


SIZE  AND  SHAPE  OF  NERVE-CELLS 


103 


best  stains,  such  as  that  of  Cajal,  show 
the  gray  matter  to  be  very  largely  filled 
with  fine  fibres,  which  appear  to  be  the 
branches  of  dendrites  and  of  axons.1 

§  6.  The  true  nerve-cells  vary  in  size 
as  much  as  in  shape;  the  limits  may, 
perhaps,  be  given  as  from  about  yj^  to 
TrsW  of  an  inch.  The  general  shape 
of  a  nerve-cell  is  chiefly  dependent  on 
the  dendrites  and  the  manner  or  place 
of  their  leaving  the  cell-body.  The  ac- 
companying figures  (Nos.  46,  47,  48,  49) 
show  several  different  forms.  The  cells 
of  the  ventral  horn  of  the  spinal  cord — 
which  through  their  axons  directly  con- 
trol the  muscles — send  out  dendrites  in 
every  direction,  and  are  accordingly 
called  "  multipolar."  The  Purkinje  cell 
from  the  cerebellar  cortex  sends  out  one 
dendritic  stem,  which  branches  into  a 
beautiful  tree,  though  this  branching  is 

1  Progress  in  tracing  the  branches  of  the  nerve- 
cells  within  the  gray  matter  has  depended  largely 
on  the  invention  of  methods  of  staining  gray 
matter  in  ways  which  bring  out  one  or  another 
of  its  features.  For  showing  the  external  form 
of  the  cells  and  their  branches,  a  method  which 
has  done  remarkable  service  is  the  silver  method 
of  Golgi,  according  to  which  pieces  of  gray  mat- 
ter that  have  first  been  soaked  in  potassium 
bichromate  are  treated  with  nitrate  of  silver, 
with  the  result  that  a  deposit  of  dark  silver 
chromate  is  precipitated  on  some  of  the  cells. 
The  peculiarity  of  this  method  is  that  com- 
paratively few  cells  are  thus  blackened,  but 
these  few  are  blackened  throughout,  even  to 
their  fine  branches;  in  this  manner  an  individual 
cell,  which  would  otherwise  be  lost  in  its  intri- 
cate interlacing  with  other  cells,  is  made  to  stand 
out  clearly.  The  internal  structure  of  the  cell  is 
not,  however,  brought  out  by  the  Golgi  method, 
which  simply  encrusts  the  surface  with  a  black 
deposit.  Other  methods  have  been  found  which 
permit  of  study  of  the  internal  structure.  The 
method  of  Nissl  consists  in  first  staining  densely 

with  a  basic  dye  such  as  methylene  blue,  and  then  dissolving  out  some  of  the 
dye  with  alcohol;  this  leaves  some  parts  of  the  cell  still  deeply  stained. 
Apdthy  and  Bethe  have  also  introduced  methods  for  selectively  staining  the 
fibrils  within  the  cells. 


G.  49.— Cells  of  the  Mid-brain 
(Optic  Lobe  of  Chick).  (Van 
Gehuchten.)  pr.  cyl.,  the  axon, 
which  here  emerges  from  a  den- 
drite;  /.  opt,  axon  entering  from 
a  distance,  and  connecting  with 
the  dendrite. 


104         ELEMENTS  OF  THE  NERVOUS  STRUCTURE 


peculiar  in  that  it  is  confined  to  one  plane.  The  pyramidal  cells 
of  the  cerebral  cortex  send  out  one  long  dendritic  stalk  toward 
the  surface  of  the  cortex,  and  other  stalks  to  the  sides.  Many 
,  small  cells  have  only  a 

small  development  of  den- 
drites.  The  cells  of  the 
spinal  ganglia,  and  of  the 
similar  ganglia  of  the  sen- 
sory cranial  nerves,  have 
no  dendrites  (see  Fig.  15, 
p.  44). 

The  nerve-cell  is  usually, 
at  first,  a  round  or  some- 
what angular  body  without 
branches.  From  this  a  band 

^}^M/AX^^>^r<-^7S/Mk  »       of  tissue  issues>  like  a  sort 

°f  P^o-podium  and  then 
breaks  up  into  branches, 
thus  forming  the  dendrites. 
Some  cells  in  the  brain  (as 
the  Pur  kin  je  and  pyramidal 
cells)  develop  dendrites  after 
birth.  Most  central  cells 
have  only  one  axon  (mon- 
axion);  but  in  the  external 
layer  of  the  cerebrum  poly- 
axion  cells  are  said  to  be 
found.  Bipolar  cells  are 
found  in  abundance  in  the 
dorsal  root  ganglia;  and  the 
peripheral  sympathetic  cells 
are  polyaxion.  Wholly  an- 
axion  cells  are  rare;  but  are 
said  to  be  found  in  the  olfac- 
tory bulb,  the  ear,  and  the 
se  of  some  animals. 
o  special  function,  either 
sensory  or  motor,  has  been 
proved  to  be  assignable  to  any  of  these  different  shapes. 

Dendrites  never  extend  far  from  their  cell-body,  but  bifurcate 
repeatedly  in  the  immediate  neighborhood  of  the  cell.  In  both 
these  respects  they  differ  from  the  typical  axon,  which  is  long, 
narrow,  cylindrical,  and  in  general  branches  but  little.  Where  it 
does  divide,  the  branch  comes  off  at  right  angles  to  the  trunk  of 


FIG.  50.— Ceil   with   Short  and    Much    Branched 
Axon.  (Van  Gehuchten.)    pr.  cyL,  the  axon. 


INTERNAL  STRUCTURE  OF  NERVE-CELLS 


105 


FIG.  51.— A  Nerve-Cell,  Stained 
by  the  Nissl  Method,  and 
Highly  Magnified.  (Ewing.) 
The  blue  of  the  stain  is  here 
represented  by  black. 


the  axon,  and  is  called  a  collateral.  The 
axon  is  peculiar  also  in  that  it  passes  into 
the  white  matter,  and  acquires  a  myelin 
sheath.  It  often  extends,  in  the  white 
matter  and  in  the  peripheral  nerves,  to  a 
length  of  several  feet.  But  there  is  another 
type  of  axon  (compare  Fig.  50),  which  is 
short,  and  branches  abundantly  near  the 
cell-body;  it  can  still  be  distinguished  from 
the  dendrites  by  its  uniform  slenderness, 
and  by  the  rectangular  character  of  its 
branching. 

§  7.  The  inner  structure  of  the  cell-body 
shows  a  nucleus  with  a  minute  nucleolus 
within  the  nucleus.  When  the  Nissl  stain 
is  employed  (compare  Fig.  51),  there  are 
seen  many  large  granules,  often  of  spin- 
dle shape,  with  spaces  between  them. 
These  "Nissl  bodies"  are  present  in  the  dendrites  as  well  as  in 
the  cell-body,  but  are  absent  from  the  axon  and  from  the  conical 
projection  of  the  cell  from  which  the  axon  arises.  It  is  not  easy 
to  make  sure  that  such  appearances  as  these  represent  structures 
which  exist  in  the  living  cell;  for  the  chemical  treatment  through 
which  the  cell  must  pass  in  order  to  show  details  may  cause  coagu- 
lations and  other  changes  of  the  substances  within  the  cell. 
What  is  certain  is  that  the  cells  contain  a  particular  substance 
wThich  has  an  affinity  for  basic  dyes;  and  that  the  same  sort  of 
nerve-cell  always  gives  the  same  sort  of  picture 
when  it  is  treated  by  the  Nissl  method,  if  it  has 
come  from  an  animal  in  normal  condition.  But 
there  are  many  conditions,  more  or  less  abnor- 
mal, which  cause  the  cells  to  present  quite  a  dif- 
ferent picture  after  staining.  Thus  poisoning 
with  lead,  mercury,  arsenic,  alcohol,  strychnine,, 
and  many  other  poisons,  asphyxia,  or  excessive 
activity  and  fatigue,  causes  a  diffusion  of  the 
stainable  substance  throughout  the  cell-body — 
a  condition  known  as  chromatolysis  (see  Fig.  52). 
Further  action  of  the  poison  may  cause  a  com- 
plete disappearance  of  the  stainable  substance. 
The  results  suggest,  but  do  not  fully  prove,  that 
FIG  52  — Chromatoiy-  *m*s  substance  is  of  the  nature  of  stored  food  or 
sis.  (Ewing.)  TO  fuel,  which  the  cell  utilizes  in  its  activity,  and 

be    compared    with 


Fig.  51. 


uses  up  in  excessive  activity. 


106 


ELEMENTS  OF  THE  NERVOUS  STRUCTURE 


§  8.  Methods  of  staining  nerve-cells  have  also  been  introduced1 
which  show  numerous  fibrils  in  the  cell-body  and  dendrites,  as  well 
as  in  the  axon  (see  Fig.  53).  Small  bundles  of  fibrils  from  each 
dendrite  enter  and  pass  through  the  cell-body  into  other  dendrites 
or  into  the  axon.  Fibrils  thus  pass  from  each  branch  of  the  cell 
into  nearly  or  quite  every  other  branch.  In  the  cells  of  the  verte- 
brate brain  and  cord,  the  fibrils 
seem  to  maintain  their  individu- 
ality through  the  cell,  not  anas- 
tomosing with  each  other.2 

In  the  spinal  ganglion  cells, 
however,  the  fibrils  unite  with 
each  other  into  a  network  or 
latticework;  and  the  same  is 
true  of  the  cells  in  the  nerve- 
centres  of  invertebrates,  as  de- 
monstrated by  Apathy. 

Occasionally  a  deposit  of  pig- 
ment is  found  in  nerve-cells ;  the 
amount  of  it  increases  with  the 
age  of  the  individual.  The  sig- 
nificance of  the  pigment  is  other- 
wise unknown. 

§  9.  The  arrangement  of  cells, 
dendrites,  and  terminations  of 
axons  in  the  gray  matter  is  of 
no  less  importance  than  the 
structure  of  the  single  cell.  In 
general,  it  may  be  said  that  the 
interweaving  of  branches  from 
different  cells  is  very  dense  and 
intricate.  The  number  of  cells 
is  great,  even  in  a  small  ganglion 
or  nucleus;  the  number  of  axons 
entering  and  terminating  is  also  great;  and  the  relations  of  the 
axons  to  the  cells  near  which  they  terminate  is  not  by  any  means 
easy  to  make  out.  It  is  fairly  certain  that  axons,  on  entering  a 
mass  of  gray  matter,  come  into  definite  functional  relations  with 
the  cells  located  there,  or  with  their  dendrites;  but  the  exact 
mode  of  connection  is  often  obscure.  In  certain  localities,  how- 
ever, the  relations  are  clear  as  to  certain  facts. 

§  10.  The  connections  of  the  fibres  of  the  olfactory  nerve,  for 
example,  are  specially  clear.     These  fibres  arise  from  sensory  cells 
1  By  Bethe,  Bielchowsky,  Cajal.  2  Bethe,  op.  cit.,  pp.  56-60. 


Fro.  53.— Nerve-Cell  Stained  for  Fibrils. 
(Bethe.)  a,  b,  c,  d,  the  stumps  of  several 
dendrites;  Ax,  stump  of  the  axon. 


STRUCTURE  AND  OFFICE  OF  THE  DENDRITES    107 

in  the  mucous  membrane  of  the  nose,  and,  passing  through  the  bone 
into  the  brain  cavity,  enter  the  olfactory  bulb,  where  each  axon 
breaks  up  into  a  little  bush  of  branches.  Interlacing  with  these 
are  the  branches  of  a  dendrite  which  belongs  to  a  cell  of  the  olfac- 
tory bulb.  This  dendrite,  extending  outward  from  its  cell-body, 
meets  the  axon  coming  in  from  the  nose,  and  the  two  break  up 


FIG.  54.— Diagram  to  Show  the  Connection  of  Axons  and  Dendrites  in  the  Olfactory  Bulb. 
(Schafer,  from  Quain's  Anatomy,  by  permission  of  Longmans,  Green  &  Co.)  olf.c.,  olfac- 
tory cells  in  the  nasal  mucous  membrane;  olf.n.,  the  olfactory  nerve,  consisting  of  axons 
from  the  cells  just  mentioned;  gl.,  "glomeruli,"  in  which  the  terminal  branches  of  these 
axons  are  interwoven  with  dendrites  of  cells  of  the  olfactory  bulb;  m.c.,  these  cells;  a, 
their  axons,  passing  further  into  the  brain. 

together  into  a  mass  of  interlacing  branches  (see  Fig.  54).  On  the 
other  side,  the  axon  of  the  cell  in  the  olfactory  bulb  runs  back  to 
other  parts  of  the  brain. 

Now  it  is  certain  that  the  line  of  communication  must,  in  this 
case,  lead  from  the  nose  to  the  brain.  It  enters  the  brain  by  the 
fibres  of  the  olfactory  nerve;  it  must  therefore  pass  from  these 
fibres  to  the  structures  into  which  they  enter  into  relation;  and 
these  structures  are  the  dendrites  of  the  cells  of  the  olfactory  bulb. 
The  case  thus  shows  communication  from  the  axon  of  one  cell 
to  the  dendrites  of  another,  and  from  these  dendrites  to  their  cell- 
body  and  its  axon.  The  dendrites  must,  therefore,  be  the  receptive 
part  of  the  cell,  i.  e.,  the  part  which  receives  the  nervous  influences 
or  impulses  from  other  more  peripheral  parts  of  the  nervous 


108         ELEMENTS  OF  THE  NERVOUS  STRUCTURE 

system.  What  is  certain  in  this  case,  is  probable  enough  in  many 
other  parts  of  the  gray  matter,  in  which  sensory  axons  terminate; 
for  example,  in  the  case  of  the  optic  path.  There  are  always 
dendrites  present  which  may  be  the  recipients  of  the  sensory  im- 
pulses. Moreover,  there  are  no  anatomical  peculiarities  of  any 
part  of  the  gray  matter  which  would  discredit  the  general  concep- 
tion of  the  dendrites  as  receptive  organs;  or  which  would  make 


Fio.  55. — Baskets  of  Axon-branches  Around  Nerve-cells.  (Veratti,  Edinger.)  The  spheri- 
cal bodies  are  nerve-cells,  the  branches  of  which  do  not  show  in  the  figure;  but  the  cell- 
bodies  are  seen  to  be  closely  enveloped  by  "baskets"  of  fibres  which  result  from  the 
splitting  up  of  axons  from  other,  distant  cells. 

more  probable  any  other  function  for  them.  That  they  have  this 
receptive  function  may,  therefore,  be  taken  as  a  highly  probable 
and  generally  accepted  view. 

There  are,  however,  indications  that  impulses  are  sometimes 
received  directly  at  the  surface  of  the  cell-body,  as  well  as  through 
the  dendrites.  In  certain  cases  the  terminal  branchings  of  an  axon 
are  closely  applied  to  the  body  of  another  cell.  The  cells  of  the 
"  trapezium,"  a  portion  of  the  bulb  and  pons  which  is  closely  con- 
nected with  the  auditory  nerve,  show  examples  of  this  (compare 
Fig.  55).  Here  the  structure  strongly  suggests  that  the  axon  in- 
fluences the  cell-body  directly. 

§  11.  The  cerebellum  affords  interesting  examples  of  various 
forms  of  communication  between  one  cell  and  another.  The  cor- 
tex of  this  organ  contains  several  varieties  of  nerve-cells,  such  as 


NERVE-CELLS   OF  THE  CEREBELLUM 


109 


the  Purkinje  cells,  with  their  richly  branched  dendrites,  granule 
cells,  "  basket  cells,"  and  cells  with  short  and  much  branched 
axons.  Of  all  these,  the  Purkinje  cells  are  those  which  send  axons 
away  from  the  cortex  of  the  cerebellum;  apparently,  therefore,  the 
influence  of  the  cerebellum  on  the  other  parts  of  the  nervous  system 
is  exerted  through  these  axons  of  the  Purkinje  cells.  Accordingly, 
the  different  nerve  impulses 
which  may  be  present  in 
the  cerebellar  cortex  must 
be  concentrated  on  the 
Purkinje  cells. 

There  are  fibres  enter- 
ing this  cortex  from  other 
parts  of  the  nervous  system, 
and  some  of  these,  called 
"  climbing  fibres,"  grow  up 
the  dendrites  of  the  cells 
of  Purkinje,  like  a  vine 
up  a  tree;  other  incoming 
fibres,  however,  do  not 
come  into  direct  relation 
with  the  Purkinje  cells  at 
all,  but  end  in  peculiar, 
mossy  terminations  in  the 
neighborhood  of  the  little 
granule  cells.  The  axons 
of  these  latter  cells  then 
pass  upward  to  the  level  of  the  Purkinje  dendrites,  and  extend 
in  great  numbers  through,  or  between,  the  branches  of  these 
dendrites.  The  "basket  cells"  lie  in  the  same  region  as  the 
Purkinje  dendrites;  but  their  axons  divide  into  several  branches 
each  of  which  splits  up  into  a  basket-like  arrangement  of  fine 
branches  around  the  cell-body  of  a  cell  of  Purkinje.  Thus  one 
basket  cell  appears  "  to  hold  the  reins  on "  several  cells  of  Purkinje. 
The  cells  with  much  branched  axons  seem  to  spread  influences, 
similarly,  over  many  granule  cells.  We  certainly  are  far  from  a 
full  comprehension  of  these  intricate  relations  of  cells;  though  the 
main  fact  that  the  cells  of  Purkinje  are  subject  to  a  combination  of 
influences  from  other  cells  is  clear;  and  it  is  also  highly  probable  that 
both  the  dendrites  and  the  cell-body  of  the  Purkinje  cell  are  recep- 
tive of  influences  from  the  axons  of  other  cells  (see  Figs.  56  and  57). 

§  12.  If  the  dendrites  are  receptive  in  function,  the  terminations 
of  the  axon  must  be  transmissive ;  i.  e.,  they  must  pass  on  the  nerve- 
impulse  to  the  dendrites  and  cell-bodies  of  other  cells.  There  is 


FIG.  56. — Fibre-baskets  Around  Purkinje  Cells. 
(Cajal.) 


110         ELEMENTS  OF  THE  NERVOUS  STRUCTURE 

much  evidence  to  favor  this  view,  and  little  opposed  to  it.  The 
nerve-cell  may  therefore  be  said  to  be  "  polarized " ;  since  one  end  of 
it  is  capable  of  taking  up  stimuli,  and  the  other  is  capable  of  giving 


FIG.  57. —  Diagram  of  the  Cells  of  the  Cerebellar  Cortex.  (Kolliker.)  gl,  neuroglia 
cell;  gr,  granule  cell;  p,  axon  of  a  Purkinje  cell;  /,  "  moss-fibre"  ;  k,  climbing  fibre,  and 
fc1,  its  termination;  ra,  small  nerve-cell;  n,  cell  with  short  and  much  branched  axon;  m1, 
basket  cell,  the  axon  of  which  branches  at  zk,  about  the  body  of  a  Purkinje  cell. 

off  stimuli.  In  case  of  the  sensory  fibres  which  have  no  dendrites 
— such  as  those  of  the  dorsal  roots  of  the  cord,  whose  cells  lie  in  the 
spinal  ganglia — the  peripheral  termination  of  the  axon  in  the  sense- 
organ  becomes  the  functional  equivalent  of  the  dendrites;  i.  e.,  it 
is  the  receptive  part.  In  the  usual  type  of  cell  in  the  centres,  the 
impulse  seems  to  enter  at  the  dendrites,  to  pass  thence  to  the  cell- 


STRUCTURE  AND  FUNCTION  OF  THE  AXON       111 

body  and  thence  into  the  axon,  and  out  at  the  terminal  arboriza- 
tions of  the  axon  and  its  collaterals. 

The  axon,  therefore,  usually  conducts  toward  its  own  branched 
ends.  Experiment  proves,  indeed,  that  the  axon  is  inherently 
capable  of  conducting  in  either  direction.  But  experiment  also 
seems  to  prove  that  conduction  from  an  axon  to  the  dendrites  of 
another  cell  can  occur  only  in  this  one,  and  not  in  the  reverse,  di- 
rection. Thus,  in  the  spinal  cord,  an  impulse  passing  in  by  the 


vr 


FIG.  58. — Diagram  of  a  Synapse  in  the  Cord,  dr,  fibre  of  the  dorsal  root,  passing  into  the 
ventral  horn,  and  connecting,  at  the  synapse,  with  a  motor  cell  from  which  arises  the 
fibre  of  the  ventral  root,  vr.  Stimulating  dr  arouses  vr  to  activity,  but  stimulation  of  vr 
does  not  arouse  dr. 

fibres  of  the  dorsal  roots  is  transmitted  over  to  the  motor  fibres  of 
the  ventral  roots;  but  an  impulse  artificially  generated  in  these 
motor  fibres,  and  conducted  back  into  the  cord,  does  not  make  its 
appearance  in  the  dorsal  roots.  Now  we  have  already  seen  (p.  89) 
that  the  connection  between  the  dorsal  and  the  ventral  fibres  lies  in 
the  gray  matter  of  the  ventral  horn;  it  appears,  therefore,  to  be 
a  connection  between  the  terminations  of  the  incoming  sensory 
axons  and  the  dendrites  of  the  large  cells  of  the  ventral  horn.  If 
this  appearance  is  correct,  the  above  experimental  result  may  be 
restated  in  the  following  terms:  Nerve-impulses  are  conducted  from 
the  terminations  of  the  sensory  axons  to  the  dendrites  of  the  motor 
cells,  but  will  not  pass  in  the  reverse  direction.  Thus  the  connection 
between  axonic  terminations  and  dendrites  acts  as  a  sort  of  valve, 
allowing  nerve-impulses  to  pass  in  only  one  direction. 

The  above-mentioned  and  similar  facts — such  as  that  conduction 
is  slower  in  the  gray  matter  than  along  the  axons  of  the  nerves, 
and  more  liable  to  interruption  by  the  action  of  drugs,  etc. — have 


112         ELEMENTS  OF  THE  NERVOUS  STRUCTURE 

led  physiologists  to  the  conception  of  a  certain  looseness  of  connec- 
tion within  the  gray  matter,  or  lack  of  such  complete  continuity  as 
obtains  between  the  parts  of  the  axon.  They  have  accordingly  given 
a  special  name  to  the  connection  between  the  terminations  of  an 
axon  and  the  dendrites  of  another  cell.  This  name,  synapse  (com- 
pare Fig.  58),  signifies  a  "fitting"  together,  as  distinguished  from 
a  "growing"  together.  In  this  way,  axon  and  dendrites  are  con- 
ceived of  as  dovetailed  together; — perhaps  very  snugly,  but  with- 
out such  complete  continuity  as  obtains  between  an  axon  and  its 
cell-body,  or  between  the  cell-body  and  its  dendrites.  A  cell-body, 
with  the  axon  and  dendrites  especially  belonging  to  it,  is  thus  thought 
of  as  a  continuous  whole,  within  which  conduction  is  easy  and  can 
occur  in  any  direction;  whereas  a  certain  degree  of  discontinuity 
or  separation  is  conceived  to  exist  between  one  cell  and  another. 
When  regarded  in  this  way,  each  cell  with  its  branches  is  called  a 
neurone;  and  the  doctrine  that  some  degree  of  discontinuity  exists 
between  the  neurones  is  called  the  neurone  theory. 

§  13.  The  history  of  the  neurone  theory  is  interesting  and  sug- 
gestive as  to  the  difficulties  and  the  nature  of  the  conclusions  in 
this  entire  field  of  inquiry.  The  first  clearly  defined  theory  re- 
garding the  connections  existing  within  the  gray  matter  was  that 
of  Gerlach,  who,  in  1870,  concluded  from  the  knowledge  then  in 
hand  that  the  dendrites  and  other  branches  of  the  nerve-cells 
united  with  one  another  into  a  dense  and  continuous  network 
throughout  the  gray  matter.  This  "nerve-net"  conception  held 
the  field  for  about  twenty  years.  Meanwhile  the  use  of  the  Golgi 
method  of  staining,  especially  in  the  hands  of  the  Spanish  investi- 
gator, S.  Ram6n  y  Cajal,  gave  pictures  of  individual  cells  with 
numerous  branches,  but  with  no  indication  of  anastomoses  between 
cells,  or  of  a  continuous  network  of  branches.  On  the  contrary, 
each  cell,  with  its  branches,  appeared  separate  from  every  other 
cell.  This  anatomical,  or  histological,  result  was  quite  in  accord 
with  the  embryological  observations  of  His, — namely  (see  above, 
p.  41),  that  the  gray  matter  starts  as  a  collection  of  separate  cells, 
out  of  which  the  branches  grow.  Summing  up  the  anatomical  and 
embryological  evidence,  Waldeyer,  in  1891,  formulated  the  theory 
that  nerve-cells,  however  much  they  might  branch,  remained,  as 
they  had  begun,  separate  units  which  he  named  "  neurones."  The 
rapidity  with  which  this  new  conception  won  its  way  among  all 
classes  of  students  is  notable  in  scientific  history.  To  the  physiolo- 
gists it  was  welcome  as  affording  an  explanation  of  the  slow  con- 
duction and  valve-like  action  of  the  connections  within  the  gray 
substance  of  the  nervous  system.  To  the  pathologist,  it  was  also 
welcome  as  affording  an  explanation  of  the  peculiar  fact  that  de- 


THE  NEURONE  THEORY  113 

generation  of  nervous  tissue  resulting  from  injury  extends  only  to 
the  terminations  of  the  axons  injured;  and  does  not  spread  freely 
over  to  other  cells.  Even  the  psychologist  found  the  neurone  theory 
useful,  since  learning  and  association  could  now  be  understood  as 
dependent  on  the  formation  of  new  synapses;  while  sleep  and  un- 
consciousness might  very  well  be  due  to  slight  influences  obstruct- 
ing the  synapse.  Many  other  mental  facts  received  help  from  this 
theory,  in  respect  of  their  psycho-physical  interpretation.  From 
about  1891,  accordingly,  the  neurone  theory  attained  great  vogue 
and  came  into  well-nigh  universal  acceptation. 

There  was  always,  however,  a  minority  of  able  neurologists  who 
did  not  fully  accept  the  neurone  theory,  and  who  were  stimulated 
by  it  to  look  more  carefully  for  direct  connections  between  the  cells 
of  the  gray  matter.  Protoplasmic  bridges  were  found  to  exist  in 
the  nervous  systems  of  very  low  orders  of  invertebrates  (compare 
p.  18);  but  in  the  higher  invertebrates  and  in  vertebrates,  such 
bridges  were  not  found;  and  all  now  admit  that  they  do  not  exist. 
The  centre  of  discussion  was,  accordingly,  shifted  with  the  new  dis- 
coveries regarding  the  inner  structure  of  the  nerve-cell  and  its 
branches  (see  p.  106).  Since  the  fibrils,  from  their  appearance, 
are  probably  the  real  conductors  of  nerve-impulses,  continuity 
between  two  cells  might  be  established  if  simply  the  fibrils,  rather 
than  larger  protoplasmic  branches  such  as  the  dendrites,  could  be 
traced  as  emerging  from  one  cell  and  entering  into  another  cell. 
In  some  invertebrates,  a  passage  of  fibrils  from  cell  to  cell  was  an- 
nounced by  Apathy;  but  this  alleged  discovery  has  not  been  con- 
firmed by  some  of  the  competent  authorities  who  have  followed 
Apathy's  methods. 

In  the  exceedingly  intricate  gray  matter  of  vertebrates,  the  task 
of  tracing  such  minute  structures  as  the  nerve-fibrils  from  one  cell  to 
another  would  at  best  be  extremely  difficult;  and  for  this  reason, 
no  strong  disproof  can  be  based  on  failure  to  trace  them.  In  point 
of  fact,  however,  the  evidence  for  such  passage  of  fibrils  from  one 
cell  to  another  in  the  gray  matter  is,  up  to  the  present,  slight  and 
dubious.  There  is,  indeed,  an  appearance  observed  on,  or  near, 
the  surface  of  many  cell-bodies,  which  has  been  called,  from  its 
discoverer,  the  "Golgi  net,"  and  which  consists  of  fine  fibrils;  and 
some  observers,  as  Bethe,1  have  found  some  evidence  of  fibrils 
issuing  from  within  the  cell  and  joining  the  Golgi  net.  On  the 
other  hand,  this  net  has  seemed  to  be  in  continuity  with  axons  ap- 
proaching the  cell  from  elsewhere.  These  observations  seem  to 
show  a  possible  path  of  fibrillar  connection  between  the  nerve-cells. 

1  Allgemeine  Anatomic  und  Physiologic  des  Nervensy  stems,  pp.  65-78  (Leipzig, 
1903). 


114         ELEMENTS  OF  THE  NERVOUS  STRUCTURE 

The  full  meaning  of  the  fibrillar  theory  may  be  summed  up  in 
this:  that  the  fibrils  are  the  real  units  of  the  nervous  system.  The 
cells,  which  are  the  units  according  to  the  neurone  theory,  are  to  the 
fibrillar  theory  merely  the  medium  in  which  the  fibrils  grow  and 
by  which  they  are  nourished.  The  fibrils  issue  freely  from  the  cells 
and  perhaps  form  a  fibril  network  in  the  spaces  between  them. 
This  network,  according  to  the  theory,  would  therefore  be  the  es- 
sential structure  of  the  gray  matter;  it  would  contain  the  connections 
between  incoming  and  outgoing  axons.  Fibrils  would  enter  the 
network  from  axons  coming  into  the  gray  matter;  and  fibrils  from 
the  network  would  enter  the  dendrites  and  cell-bodies,  and  then 
pass — at  least  some  of  them — into  the  axons  of  these  cell-bodies 
and  so  away  to  other  parts. 

As  between  the  neurone  theory  and  the  fibrillar  theory,  it  is  im- 
possible at  present  to  decide  with  certainty;  but  there  is  no  doubt 
that  the  neurone  theory  still  commands  the  support  of  the  majority 
of  authorities;  and  that  it  serves,  for  the  present,  the  useful  purpose 
of  summing  up  a  large  proportion  of  the  known  facts  that  have  a 
bearing  on  the  connections  within  the  gray  matter  of  the  nervous 
system. 

§  14.  It  has  already  been  said  (p.  40)  that  a  considerable  part 
of  the  central  nervous  system  consists  of  a  substance  which  is  char- 
acteristically different  from  either  of  the  definitely  recognized  nerve 
elements,  and  to  which  the  name  of  "neuroglia"  has  been  given. 
This  name  was  originally  designed  to  embody  the  opinion  that  it 
acted  as  a  kind  of  "  nerve-cement "  (nerven-kiti);  and  neuroglia  has 
frequently  been  classified  with  the  connective  tissue.  But  as  long 
ago  as  1880,  Henle  said  of  this  substance:  "It  is  at  all  events  to 
be  distinguished  from  connective  tissue  on  account  of  its  chemical 
properties." l  Microscopic  examination  shows  that  the  neuroglia 
is  by  no  means  a  homogeneous  mass,  but  is  composed  of  innumer- 
able minute  cells  which  differ  in  their  physical  characteristics  from 
the  typical  nerve-cells.  The  cell-body  is  small;2  the  branches  are, 
in  general,  of  tolerably  uniform  size,  and  at  the  very  edge  the  fibres 
end  in  minute  balls  or  knobs.  The  method  of  the  branching  of 
the  neuroglia  cells,  when  any  branching  occurs,  enables  the  ob- 
server to  differentiate  them  from  the  dendrites  of  the  true  nerve- 
cell.  Thus  their  appearance  has  caused  some  of  them  to  be  called 
"spider  cells"  (see  Fig.  59). 

Neuroglia  cells  differ  both  in  size  and  in  distribution,  in  the  differ- 
ent parts  of  the  central  nervous  system.  They  are  most  numerous 

1  Anatomie  d.  Menschen,  text,  p.  306. 

2  Some  of  these  cells  are,  indeed,  exceedingly  minute,  being  scarcely  more  than 
fuVu  or  ssVff  of  an  inch  in  diameter. 


THE  NEUROGLIA  CELLS 


115 


where  there  are  fewest  nervous  elements — as  though  their  office  were 
to  fill  in  the  interstices,  and  to  constitute  the  background  or  soil  for 
holding  the  nerve-cells  and  nerve-fibres.  They  sometimes  arrange 
themselves  in  regular  order,  as,  for  example,  around  the  central 
commissural  region  of  the  cord.  The  distribution  of  their  branches 
is  fairly  uniform  in  the  middle  of  the  cord,  but  changes  at  the  edge, 
where  the  massing  of  the  fibres  constitutes  the  outer  wall  of  the  cord 
and  the  posterior  fissure. 

As  respects  their  origin,  these  spider  cells  seem  to  be  ectodermal 
and  not  connective  tissue  cells.     They  therefore  arise  from  the  same 


FIG.  59.— A  Spider  Cell  from  the  Spinal  Cord.     (V.  LenhossSk.) 

embryological  elements  as  the  nerve-cells.  This,  however,  does 
not  prove  that  they  are,  functionally  considered,  true  nerve-cells; 
for  cells  from  the  same  source  may  have  altogether  different  func- 
tions in  their  developed  form.  In  general,  the  supporting  cells  of 
the  nervous  substance  are  older,  ontogenetically,  than  the  true 
nervous  elements.  They  begin  in  primitive  cells,  which  are  ar- 
ranged radially  around  the  cord  centre.  But  these  primitive  cells 
gradually  undergo  modifications  of  shape,  before  their  place  ap- 
pears to  be  taken  by  the  spider  cells.  On  the  other  hand,  some  of 
the  spider  cells  seem  to  come  directly  from  the  ectodermal  cells,  with- 
out passing  through  the  intermediate  cell  period. 


116         ELEMENTS  OF  THE  NERVOUS  STRUCTURE 

It  thus  appears  that  we  cannot  as  yet  be  perfectly  certain  as  to 
what  is  the  whole  functional  relation  of  the  neuroglia  to  the  acknowl- 
edged elements  of  the  nervous  system.  And,  indeed,  although  we 
now  know  much  more  than  when  these  words  were  written,  it  still 
remains  true,  as  Eckhard1  said  years  ago:  "If  we  start  the  inquiry, 
what  formal  elements  of  the  brain  and  cord  take  part  in  the  activi- 
ties of  these  organs,  and  in  what  way  they  do  take  part,  we  are 
able  to  give  to  it  only  a  very  unsatisfactory  answer."  2 

1  Hermann,  Handbuch  d.  Physiologic,  II,  ii,  p.  15. 

2  The  modern  view  of  the  nature  of  neuroglia  was  first  hinted  at  by  Kolliker 
(Handb.  d.  Gewebelehre,  4th  Aufl.,  1862,  pp.  304  ff.);  it  was  worked  out  more  fully 
by  Golgi  in  contributions  to  the  histology  of  the  central  nervous  system,  published 
in  the  Rivista  clinica  di  Bologna,  during  1871-1872.     Further  important  contri- 
butions to  our  knowledge  were  made  by  F.  Boll  ("  Die  Histologie  u.  Histiogenese 
d.  nervosen  Centralorgane,"  Archiv.  /.  Psychiatric  u.  Nervenkrankh.,  Bd.  IV,  p.  1); 
by  H.  Gieke  ("Die  Stiitzsubstanz  d.  Centralsystems,"  Archiv.  f.  mikr.  Anatomie, 
Bd.  XXV,  pp.  441  ff.,  and  Bd.  XXVI,  pp.  129  ff.);  and  by  W.  Vigual  ("Sur  le 
deVelloppement  des  elements  de  la  moelle  des  mammiferes,"  Archives  de  PhysioL 
normale  et  pathol.,  I,  1884,  pp.  230  ff.).    The  conclusions  of  these  investigators, 
which  were  at  first  rather  vague,  have  subsequently  been  on  the  whole  con- 
firmed by  His,  Cajal,  and  other  later  workers. 


CHAPTER  V 
CHEMISTRY  OF  THE  NERVOUS  SYSTEM 

§  1.  It  is  conceivable  that,  in  the  progress  of  the  sciences  concerned, 
the  action  of  the  nervous  substance  should  be  brought  into  such  re- 
lations with  its  chemistry  and  physics,  that  nervous  functions  might 
be  stated  very  largely  in  terms  of  physical  and  chemical  processes. 
At  present  so  engaging  a  prospect  seems  indefinitely  remote;  partly 
because  of  our  meagre  knowledge  of  the  chemistry  of  the  nervous 
tissues,  and  partly  because  of  insufficient  knowledge  regarding  the 
intimate  functions  of  the  nervous  elements.  Much  the  same  thing 
is  true,  indeed,  of  the  muscles  and  the  glands :  the  known  functions 
of  these  organs,  too,  cannot  as  yet  be  explained  in  terms  of  physics 
and  chemistry.  In  regard  to  nervous  tissue,  we  know  fairly  well 
the  chemical  elements  which  enter  into  its  composition;  and  we 
know  some  of  the  compounds  which  exist  there.  In  general,  the 
same  chemical  elements  are  represented  in  nervous  as  in  other 
tissues,  though  in  somewhat  different  proportions.  Also,  the  same 
general  classes  of  compounds  occur  in  nervous  and  in  other  tissues. 
In  respect  neither  of  their  elements  nor  of  their  compounds,  there- 
fore, do  the  different  tissues  present  such  striking  differences  as 
might  be  expected  from  their  various  functions. 

§  2.  The  general  chemistry  of  the  brain  and  nerves  is  still  in  an 
undeveloped  condition.  Apparently,  a  great  number  of  somewhat 
similar  compounds  exist  in  the  brain,  the  isolation  and  analysis  of 
which  is  attended  with  great  difficulty.  These  difficulties  are  not 
due  simply  to  the  complex  constitution  of  most  of  the  substances 
with  which  we  have  to  deal.  They  are  also  very  largely  due  to  the 
fact  that  these  substances  are  products  of  life;  and  living  tissue  can- 
not be  at  the  same  time  kept  in  normal  condition  and  subjected  to 
the  handling  necessary  for  chemical  analysis.  As  soon  as  it  is  no 
longer  alive,  or  at  any  rate  long  before  any  chemical  analysis  can  be 
completed,  the  constitution  of  such  tissue  is  changed.  However 
carefully  the  chemical  elements,  the  constituents,  which  enter  into 
the  nervous  substance  may  be  preserved,  their  constitution,  their 
chemical  arrangement  and  behavior,  cannot  be  preserved.  It  is 
impossible — for  example — for  the  chemist  even  to  determine  the 
specific  gravity  of  uncoagulated  blood,  "except  by  operating  with 
extreme  expedition  and  at  temperatures  below  0°  C." 

117 


118  CHEMISTRY  OF  THE  NERVOUS  SYSTEM 

The  specific  gravity  of  the  white  matter  is  somewhat  greater  than 
that  of  the  gray.  Danilewski  found  the  specific  gravity  of  the  gray 
matter  in  the  human  brain  to  vary  from  1.029  to  1.039;  that  of  the 
white  matter  from  1.039  to  1.043.1  Other  authors  have  assigned 
slightly  lower  values  for  both.  This  difference  in  the  weight  of  the 
white  and  the  gray  matter  is  chiefly  due  to  difference  in  the  relative 
amounts  of  water  and  of  solids  which  they  respectively  contain. 
Determinations  by  several  authors2  agree  in  assigning  about  86 
per  cent,  of  water  and  14  per  cent,  of  solids  to  the  gray  matter  (cere- 
bral cortex);  whereas  the  white  matter  shows  only  from  70  to  71 
per  cent,  of  water.  The  water-content  of  the  peripheral  nerves  is 
still  less,  ranging  from  40  to  70  per  cent. 

§  3.  The  chemical  elements  chiefly  present  in  the  brain,  as  in  all 
living  cells  and  tissues,  are  carbon,  hydrogen,  oxygen,  nitrogen  and 
phosphorus,  chlorine,  sodium,  and  potassium. 

Of  chemical  compounds,  the  brain,  like  all  living  cells,  contains 
water,  salts  (such  as  sodium  chloride  and  potassium  phosphate), 
proteids,  and  lipoids. 

The  proteids  are  highly  complex  organic  compounds  of  carbon, 
hydrogen,  oxygen,  and  nitrogen;  they  form  a  large  proportion  of 
the  substance  of  all  living  cells,  whether  animal  or  vegetable,  and  ap- 
pear to  play  a  very  important  part  in  the  life  of  the  cells.  They 
have,  indeed,  been  regarded  as  the  essentially  living  parts  of  the 
cells;  but  it  is  probable  that  the  role  of  the  salts  held  in  solution, 
and  of  the  lipoids,  is  no  less  essential  to  life. 

Of  the  solids  contained  in  the  nervous  centres,  more  than  one-half 
in  the  gray,  and  about  one-third  in  the  white,  consist  of  these  pro- 
teid  substances.  Four  such  bodies  are  now  generally  recognized; 
of  which  the  most  abundant  is  a  nucleo-proteid  containing  phos- 
phorus, which  is  soluble  in  water;  it  is  believed  to  come  in  large 
measure  from  the  nuclei  of  the  cells,  and  is  analogous  to  the  nucleo- 
proteids  derived  from  all  other  cellular  tissues.  Two  other  pro- 
teids found  in  the  brain  substance  are  globulins,  differing  in  their 
solubilities  and  in  the  temperatures  at  which  they  coagulate  (47° 
C.  and  70°  C.  respectively);  they  are  also  quite  similar  to  the  globu- 
lins derived  from  all  other  cellular  tissues,  such  as  muscle,  liver,  and 
kidney.3  It  will  be  noticed  that  all  these  three  proteids  have  been 
found  to  be  soluble  and  coagulable  at  varying  temperatures. 

1  Med.  CentralbL,  xviii,  p.  241. 

2  Cited  by  S.  Frankel,  in  his  "Gehirn-Chemie,"  in  Asher  and  Spiro's  Ergeb- 
nisse  der  Physiologic,  1909,  VIII,  p.  217.     This  work  has  been  followed  largely  in 
the  statements  made  in  the  following  pages  with  regard  to  the  chemistry  of  the 
nervous  substance  of  the  brain. 

3  Bernhardt;  also  Halliburton,  Journal  of  Physiology,  1893,  XV,  90;  Chemistry 
of  Muscle  and  Nerve,  1904,  p.  61. 


LIPOIDS  OF  THE  NERVOUS  SUBSTANCE  119 

Besides  these  three  soluble  proteids,  the  brain  yields  a  small  pro- 
portion of  an  insoluble  proteid,  called  neurokeratin,1  which  is 
similar  in  its  properties  to  the  keratin  that  forms  so  large  a  pro- 
portion of  the  hair,  horn,  nails,  and  other  like  appendages  of  the  skin. 
This  substance  is  found  both  in  the  medullated  nerves  and  in  the 
central  organs — according  to  Kuhne  in  both  gray  and  white  matter, 
but  according  to  W.  Koch2  only  in  the  white.  As  the  researches 
of  Kiihne  and  his  pupils  showed  some  years  ago,  neurokeratin  oc- 
curs also  in  the  retina.  It  contains  carbon,  hydrogen,  nitrogen,  and 
sulphur,  besides  inorganic  constituents. 

§  4.  It  is  the  lipoids  which  are  likely  to  be  of  most  interest  in 
their  relation  to  the  physiology  and  psycho-physics  of  the  brain. 
Indeed,  the  brain  is  chiefly  characterized,  chemically  considered, 
by  its  exceptionally  large  content  of  these  substances.  It  is  the 
white  matter,  however,  rather  than  the  gray,  which  is  peculiarly 
rich  in  them;  and  this  is  the  same  thing  as  saying  that  the  white 
matter  is  chemically  more  specialized  than  is  the  gray;  while  the 
latter  seems  to  differ  but  little  from  the  substance  of  other  living 
cells.  Since  the  myelin  sheath  of  the  nerve-fibres  is  the  distin- 
guishing feature  of  the  white  nervous  matter,  it  appears  probable 
that  the  lipoids  are  constituents  of  the  myelin  sheath;  this,  indeed, 
may  be  regarded  as  certain;  and  it  is  likely  also  that  a  large  share  of 
the  lipoid  content  of  gray  matter  is  due  to  the  fine  myelinated 
fibres  which  penetrate  it.  In  a  word,  the  chemistry  of  the  brain, 
so  far  as  it  presents  anything  peculiar,  appears  for  the  most  part 
to  be  the  chemistry  of  the  myelin  sheath.  And  the  chemistry  of 
the  myelin  sheath  is  for  the  most  part  the  study  of  its  "  lipoids." 

§  5.  The  term  "lipoid"  means,  etymologically,  a  "fat-like  body"; 
it  covers  a  considerable  number  of  organic  compounds  which  are 
not  exactly  fat,  but  which  resemble  fat  in  some  of  its  physical  and 
chemical  properties.  To  understand  the  use  of  the  term,  it  is  neces- 
sary to  consider  briefly  the  chemistry  of  living  cells  in  general. 

As  we  have  already  seen  (p.  14),  one  of  the  most  essential  prop- 
erties of  a  living  cell  is  its  power  of  taking  in  and  letting  out  dis- 
solved substances  through  its  surface;  while,  nevertheless,  not  al- 
lowing the  passage  of  such  substances  to  be  perfectly  free.  This 
we  have  called  "the  selective,"  or  "preferential,"  power  of  the  living 
cell.  Such  a  product  could  not  live  without  exchange  of  substances 
with  the  medium  in  which  it  lives;  but,  at  the  same  time,  it  could 
not  live  if  the  diffusion  of  these  substances  were  perfectly  free,  for 
then  it  would  soon  differ  in  no  respect  from  its  surrounding  medium. 

1  Ewald  and  Kiihne,  Verhandlungen  der  nat.-hist.-med.  Vereins,  N.  F.  I,  357 
(Heidelberg,  1877). 

2  Zeitschrift  fur  physiologische  Chemie,  1902,  XXXVI,  p.  134. 


120  CHEMISTRY  OF  THE  NERVOUS  SYSTEM 

The  individuality  of  the  cell  depends  on  its  power  of  resisting  free 
osmosis  through  its  surface.  Its  surface  is  not  freely  permeable, 
but  "semi-permeable."  In  many  cells,  accordingly,  the  outer  sur- 
face consists  of  a  more  or  less  definite  membrane,  and  the  necessary 
"semi-permeability"  is  very  likely  a  property  of  this  membrane. 
Certain  chemical  agents  dissolve  or  otherwise  alter  the  membrane; 
among  these  are  the  cell-narcotics,  ether,  chloroform,  benzol,  and 
others.  From  this  it  is  inferred  that  substances  which  are  dis- 
solved out  of  the  cell  by  the  application  of  ether,  etc.,  have  special 
importance  in  the  membrane  of  the  living  cell,  and  probably  have 
much  to  do  with  the  resistance  offered  by  the  cell  to  free  osmosis.1 

Now  since  substances  extracted  by  ether,  etc.,  include,  besides 
the  true  fats,  the  class  of  bodies  called  lipoids,  the  definition  of  the 
term  ("lipoid")  may  be  given  in  terms  of  this  property  of  being 
extracted  by  ether  and  other  cell-narcotics;  or,  more  biologically 
but  less  definitely,  in  terms  of  the  probable  function  of  these  bodies 
in  the  cell  membrane.  This  function  may  be  shared  by  the  fats 
proper. 

§  6.  The  brain  contains  a  large  amount  of  substances  which  can 
be  extracted  by  ether,  etc.,  but  which  are  not  fats.  It  is  probable, 
as  was  said  before,  that  these  substances  come  chiefly  from  the  myelin 
sheath,  which  may  perhaps  be  regarded  as  a  highly  developed  cell 
membrane.  Thudicum,2  whose  work  on  the  chemistry  of  the 
brain  is  very  elaborate  and  fundamental,  recognizes  three  well- 
defined  classes  of  brain  lipoids,  besides  others  less  well  understood. 
The  compounds  of  the  first  class  contain  both  nitrogen  and  phos- 
phorus; those  of  the  second  contain  nitrogen  but  no  phosphorus; 
and  those  of  the  third  class  contain  neither  of  these  elements,  but 
only  carbon,  hydrogen,  and  oxygen.  He  is  further  able  to  subdi- 
vide these  classes,  and  to  point  out  several  members  existing  within 
some  of  the  groups;  but  inasmuch  as  most  of  these  substances  are 
as  yet  imperfectly  defined,  and  as  great  difference  of  opinion  re- 
garding some  of  them  exists  among  chemical  authorities,  it  will 
not  be  worth  our  while  to  give  a  full  list  of  them  here;  we  may  con- 
fine our  attention  to  a  few  of  the  best-established. 

§  7.  Cholesterin  is  the  best  established  of  all  the  brain  lipoids ; 
it  belongs  to  the  third  of  the  classes  just  mentioned,  containing 
neither  phosphorus  nor  nitrogen.  Its  composition  corresponds 

1  See  Ivar  Bang,  "Biochemie  der  Zell-lipoide,"  in  Asher  and  Spiro's  Ergebnisse 
der  Physiologic,  1907,  VIII,  1  and  2  Abth.,  pp.  134  ff. 

a  "Researches  on  the  Chemical  Constitution  of  the  Brain,"  in  Reports  of  the 
Medical  Officer  of  the  Privy  Council  and  Local  Government  Board,  1874;  and 
Die  chemische  Konstitution  des  Gehirns  des  Menschen  und  der  Tiere  (Tubingen, 
1901). 


THE  PHOSPHORIZED  LIPOIDS  121 

closely  to  the  formula  C27H44O,  which  may  also  be  written,  as 
a  slight  indication  of  its  structure,  C27H48OH.  It  belongs  to 
the  alcohols,  being  the  hydroxide  of  a  hypothetical  hydrocarbon, 
C27H44,  which  has  not  yet  been  prepared.  It  is  a  "monatomic" 
alcohol,  having  one  OH  group  by  which  it  combines  with  acids, 
as  with  the  fatty  acids,  and  thus  forms  compounds  which  are  prob- 
ably present  in  the  brain.  Cholesterin  is  also  widely  present  in 
other  tissues,  and  indeed  in  all  tissues,  both  animal  and  vegetable; 
but  is  especially  abundant  in  nervous  tissue.  Cholesterin  is  a  solid 
at  body  temperature,  its  melting-point  being  147°  C.  Its  specific 
gravity  is  1.046.  It  is  insoluble  in  water,  though  a  mixture  of  it 
and  other  lipoids  is  soluble.  It  is  soluble  in  hot  alcohol,  in  ether, 
chloroform,  etc.,  and  crystallizes  out  of  these  solutions  in  the  form 
of  fine  needles  (out  of  ether)  or  of  rhombic  tables  (out  of  alcohol). 
It  is  electrically  a  non-conductor. 

To  the  class  of  lipoids  which  contain  nitrogen  but  no  phospho- 
rus are  to  be  reckoned  a  considerable  number  of  substances,  such 
as  cerebrin,  cerebron,  phrenosin,  and  kerasin.  Some  of  these 
differ  but  little  among  themselves,  according  to  the  analyses,  and 
may  even  be  identical,  if  allowance  be  made  for  accidental  impuri- 
ties.1 

§  8.  Most  interest,  however,  attaches  itself  to  the  class  of  phos- 
phorized  lipoids.  The  fact  that  the  brain  is  rich  in  phosphorus 
has  long  been  deemed  significant,  and  at  one  time  the  rather  crude 
dictum,  "No  thought  without  phosphorus,"  attained  considerable 
vogue.  But  on  the  one  hand,  there  are  many  other  elements  which 
are  essential  to  the  brain's  activity;  and,  on  the  other  hand,  phos- 
phorus is  present  in  all  living  cells.  Phosphorus  as  a  chemical 
element  of  the  nervous  substance  has,  therefore,  no  monopoly; 
and  the  statement,  "  No  life  without  phosphorus,"  would  more  ade- 
quately represent  the  facts  of  the  case.  The  phosphorus  of  the 
brain  is  for  the  most  part  contained  in  its  lipoids,  that  is  to  say,  in 
the  myelin  sheaths  of  its  nerve  fibres;  and,  instead  of  hoping  to 
discover  some  direct  connection  between  phosphorus  and  con- 
sciousness, we  should,  if  we  desire  to  follow  a  scientific  and  promis- 
ing course  of  investigation,  seek  first  for  the  part  played  by  phos- 
phorus-containing lipoids  in  the  myelin  sheath. 

While  a  large  share  of  the  lipoid  substances  which  have  been  ex- 
tracted from  the  brain  do,  without  doubt,  contain  phosphorus,  and 
while  a  considerable  number  of  different  compounds,  belonging 
to  this  class,  have  been  obtained,  analyzed,  and  named  by  chemists, 
the  difficulties  of  satisfactory  determination  are  here  so  great  that 

1  See  Posner  and  Gies,  Journal  of  Biological  Chemistry,  I,  59;  and  S.  Frankel, 
in  Asher  and  Spiro's  Ergebnisse  der  Physiologic,  1909,  VIII,  249. 


122  CHEMISTRY  OF  THE  NERVOUS  SYSTEM 

there  is  no  one  of  these  bodies  which  has  been  generally  accepted 
as  a  preformed  constituent  of  the  brain.  There  is  no  doubt  con- 
cerning some  of  the  decomposition  products  of  these  lipoids,  and 
none,  therefore,  regarding  some  essential  facts  of  their  structure; 
but  the  agreement  ceases  when  the  exact  composition  and  formula 
of  the  brain  constituents  is  in  question.  Controversy  between  au- 
thorities has  been  especially  bitter  in  this  field,  and  the  substances 
around  which  the  controversy  has  principally  centered  are  these  two: 
lecithin  and  protagon. 

§  9.  Lecithin  is  a  well-recognized  and  fairly  well-understood 
substance;  but  its  occurrence  preformed  in  the  living  brain  is  not 
established  beyond  dispute.  It  occurs  abundantly  in  the  yolk  of 
eggs,  and  is  known  also  to  occur  in  muscle  tissue;  it  probably  also 
occurs  quite  widely  in  cells  of  different  kinds.  The  formulae  as- 
signed for  it  by  different  authors  differ  somewhat,  possibly  because 
there  is  more  than  one  lecithin.  Thudicum's  formula  for  brain 
lecithin  is  C43H84NPO9.  It  is  described  as  a  phosphorized  fat, 
and  its  relation  to  the  ordinary  fats  may  be  roughly  expressed  by 
saying  that,  whereas  these  consist  of  glycerin  combined  with  fatty 
acids,  lecithin  contains,  in  addition,  phosphoric  acid  and  the  alka- 
loid cholin.1 

Lecithin  is  an  unstable  compound,  easily  breaking  up  into  simpler 
compounds,  while  on  the  other  hand  it  apparently  enters  into  still 
more  complex  compounds  with  proteids  and  with  cholesterin. 
Physically,  lecithin  is  a  yellowish-white,  waxy,  hygroscopic  solid, 
which  in  thin  layers  shines  with  a  silky  lustre.  It  is  soluble  in 
alcohol,  ether,  etc.,  but  in  water  it  swells  to  a  sort  of  paste,  much  like 
starch.  Electrically  it  is  a  non-conductor;  and  bound  up  with  this 
is  its  property  of  resisting  free  osmosis  through  any  membrane  into 
which  it  enters.  It  may  also  be  called  a  highly  sensitive  substance, 
since  its  physical  condition,  while  in  solution,  is  readily  changed  (as 
by  the  action  of  ions).2  All  of  these  properties  are  interesting  and 
suggestive  of  the  probable  importance  of  lecithin  in  the  life  of  the 
cells.  As  was  stated  above,  the  existence  of  lecithin  preformed  in 
the  brain  is  not  completely  made  out;  but  if  it  does  not  occur  there, 
then  other  similar  substances,  such  as  Thudicum's  kephalin,  or  else 
more  complex  compounds  into  which  lecithin  and  kephalin  enter, 

1  The  complex  structure  of  lecithin  is  more  precisely  stated  by  saying  that 
glycerin  contains  three  hydroxyl  groups;  by  one  of  which  it  takes  up  phosphoric 
acid,  and  by  the  others,  two  molecules  of  fatty  acid;  meanwhile,  the  phosphoric 
acid  also  combines  with  the  base,  cholin,  which  has  itself  a  rather  complex  struct- 
ure, as  expressed  by  its  analytic  name,  trimethyl-oxyethyl-ammonium  hydroxide. 

2  W.  Koch,  "The  Lecithans,"  Decennial  Publications  of  the  University  of  Chicago, 
1902,  vol.  X;  American  Journal  of  Physiology,  1904,  XI,  303. 


THE  PHOSPHORIZED  LIPOIDS  123 

must  exist  in  the  brain.  In  other  words,  the  phosphorus  of  the  brain 
is  partly  contained  in  compounds  presenting  the  general  character- 
istics which  have  been  described  as  those  of  lecithin. 

§  10.  Protagon  was  discovered,  as  a  new  proximate  principle 
that  can  be  separated  from  the  brain,  in  1864,  by  Dr.  Oscar  Lieb- 
reich;  his  discovery  was  announced  in  a  paper1  published  in  1865. 
This  investigator  gave  to  this  substance  the  name  which  it  still  bears, 
as  in  his  opinion  the  first  to  be  definitely  ascertained  among  the 
specific  constituents  of  the  brain  (TrpwTayds,  leading  the  van).  He 
assigned  to  it  the  formula  C116H24iN4O22P.  More  recent  work 
has  changed  this  formula  to  some  extent  —  especially  in  assigning 
five  atoms  of  N  to  each  one  of  P,  and  in  adding  a  small  proportion 
of  sulphur  to  the  molecule. 

The  process  by  which  he  obtained  it  from  the  brain  may  be  thus 
briefly  described  (the  description  will  serve  to  illustrate  in  general 
the  processes  of  physiological  chemistry):  Perfectly  fresh  ox's 
brains  are  freed  from  the  blood  and  membranes,  and  are  then  di- 
gested for  about  a  day  in  eighty-five  per  cent,  alcohol;  from  this 
fluid,  when  filtered,  a  quantity  of  white  flocculent  precipitate  is 
obtained,  and  the  cholesterin  and  other  bodies  soluble  in  ether  are 
dissolved  out;  from  the  substance  left  undissolved,  when  dried  and 
reduced  to  powder  and  digested  for  many  hours  with  alcohol,  and 
then  filtered  and  cooled,  microscopic  crystals  separate  themselves, 
arranged  for  the  most  part  in  rosettes.  The  substance  thus  crystal- 
lized is  protagon.  Repetition  of  the  alternate  processes  of  solution 
and  crystallization  is  resorted  to  for  purifying  the  protagon. 

§  11.  The  history  of  the  protagon  problem  is  interesting  and  curi- 
ous in  several  ways,  even  apart  from  the  importance  which  has  been 
assigned  this  substance  as  the  most  characteristic  chemical  sub- 
stance of  the  brain,  and  the  one  which  would  therefore  have  espe- 
cially to  be  considered  in  any  attempt  to  correlate  the  composition 
of  the  brain  with  its  functions.  A  substance  apparently  identical 
with  Liebreich's  protagon  had  been  obtained  by  several  previous 
observers  (1834-1850),  with  whose  work  Liebreich  was  unac- 
quainted. The  consensus  of  expert  opinion  on  the  chemical  unity 
of  the  substance  has  swung  back  and  forth  several  times  in  the  last 
half  century.  At  first,  on  Liebreich's  discovery,  it  was  accepted, 
and  lecithin  was  regarded  as  probably  non-existent  in  the  fresh 
brain;  then  the  work  of  Diaconow  (1868)  and  of  Thudicum  (1874) 
discredited  protagon,  by  tending  to  show  that  it  was  a  mixture  of 
no  constant  composition;  and  for  years  the  belief  in  its  unity  was 
relegated  to  the  limbo  of  exploded  views.  Next,  Gamgee  (1879)  re- 


die  chemische  Beschaffenheit  der  Gehirnsubstanz,"  Annalen  der 
Chemie  und  Pharmacie,  CXXXIV,  pp.  29-44. 


124  CHEMISTRY  OF  THE  NERVOUS  SYSTEM 

suscitated  it,  and  it  enjoyed  for  two  decades  general  acceptance. 
Beginning  about  1900,  results  supporting  Thudicum  have  been  ob- 
tained by  several  writers,  who  do  not  hesitate  to  say  that  the  chemi- 
cal unity  of  protagon  has  been  fully  disproved.  Gies  and  his  col- 
laborators1 have  shown  that  if  the  method  employed  for  purify- 
ing protagon  is  repeated  time  after  time,  instead  of  approximating 
to  a  constant  composition,  the  protagon  changes  at  each  repetition 
of  the  process.  They  infer  that  pure  protagon,  if  it  exists,  has  never 
been  prepared,  and  that  probably  it  does  not  exist  as  a  definite  com- 
pound. Their  results  have  been  confirmed  and  accepted  by  other 
observers.2 

On  the  other  hand,  Cramer  has  found  it  possible  to  prepare  from 
the  brain,  by  an  entirely  different  method,  a  substance  which  has  the 
same  quantitative  composition  as  Liebreich's  protagon.  This  con- 
stancy of  composition  under  different  methods  of  preparation,  taken 
in  connection  with  the  crystalline  form  of  the  substance,  leads  au- 
thorities such  as  Hammersten3  and  Halliburton4  to  conclude  that  its 
chemical  identity  has  not  been  disproved.  The  former  of  these  two 
authorities  regards  protagon  as  a  crystalline  substance  which  is 
extremely  difficult  to  separate  from  other  substances  that  are  in 
part  its  decomposition  products.  But  FrankePs  position5  is  alone 
tenable;  for  he  regards  the  whole  state  of  the  subject  as  at  present 
thoroughly  unsatisfactory.  To  illustrate  and  enforce  this  truth 
may  be  regarded  as  the  principal  justification  for  the  attention 
which  we  have  ourselves  bestowed  upon  so-called  protagon. 

§  12.  The  specific  chemistry  of  the  histological  elements  of  the 
nervous  system,  or  of  the  various  parts  of  such  elements,  is  yet  more 
meagre  and  doubtful  than  its  general  chemistry.  The  differences 
between  the  composition  of  gray  and  of  white  matter  indicate  that 
the  nerve-cell  body  is  rich  in  proteids,  whereas  the  myelin  sheath 
consists  very  largely  of  lipoids.  The  internal  structure  of  the  cells 
and  axons,  as  revealed  by  selective  stains,  is  an  indication  of  chemi- 
cal differences  between  the  parts  differentiated  by  the  stains;  but 
the  interpretation  of  the  reactions  on  staining  is  by  no  means  easy, 
and  little  can  at  present  be  said  regarding  the  chemistry  of  parts  of 
the  cells. 

§  13.  It  need  scarcely  be  said,  in  conclusion,  that  we  have  little 
knowledge  respecting  the  relation  which  exists  between  the  chemical 
constitution  and  chemical  processes  of  the  nervous  system,  on  the 
one  hand,  and,  on  the  other,  the  phenomena  of  so-called  mind. 

1  See  Posner  and  Gies,  Journal  of  Biological  Chemistry,  1905,  vol.  I,  p.  98. 
a  Rosenheim  and  Tebb,  1907,  Journal  of  Physiology,  XXXVI,  pp.  1  ff. 

3  Lehrbuch  der  physiologischen  Chemie,  1907,  p.  483. 

4  Biochemistry  of  Muscle  and  Nerve,  1904,  p.  65.  5  Op.  cit.,  p.  251. 


SPECIFIC   CHEMISTRY  OF  NERVOUS  ELEMENTS    125 

Nevertheless,  certain  important  general  relations  may  be  point- 
ed out  between  the  chemical  nature  of  the  nervous  mechanism  and 
its  psycho-physical  functions.  The  extremely  high  organization 
and  chemically  sensitive  constitution  of  this  mechanism  are  beyond 
doubt  related  to  all  its  distinctive  activities.  Like  every  other  nat- 
ural material  structure,  the  nervous  system  is  obviously  adapted 
to  its  peculiar  kind  of  work.  Chemically  considered,  it  appears  as 
composed  of  a  number  of  extremely  complex  and  highly  unstable 
compounds.  It  therefore  holds  in  its  chemical  constitution  a  large 
amount  of  disposable  energy;  this  energy  it  yields  readily  when  the 
equilibrium  of  its  molecules  is  in  any  way  disturbed.  Within, cer- 
tain limits,  it  explodes  with  increasing  surrender  of  its  disposable 
energy  as  the  number  and  intensity  of  the  demands  upon  it  are  in- 
creased— very  much  as  would  a  gun  which  should  be  arranged  so 
as  to  go  off  with  greater  energy  as  the  pressure  of  the  finger  on  its 
trigger  is  repeated  or  increased. 

It  is  probable  that  the  substance  of  the  nerves  is  the  seat  of  a 
chemical  synthesis,  as  the  result  of  which  still  more  complex  bodies 
are  constructed  from  the  already  complex  alimentary  material  fur- 
nished by  the  blood;  such  bodies  have  a  high  value  as  combus- 
tibles, and  thus,  as  has  been  said,  possess  a  significant  amount  of 
disposable  energy.  The  relation  of  a  supply  of  oxygen  to  the 
nerve-centres  is  also  important  to  notice.  The  nerve-fibres  require 
comparatively  a  small  amount  of  oxygen.  It  may  be  conjectured 
that  in  their  case,  as  in  the  case  of  muscle-fibre,  intra-molecular 
oxygen  is  of  some  use  in  preparing  explosive  materials.  But  at 
present  we  must  be  satisfied  with  conjecture  on  this  point.  On  the 
contrary,  the  vascular  nature  of  the  central  organs  creates  a  pre- 
sumption that  the  chemical  processes  which  have  their  seat  in  them 
require  an  abundance  of  oxygen.  Experience  confirms  this  pre- 
sumption. The  respiratory  centre  in  the  medulla  oblongata  is 
chiefly  controlled  in  its  action  by  the  amount  of  oxygen  which 
reaches  it  in  the  blood.  The  phenomena  of  consciousness  vanish 
when  the  supply  of  oxygenated  blood  is  cut  off  from  the  brain. 

Although  we  are  still  in  the  dark  as  to  the  precise  significance  of 
the  visual  purple,  the  phenomena  which  the  study  of  it  has  brought 
to  light  are  suggestive  of  unseen  chemical  processes  that  are  set  up 
in  the  retina,  and  so  serve  as  stimulus  for  the  fibrils  of  the  optic 
nerve.  In  general  we  know  that  certain  sensations  are  dependent 
upon  the  chemical  constitution  and  activity  of  the  various  end- 
organs  of  sense. 

Further  researches  can  scarcely  fail  to  enlarge  our  knowledge 
of  those  facts  of  relation  which  exist  between  the  chemical  constitu- 
tion and  changes  of  the  nervous  mechanism  and  the  phenomena  of 


126  CHEMISTRY  OF  THE  NERVOUS  SYSTEM 

psychical  life.  Perhaps  the  more  particular  statements  of  fact  may 
ultimately  be  gathered  into  those  more  general  statements  of  fact, 
more  or  less  verifiable  by  experiment,  which  we  consider  sufficient 
to  constitute  scientifically  verifiable  laws.  But  why  certain  chemi- 
cal constituents,  when  combined  and  changed  in  definite  fashion, 
should  be  specifically  connected  with  certain  conscious  experiences, 
will  probably  always  remain  a  quite  unanswerable  inquiry. 


CHAPTER  VI 

THE  NERVES  AS  CONDUCTORS 

§  1.  In  that  threefold  economy  of  organs  which  characterizes  the 
developed  nervous  mechanism,  the  office  of  propagating  the  neural 
process  between  the  central  organs  and  the  end-organs  has  been 
assigned  to  the  nerves.  The  power  to  originate  this  process  under 
the  action  of  external  stimuli,  although  experiment  shows  that  it 
belongs  to  the  nerves,  is  not  exercised  by  them  while  in  their  nor- 
mal place  within  the  mechanism.  It  is  the  office  of  the  end-organs 
to  transmute  the  physical  molecular  processes,  which  are  their  stim- 
uli, into  the  physiological  and  neural  process,  and  hand  it  over,  as  it 
were,  to  these  conducting  cords.  But  the  office  of  the  nerves  as 
conductors  is,  of  course,  not  like  that  of  a  tube  which  conducts  along 
its  channel  some  kind  of  fluid,  nor  is  it  like  that  of  the  wire  or  bell- 
metal  which  is  thrown  into  vibration  throughout.  It  is  a  molecular 
commotion  which,  when  started  at  any  point  in  the  nerves,  moves 
in  both  directions  from  point  to  point  along  its  course.  The  in- 
timate connection  between  the  two  functions  of  excitation  and  con- 
duction becomes,  then,  at  once  apparent.  Indeed,  excitation  may 
be  considered  as  the  setting-up  of  the  process  of  conduction;  con- 
duction as  the  uninterrupted  continuance,  or  propagation  from 
point  to  point,  successively,  of  the  process  of  excitation.  Each 
minute  subdivision  of  the  nerve,  then,  must  be  regarded  as  consti- 
tuting, in  some  sort,  a  source  or  centre  of  stimulation  with  respect 
to  its  neighboring  subdivisions.  If  the  nerve-commotion  is  to 
move  along  the  nerve  N,  between  two  distant  portions  of  its  struct- 
ure, a  and  z,  then  a  must  act  upon  its  neighbor  b  as  a  stimulus,  b 
upon  c,  and  so  on  successively  until  y  is  found  stimulating  2,  and 
the  process  of  progressive  excitation  or  conduction  is  complete. 

§  2.  It  follows  from  what  has  just  been  said  that,  in  considering 
the  nerves  as  conductors,  the  conditions  and  laws  of  the  origination 
of  that  process  of  excitation  which  they  conduct  must  be  taken  into 
account.  It  is  neither  necessary  nor  convenient,  however,  to  carry 
throughout  a  distinction  between  the  two  functions — the  excitabil- 
ity and  the  conductivity — of  the  nerves;  it  is  better  to  regard  them 
as  one  process,  looked  at  from  somewhat  different  points  of  view. 

127 


128  THE  NERVES  AS  CONDUCTORS 

Clearly,  if  the  "nerve  impulse"  is  to  be  propagated,  it  must  first  be 
aroused  at  some  point  in  the  nerve;  and  under  normal  conditions, 
by  the  action  on  it  of  other  organs.  In  the  case  of  sensory  nerve- 
fibres,  the  stimulus  is  furnished  by  the  action  of  modified  receptive 
cells;  as,  for  example,  the  optic  nerve  is  normally  excited  by  the  rods 
and  cones,  which  themselves  have  been  aroused  by  the  action  of 
light.  In  the  case  of  motor  nerve-fibres,  the  normal  source  of  their 
activity  lies  in  the  spinal  cord  or  brain-stem;  here  they  are  aroused 
by  the  action  on  them  of  other  nerve-fibres — such  action  occurring 
at  the  synapse,  where  the  fine  terminations  of  an  axon  come  into 
relation  with  the  dendrites  of  another  cell  and  so,  indirectly,  with 
the  axon  of  that  cell. 

§  3.  In  studying  the  physiology  of  the  nerves,  it  is  often  neces- 
sary to  separate  them  from  their  normal  connections  in  the  body, 
and  arouse  them  by  artificial  means.  At  first  sight,  this  artifici- 
ality in  the  experimental  procedure  would  seem  to  vitiate  the  results; 
but  control  experiments  show  that  the  nerve  impulse  behaves  in 
the  same  way — so  far  as  can  be  observed — whether  it  is  aroused 
in  the  natural  or  in  an  artificial  manner.  Apparently  a  nerve-im- 
pulse is  a  nerve  impulse,  however  aroused.  The  objection  that  the 
experiments  are  made  on  the  nerves  of  animals,  from  which  an  in- 
ference to  human  nerves  may  be  doubtful,  is  also  met  by  the  state- 
ment that  a  nerve  is  a  nerve,  from  whatever  species  of  animal.  Such 
observations  as  have  been  possible  on  human  nerves  show  no  differ- 
ence between  them  and  the  nerves  of  other  mammals;  and  there  are 
only  minor  differences  between  the  latter  and  the  nerves  of  the  frog, 
which  have  most  often  been  used  in  experiments.  While  this  is 
true  of  the  medullated  nerves,  unmedullated  nerves  form,  in  many 
respects,  a  separate  class. 

In  experimenting  with  a  nerve,  some  sort  of  indicator  is  needed 
to  show  when  it  is  active;  for  an  active  nerve  does  not  differ  visibly 
from  an  inactive  one.  For  a  motor  nerve,  a  good  indicator  is  af- 
forded by  the  attached  muscle,  which  contracts  when  the  nerve 
impulse  reaches  it,  and  so  indicates  the  activity  of  the  nerve.  For 
a  sensory  nerve  a  reflex  movement  or  a  sensation  (in  the  human 
subject)  may  be  used  as  the  indicator.  Since,  however,  reflexes  and 
sensations  involve  the  complex  and  variable  activity  of  nerve  cen- 
tres, the  physiologist  prefers  the  motor  nerves,  and  makes  a  large 
share  of  his  experiments  on  what  he  calls  a  "  nerve-muscle  prepara- 
tion." 

This  preparation  consists  of  a  muscle  freshly  taken  from  the  liv- 
ing animal  with  its  attached  nerve  dissected  out;  for  example,  the 
gastrocnemius  muscle  of  the  frog  with  the  attached  sciatic  nerve. 
Such  a  preparation  may  be  kept  alive  for  some  time  in  a  moist 


METHODS  OF  EXPERIMENTATION  129 

chamber.  By  the  simple  contrivance  of  connecting  the  end  of  the 
muscle  with  a  lever,  arming  the  lever  with  some  means  of  making 
a  mark — either  pen,  or  bristle,  or  needle — and  bringing  its  point 
thus  armed  to  bear  on  a  rapidly  travelling  surface  (plain  paper, 
or  smoked  paper  or  glass),  the  time  and  amount  of  the  contrac- 
tions of  the  muscle  may  be  recorded.  The  most  refined  means 
for  noting  the  exact  instant  when  the  stimulus  is  applied,  and  also 
the  state  of  the  effects  produced  at  every  succeeding  instant  of  their 
duration,  are  of  first  importance.  The  nerve  may  be  stimulated 
with  different  kinds,  degrees,  and  directions  of  the  electrical  cur- 
rent (or  with  other  forms  of  stimuli)  at  any  points  preferred  in  its 
stretch,  and  under  a  great  variety  of  conditions  with  respect  to  tem- 
perature, moisture,  mechanical  pressure  or  stricture,  integrity  and 
vitality  of  its  structure,  etc.;  and  the  effects  of  such  stimulations 
upon  the  contractions  of  the  muscle  may  be  noted  and  compared 
as  they  have  been  recorded.  Means  for  testing  the  most  delicate 
and  rapid  changes  in  the  electrical  or  thermometric  conditions  of 
the  nerve  may  be  applied  to  it  at  any  point  of  its  stretch.  Varia- 
tions and  refinements  of  experiments  essentially  the  same  may  be 
almost  indefinitely  multiplied;  the  experiments  may  be  repeated, 
and  verified  or  corrected,  by  the  same  observer  or  by  others.  In- 
asmuch as  the  preparation  is  both  muscle  and  nerve,  an  acquaint- 
ance with  the  behavior  of  the  muscle,  and  with  the  laws  of  its  con- 
traction, is  necessary  in  order  that  it  may  be  known  how  much  of 
the  complex  phenomena  is  to  be  ascribed  to  the  functional  activity 
of  muscle,  how  much  to  that  of  nerve.  But  into  a  statement  of  the 
general  laws  of  contractile  tissues,  and  of  the  nature  and  explana- 
tion of  the  behavior  of  muscle  when  irritated,  we  cannot  enter  in 
great  detail.1 

Being  thus  provided  with  a  fresh  nerve  and  a  muscle  to  serve  as 
indicator  of  the  nerve's  activity,  the  physiologist  is  able  to  observe 
and  draw  inferences  with  regard  to  the  excitability  and  conduc- 
tivity of  the  nerve. 

§  4.  As  respects  its  excitability,  the  nerve  is  found  to  be  easily 
aroused  by  stimuli  applied  directly  to  it  at  any  point.  Pinch  the 
nerve,  and  the  muscle  instantly  contracts;  drop  a  light  weight  on 
the  nerve,  and  again  the  muscle  contracts.  These  are  examples 
of  "mechanical  stimuli,"  which  are  thus  proved  to  be  capable  of 
arousing  the  nerve  to  activity.  A  variety  of  chemical  substances 
also  excite  the  nerve;  acids  are  especially  effective,  bases  less  so; 

1  For  a  description  of  the  method  and  results  of  experimenting  with  the  nerve- 
muscle  preparation,  more  accessible  to  the  general  reader  than  the  books  to  which 
reference  will  chiefly  be  made,  see  Howell's  Text-book  of  Physiology,  chap.  I, 
p.  5. 


130  THE  NERVES  AS  CONDUCTORS 

of  salts,  some,  like  sodium  chloride,  act  as  stimuli,  while  others, 
such  as  most  of  the  salts  of  the  heavy  metals,  kill  the  nerve  with- 
out exciting  it.  The  same  effect  follows  use  of  too  strong  a  solu- 
tion of  any  of  these  chemical  agents.  Sudden  heating  of  the  nerve 
acts  as  a  stimulus.  Electric  shocks,  especially  in  the  form  of  in- 
duced currents,  are  highly  effective.  These  four  classes  of  agents 
— mechanical,  chemical,  thermal,  and  electrical — are  called  "gen- 
eral stimuli,"  because  they  are  capable  of  arousing,  not  only  nerves, 
but  a  great  variety  of  forms  of  living  matter,  including  the  muscles 
and  glands  of  the  higher  animals  and  the  less  specialized  protoplasm 
of  protozoa. 

§  5.  The  effectiveness  of  any  of  these  stimuli  depends  in  part 
on  its  suddenness.  Slow  changes  of  temperature,  for  example,  do 
not  excite  the  nerve;  and  very  gradually  increasing  pressure  may 
crush  the  nerve  without  exciting  it.  The  electric  current,  too, 
must  have  a  degree  of  suddenness  in  its  changes,  if  it  is  to  act  as  a 
stimulus  to  nerve. 

In  regard  to  this  last  point,  Du  Bois-Reymond,  one  of  the  pio- 
neers in  "  electrophysiology,"  announced  in  1845 l  the  discovery  of 
two  closely  related  laws  of  the  electrical  excitation  of  nerve.  The 
first  law  is  that  the  mere  passage  of  a  current  through  the  nerve 
does  not  arouse  it;  only  changes  in  the  electric  current  act  as  stimuli; 
but  these  changes  may  consist  either  in  increasing  the  current  or  in 
decreasing  it.  This  law  is  the  expression  of  a  fact  which  is  easily 
verified  on  the  nerve-muscle  preparation.  When  the  current  from 
a  battery  is  passed  through  a  nerve,  the  muscle  gives  a  twitch  at 
the  starting  of  the  current,  and  then,  usually,  remains  at  rest  dur- 
ing the  passage  of  the  current,  only  to  twitch  again  at  the  stopping 
of  the  current.  Or,  if  the  current  is  not  entirely  stopped,  but  is 
suddenly  cut  down  in  strength,  still  the  muscle  twitches,  as  it  does 
also  if  the  current  is  suddenly  increased.  The  second  law  of  Du 
Bois-Reymond  is  that  the  exciting  effect  of  any  change  of  current 
is  greater,  the  more  sudden  the  change.  If  the  current  is  gradually 
increased  by  a  certain  amount,  the  muscle  does  not  twitch,  whereas 
if  the  same  amount  of  increase  is  effected  quickly,  the  muscle 
twitches. 

The  validity  of  the  first  of  these  laws  has  not  been  much  affected 
by  later  investigations,  at  least  so  far  as  concerns  the  nerves.  Cer- 
tain other  tissues  are  excited  by  the  passage  of  a  steady  current,  but 
the  nerve  is  usually  not  excited  in  such  a  way  as  to  arouse  the  muscle 
which  is  attached  to  it.  The  nerve  is  not,  however,  by  any  means 
in  its  normal  quiescent  state  during  the  passage  of  a  current,  but  is 

1  In  a  paper  communicated  to  the  Physiological  Society  in  Berlin;  see  also 
his  Untersuchungen  uber  thierische  Electricitat,  I,  258. 


THE  LAWS  OF  DU  BOIS-REYMOND  131 

in  quite  an  unusual  condition  as  concerns  its  excitability  and  con- 
ductivity. This  fact  will  be  brought  out  later  in  the  chapter. 

The  second  law  of  Du  Bois-Reymond  needs  qualification  in  the 
light  of  further  experience.  With  the  introduction  of  very  rapidly 
oscillating  currents  ("Tesla"  or  "d'Arsonval  currents"),  it  has 
been  found  that  this  effectiveness  as  stimuli  by  no  means  increases 
indefinitely  with  the  rapidity  of  the  oscillations.  Oscillations  up 
to  1,000,000  per  second  will  indeed  excite  the  nerve,  but  only  if  the 
current  is  very  intense.1  Beyond  a  certain  point,  the  effectiveness 
of  the  oscillations  decreases  as  their  rate  increases.  It  seems  prob- 
able, therefore,  that  in  order  to  have  the  greatest  exciting  effect  on 
the  nerve,  the  change  of  current  must  have  a  certain  high  speed, 
on  either  side  of  which  its  effectiveness  decreases.  Nerves  would 
be  in  this  respect  like  other  irritable  tissues,  some  of  which  are  best 
stimulated  by  rather  slow  changes  in  the  current,  while  others  are 
best  excited  by  more  rapid  changes.  Each  kind  of  tissue  is  best 
excited  by  a  certain  speed  of  change  of  the  current;  but  the  nerve 
has  the  distinction  of  being  that  tissue  which  is  adapted  to  the 
most  rapid  change. 

§  6.  Another  similar  distinction  of  the  nerve  is  its  very  short 
"refractory  period."  An  organ  which  is  excited  by  a  stimulus  is 
apt  to  be  inexcitable  by  another  stimulus  which  follows  immedi- 
ately; it  is  said  to  be  refractory  during  a  certain  period  after  commenc- 
ing its  response  to  the  first  stimulus.  The  length  of  this  refractory 
period  differs  in  different  organs.  In  the  heart,  in  which  the  phe- 
nomenon is  most  striking,  the  refractoriness  lasts  undiminished 
until  the  full  strength  of  the  muscular  contraction  is  reached  (or 
about  0.4  second),  after  which  it  gradually  passes  away.  In  the 
spinal  cord  and  brain,  the  refractory  period  of  some  groups  of  nerve- 
cells  may  be  as  long,  or  even  longer,  than  that  of  the  heart.  On  the 
contrary,  skeletal  muscle  has  a  much  shorter  period,  not  exceeding 
.01  second;  while  nerve-fibres  have  a  refractory  period  of  not  over 
.002  second.2  The  nerve  is,  then,  in  all  respects  a  quick-acting 
organ :  it  is  best  excited  by  a  sudden  stimulus ;  its  response  is  prompt 
and  brief,  and  it  is  very  soon  ready  for  a  fresh  stimulus. 

§  7.  In  still  another  and  very  important  respect  the  nerve  is  dis- 
tinguished for  its  quick  action;  and  that  is  the  speed  of  its  con- 
duction. In  measuring  the  speed  of  conduction  in  a  motor  nerve, 
the  nerve-muscle  preparation  is  again  employed.  The  determina- 
tion of  the  rate  of  transmission  in  nerves  was  an  achievement  of 


1  Einthoven.     Cited  by  Biedermann,  Ergebnisse  der  Physiologie,  1903,  II, 
part  2,  114. 

a  See  Gotch  and  Burch,  Journal  of  Physiol.,  1899,  XXIII,  p.  xxii. 


132  THE  NERVES  AS  CONDUCTORS 

Helmholtz,1  and  it  was  the  more  noteworthy  because  it  followed 
close  on  the  prediction  by  Johannes  Mu'ller,2  another  eminent 
physiologist,  that  the  speed  of  nerve-conduction  would  never  be  de- 
termined, because,  as  he  was  led  to  suppose,  its  speed  was  compar- 
able with  the  speed  of  light.  The  method  of  Helmholtz  was,  how- 
ever, simple  enough.  He  measured  the  time  elapsing  between  the 
moment  of  stimulation  and  the  beginning  of  the  muscular  contrac- 
tion, when  the  stimulus  was  applied  as  near  the  muscle  as  possible ; 
and  he  then  compared  this  with  the  corresponding  time,  when  the 
stimulus  was  applied  as  far  from  the  muscle  as  possible.  The 
difference  in  time  and  the  length  of  nerve  between  the  two  points 
of  stimulation  enabled  him  to  calculate  the  speed  of  transmission 
between  the  same  two  points.  Helmholtz's  work  has  often  been 
repeated  and  confirmed;  and  in  this  way,  the  rate  of  transmission 
in  frog's  nerve  is  known  to  be  approximately  28  metres  per  second. 
In  the  human  subject,  similar  experiments,  in  which  a  motor  nerve 
is  stimulated  through  the  skin,  at  two  distant  points,  have  led  to 
slightly  higher  values,  or  about  33  metres  per  second.  For  a  con- 
venient approximation,  in  English  measures,  we  may  take  100  feet 
per  second  as  the  rate  in  the  motor  nerve.  In  sensory  nerves,  the 
rate  is  much  harder  to  measure,  because  the  nerve  leads  into  the 
centre,  and  the  activity  of  the  centre,  which  consumes  a  considerable 
and  variable  time,  intervenes  before  any  movement,  reflex  or  vol- 
untary, can  be  made  to  indicate  that  the  nerve-impulse  has  arrived.3 
When  stimulation  is  confined  to  single  "touch  spots,"  and  spots  of 
equal  sensitivity  are  chosen,  fairly  precise  results  have  been  ob- 
tained,4 indicating  a  speed  of  about  30-33  metres  per  second  in  the 
sensory  nerves  of  the  arms  and  legs. 

This  speed  of  about  100  feet  per  second  is  certainly  modest  in 
comparison  with  the  speed  of  light.  Yet,  in  this  respect,  as  well 
as  in  the  others  mentioned  in  preceding  paragraphs,  nerves  occupy 
a  place  of  distinction  among  living  tissues.  This  is  specially  true 
of  medullated  nerves.  The  non-medullated  nerves  have  a  much 
slower  rate  of  transmission,  which  varies  greatly  in  different  ani- 
mals. Mammalian  unmedullated  nerve  conducts  at  about  the  speed 
of  8  metres  per  second;  in  the  lobster,  figures  have  been  obtained 
ranging  from  8  to  12  metres;  but  in  some  other  invertebrates, 
speeds  as  low  as  a  centimetre  or  even  a  millimetre  per  second  have 
been  observed.  In  human  "striped  "  muscle,  the  rate  of  trans- 
mission of  activity  is  10-13  metres  per  second;  in  the  cold-blooded 

1  Archiv  f.  Anat.  u.  PhysioL,  1850,  pp.  276-364. 
8  See  his  Handbuch  der  Physiologic,  I,  pp.  581  f.  (Coblenz,  1844). 
3  Cf.  Dolley  and  Cattell,  Psychological  Review,  1894,  I,  159. 
*Kiesow,  Zeitschr.  /.  Psychol,  1903,  XXXIII,  444. 


ALTERATIONS  IN  CONDUCTIVITY  133 

frog,  the  speed  is  less  than  one  half  of  this;  and  in  the  "unstriped" 
muscles  of  the  stomach  and  other  internal  organs  it  is  much  slower 
still.1 

§  8.  The  conductivity  of  a  nerve  is  altered  by  the  physical  and 
chemical  agents  applied  to  it.  For  example,  cooling  a  nerve  slows 
its  rate  of  conduction,  and  warming  it,  within  narrower  limits, 
hastens  the  conduction.  The  anaesthetics,  alcohol,  ether,  and 
chloroform,  when  applied  in  the  form  of  vapor  to  a  length  of  nerve 
enclosed  in  a  small  gas-tight  chamber,  lower  the  conductivity,  and 
may  entirely  abolish  it.  The  passage  of  an  electric  current  through 
a  length  of  nerve  also  interferes  with  its  conductivity,  as  will  be  more 
fully  explained  later. 

It  is  a  curious  fact  that  some  of  these  agents  act  differently  on 
the  conductivity  and  on  the  excitability  of  the  nerve.  Thus,  al- 
cohol vapor,  though  lowering  the  conductivity,  may  even,  at  a  cer- 
tain stage  in  its  action,  raise  the  excitability  of  the  portion  of  nerve 
to  which  it  is  applied,  so  that  weaker  stimuli  will  take  effect  there 
than  in  the  portions  under  normal  conditions.  Carbon  dioxide 
gas  has  the  opposite  effect:  if  applied  to  a  short  length  of  nerve, 
it  lowers  or  abolishes  the  excitability  of  that  portion  without,  for 
a  time,  interfering  with  the  conduction  through  it  of  nerve  impulses 
generated  elsewhere.  Cold,  also,  though  lowering  the  speed  and  ef- 
fectiveness of  conduction,  may  even  raise  the  excitability  to  certain 
classes  of  stimuli;  especially,  to  those  which  consist  of  relatively 
slow  changes.  Cold,  that  is  to  say,  makes  the  nerve  a  slower-act- 
ing mechanism,  and  so  adapts  it  to  take  up  slower  changes  in  the 
agents  which  are  applied  to  it. 

It  appears  from  these  results  that  the  excitability  of  a  nerve  is  to 
some  degree  independent  of  its  conductivity,  and  a  separation  of 
the  two  functions  is  indicated.  Perhaps  a  better  generalization 
of  the  results  would  be  to  say  that  the  taking  up  of  stimuli  arti- 
ficially applied  to  the  surface  of  the  nerve  is  subject  to  different 
conditions  from  those  which  determine  its  normal  activity  in  taking 
up  stimuli  from  adjoining  parts  of  itself.2 

§  9.  Another  important  fact  regarding  the  conductivity  of  nerve- 
fibres  is  that  they  conduct  in  both  directions.  Under  the  normal 
conditions  of  stimulation  a  nerve-fibre  has  occasion  to  conduct  only 
in  one  direction.  Sensory  fibres  receive  stimulation  only  from  the 
receptive  cells  with  which  their  peripheral  ends  are  in  connection; 
and  they  conduct  only  toward  the  nervous  centres.  Motor  nerves, 

1  The  figures  are  from  several  authorities;   some  of  them  have  been  brought 
together  by  Biedermann,  Ergebnisse  der  Physiologie,  1903,  II,  part  2, 147,  and 
by  Gotch  in  Schdfer's  Textbook  of  Physiology,  1900,  II,  p.  482. 

2  For  these  results,  and  a  discussion  of  them,  see  Gotch,  op.  cit.,  pp.  484  ff. 


134  THE  NERVES  AS  CONDUCTORS 

in  normal  conditions,  receive  stimuli  only  at  their  central  ends  in 
the  cord  or  brain  stem;  and  they  conduct  only  outward  to  the 
muscles.  But  when,  in  experimental  work,  a  nerve-fibre  is  excited 
somewhere  along  its  course,  and  not  at  its  natural  end,  it  is  found 
to  conduct  in  both  directions;  this  it  does,  apparently,  as  perfectly 
in  one  direction  as  in  the  other.  The  evidence  for  these  statements 
is  not  quite  so  easy  to  obtain  as  would  at  first  appear,  because  most 
nerves  are  composed  partly  of  sensory  and  partly  of  motor  fibres, 
which,  normally,  conduct  in  opposite  directions.  The  dorsal  roots 
of  the  spinal  nerves  are,  indeed,  composed  almost  or  quite  entirely 
of  sensory  fibres,  and  the  ventral  roots  of  motor  fibres.  But  in  at- 
tempting to  use  these  roots  for  experimental  purposes,  the  difficulty 
arises  of  finding  some  sure  indicator.  If  the  fibres  in  the  sensory 
root  did  conduct  outward  to  the  sense-organ,  they  could  produce 
no  movement  or  other  change  in  that  organ  to  show  their  outward 
conduction.  And  if  the  motor  roots  did,  when  excited,  conduct  in- 
ward to  the  cord,  there  is  nothing  to  show  this;  because  these  fibres 
end  in  synapses  in  the  gray  matter  of  the  cord,  and  no  conduction 
occurs  across  these  synapses,  except  in  the  outward  direction.1 

There  is,  however,  an  indicator  in  the  wave  of  electrical  change, 
which  attends  the  impulse  in  its  passage  along  a  nerve,  and  which 
can  be  detected  by  a  galvanometer.  When  this  electrical  "  current 
of  action"  is  used  as  an  indicator,  excitation  of  a  sensory  root  is 
found  to  cause  an  outward-travelling  impulse,  and  excitation  of  a 
motor  nerve  is  found  to  cause  an  inward-travelling  impulse  which 
betrays  itself  by  an  electrical  change  in  the  motor  roots. 

Additional  evidence  of  double  conduction  in  nerves  is  afforded 
by  experiments  in  which  the  muscle  is  utilized  as  the  indicator. 
When,  as  happens  in  certain  muscles,  the  motor  axons  split  on  ap- 
proaching the  muscle,  one  branch  of  each  axon  going  to  one  part 
of  the  muscle,  and  the  other  branch  to  another  part,  it  may  be  possi- 
ble to  cut  the  muscle  in  two  without  injuring  the  nerve,  and  then 
to  excite  one  branch  of  the  nerve  and  obtain  contraction  of  both 
parts  of  the  muscle.  The  excitation  has  been  conveyed  by  the 
branched  axons,  which  have  conducted  up  along  one  branch  to 
the  point  of  bifurcation,  and  then  down  along  the  other  branch. 
The  motor  axons  have,  therefore,  conducted  in  both  directions. 
To  make  this  result  convincing,  the  nerve  must  be  severed  from  the 
spinal  cord;  otherwise  the  farther  part  of  the  muscle  might  be  ex- 
cited by  reflex  action — the  upward  conduction  being  afforded  by 
sensory  axons,  which  are  sure  to  be  interspersed  with  the  motor 
fibres  in  the  nerve.  But  sensory  axons  can  transmit  their  excita- 

1  In  regard  to  the  irreversibility  of  conduction  at  the  synapse,  we  have 
spoken  in  a  preceding  chapter. 


NATURE  OF  PROCESS  CONDUCTED  135 

tion  to  motor  axons  only  through  the  nerve-centres,  and  severing 
the  nerve  from  the  cord  destroys  this  possibility.  When  these  con- 
ditions are  complied  with,  the  positive  result  of  the  experiment, 
combined  with  the  positive  result  obtained  on  using  the  current  of 
action  as  an  indicator,  leaves  no  room  for  doubting  the  double  con- 
ductivity of  nerve-fibres. 

§10.  What  is  it  that  is  conducted  along  the  nerve  with  the  speed 
of  100  feet  per  second,  but  which  is  blocked  by  a  cold  stretch  in 
the  nerve,  and  by  ether?  Any  satisfactory  answer  to  this  question 
is  extraordinarily  difficult.  There  is  no  visible  motion  of  the  nerve ; 
no  pull  or  push  is  exerted  through  it;  no  "animal  spirits"  circulate 
rapidly  through  it,  to  constitute  the  nerve-impulse.  What  is  con- 
ducted must,  therefore,  be  of  a  much  more  subtle  nature. 

By  analogy  with  other  living  tissues,  the  activity  of  a  nerve  is 
readily  thought  of  as  a  chemical  process.  It  might  be  likened, 
in  a  measure,  to  the  setting-off  of  a  train  of  gunpowder  or  of  a  fuse, 
in  which  the  oxidation  of  one  part  acts  to  ignite  the  next.  The  simile 
would  halt,  since  the  fuse  is  burnt  out  in  its  activity,  while  the  nerve 
remains  in  good  condition  and  ready  for  further  activity.  A  com- 
parison with  other  living  tissues,  such  as  the  muscle,  is  more  appro- 
priate. The  muscle,  after  one  contraction,  remains  in  good  con- 
dition, and  yet  oxidation  has  gone  on  in  the  muscle;  explosive 
material  has  been  consumed;  in  a  word,  catabolism  has  occurred  in 
the  active  tissue.  Since  living  cells  are  subject  to  the  law  of  the  con- 
servation of  energy,  and  cannot  perform  work  without  the  utiliza- 
tion of  stored  energy,  the  conclusion  seems  unavoidable  that  there 
must  be  catabolism  also  in  an  active  nerve. 

If  the  activity  of  the  nerve  is  like  other  chemical  and  catabolic 
processes,  it  should  give  rise  to  the  waste  products  of  catabolism, 
even  as  the  active  muscle  gives  rise  to  carbon  dioxide,  lactic  acid, 
etc.  With  this  in  view  physiologists  have  looked  for  the  carbon 
dioxide  produced  by  nerve  action,  but  the  most  delicate  tests  have 
been  unable  to  detect  any.  They  have  looked  for  the  acid  reaction 
which  should  betray  catabolism,  but  have  got  no  clear  evidence  of 
the  production  of  acid  products.  They  have  looked  for  the  heat 
which  is  evolved  in  all  known  instances  of  oxidation,  and  have  ap- 
plied instruments  capable  of  detecting  a  rise  in  temperature  of 
•x-fos  of  a  degree  centigrade,  but  have  not  been  able  to  detect  the 
slightest  production  of  heat.  These  negative  results  impress  differ- 
ent physiologists  differently.  Some  conclude  that  the  activity  of  the 
nerve  is,  after  all,  not  a  chemical  process,  or  at  least  not  a  process 
involving  catabolism.  Others  are  still  confident  that  there  is  catabol- 
ism there,  only  that  it  is  too  slight  for  our  present  means  of  de- 
tection. 


136  THE  NERVES  AS  CONDUCTORS 

§  11.  There  is  another  line  of  evidence  which  is  of  special  in- 
terest. A  universal  result  of  catabolic  activity  in  any  organ  is,  ap- 
parently, fatigue.  The  using-up  of  the  store  of  available  energy 
weakens  a  muscle  or  gland;  and  the  accumulation  of  the  waste 
products  of  activity  also  depresses  the  activity  of  the  organ.1  If, 
therefore,  nerve  action  is  catabolic,  the  nerve  should  be  fatigued 
by  any  long-continued  activity. 

In  examining  the  question  of  fatigue  in  nerves,  great  pains  must 
be  taken  to  find  a  suitable  indicator.  We  cannot  simply  take  the 
nerve-muscle  preparation,  and,  after  repeatedly  exciting  the  nerve, 
look  for  evidence  of  its  fatigue  in  a  weakening  of  the  muscular  con- 
tractions; for  the  muscle  is  itself  subject  to  fatigue,  and  its  fatigue 
is  quite  sufficient  to  mask  the  fatigue  of  the  nerve.  To  use  the  mus- 
cle as  an  indicator,  it  must  itself  be  protected  from  excitation,  while 
the  nerve  is  continually  excited.  This  result  has  been  reached  in 
several  different  ways.  Bowditch2  used  the  drug  curare,  which 
blocks  the  passage  of  excitation  from  the  motor  nerve  to  its  muscle, 
without  paralyzing  either  the  nerve  or  the  muscle.  He  kept  the 
animal  alive  by  artificial  respiration  for  four  or  five  hours,  exciting 
the  nerve  many  times  a  second  during  all  this  time,  at  the  end  of 
which,  as  the  effect  of  the  drug  began  to  pass  off,  the  muscle  began 
to  contract.  Thus  the  nerve  had  not  been  exhausted  by  several 
hours  of  continued  activity — many  times  as  much  as  would  have 
reduced  the  muscle  to  an  extreme  condition  of  exhaustion.  The 
same  experiment  has  recently  been  repeated  with  an  improvement, 
which  consists  in  giving  an  antidote  to  the  curare,  which  acts  with 
great  promptness.  In  this  way  the  block  between  the  active  nerve 
and  the  protected  muscle  can  be  instantly  removed,  and  it  is  then 
found  that  four  to  ten  hours  of  continued  activity  have  left  the  nerve 
still  with  unimpaired  powers  of  functioning.3  The  non-medullated 
nerves  of  mammals  seem  to  have  the  same  absence  of  fatigue:4 
it  has  been  claimed,  however,  that  the  olfactory  nerve  of  the  pike, 
a  non-medullated  nerve,  gives  signs  of  fatigue  as  tested  by  the  "  cur- 
rent of  action."5  The  action  current  has  also  been  used  in  medul- 
lated  nerves  as  an  indicator,  and  has  been  found  not  to  weaken  in 
the  course  of  long-continued  excitation  of  the  nerve  at  the  rate  of 
many  shocks  per  second.8 

1  A  fuller  discussion  of  the  phenomena  of  fatigue  will  be  given  later  on. 

8  Journal  of  Physiology,  1885,  VI,  133. 

8  This  form  of  the  classic  experiment  is  due  to  Durig,  and  the  antidote  is 
the  salicylate  of  physostigmin.  See  Zentralblatt  f.  Physiol,  1902,  XV,  751. 

4Brodie  and  Halliburton,  Journal  of  Physiology,  1902,  XXVIII,  181. 

6  Sowton,  Proceedings  of  the  Royal  Society  of  London,  1900,  LXVI,  379; 
Garten,  Pfliiger's  Archiv  f.  d.  gesammte  Physiol.,  1899,  LXXVII. 

6  Edes,  Journal  of  Physiology,  1892,  XIII,  431. 


PHENOMENA  OF  FATIGUE  137 

The  evidence,  therefore,  all  goes  to  demonstrate  the  practical  in- 
defatigability  of  nerve-fibres.  It  may  be,  of  course,  that  this  re- 
markable trait  is  after  all  only  relative ;  that,  in  truth,  the  catabolism 
of  the  nerve  is  excessively  slight,  and  it  therefore  recovers  with  ex- 
traordinary promptness  from  the  effects  of  each  phase  of  activity, 
just  as  the  heart  beats  at  a  moderate  rate  for  many  hours  without 
fatigue,  recovering  from  the  effects  of  each  beat  in  the  pause  that 
intervenes  before  the  next.  And  this  is  the  view  of  Biedermann,1 
who  also  brings  forward  some  evidence,  not  wholly  convincing, 
that  a  perfectly  continuous  electric  stimulation,  as  distinguished 
from  a  rapid  series  of  brief  shocks,  produces  fatigue  of  nerve. 

There  are  one  or  two  facts  of  a  different  order  which  bear  on  the 
same  problem.  The  blood  supply  of  the  nerves  is  very  slight;  and 
their  need  of  oxygen  is  also  very  slight.  It  has  been  shown,2  in- 
deed, that  several  hours'  confinement  in  an  atmosphere  perfectly 
free  from  oxygen  destroys  the  excitability  of  a  nerve,  and  that  there 
is  prompt  recovery  as  soon  as  oxygen  is  admitted;  and  this  is  taken 
to  mean  that  the  nerve  consumes  oxygen  and  therefore  undergoes 
catabolism;  but  some  doubt  still  remains  as  to  whether  it  is  the 
activity  of  the  nerve  which  demands  oxygen,  since  even  an  inactive 
nerve  gets  out  of  condition  in  the  absence  of  oxygen. 

Another  line  of  evidence  in  favor  of  assuming  catabolism  in  the 
active  nerve  is  adduced  by  Waller.3  The  action  current  of  a  nerve 
is  increased  by  activity  of  the  nerve,  as  induced  by  exciting  it  with 
a  rapid  series  of  induction  shocks.  If,  then,  the  character  of  the 
activity  of  the  nerve  is  changed  by  its  own  action,  this  is  prima 
facie  evidence  that  the  nerve  has  itself  changed;  and  the  change  is 
probably  chemical.  Now  the  same  increase  in  the  action  current 
is  caused  by  exposing  the  nerve  to  a  very  slight  dose  of  carbon 
dioxide;  and  Waller  is  therefore  inclined  to  infer  that  the  increased 
action  current  after  activity  is  due  to  the  production  within  the  nerve 
of  very  small  amounts  of  this  gas.  The  evidence,  however,  is  at 
best  indirect. 

§  12.  In  the  absence  of  mechanical,  thermal,  or  chemical  mani- 
festations of  the  nerve-impulse,  we  should  be  wholly  at  a  loss  re- 
garding its  physical  nature,  were  it  not  for  the  fact  that  there  is 
an  electrical  manifestation,  in  the  "current  of  action,"  to  which 
reference  has  already  several  times  been  made.  If  a  nerve  is  ex- 
cited by  any  means  at  one  point,  and  if  the  poles  of  a  galvanometer  or 
capillary  electrometer  are  laid  on  a  distant  part  of  the  nerve,  every 
excitation  of  the  nerve  is  followed  by  a  swing  of  the  galvanometer 

1  In  Ergebnisse  der  Physiologie,  1903,  II,  part  2,  131  f. 

2  A.  v.  Bayer,  Zeitschrift  /.  allgem.  Physiologie,  1902,  II,  169. 

*  Philosophical  Transactions  of  the  Royal  Society  of  London,  1896. 


138  THE  NERVES  AS  CONDUCTORS 

needle,  or  by  a  movement  of  the  mercury  in  the  electrometer — 
thus  indicating  the  existence  of  an  electric  current  in  the  nerve 
itself.  The  rate  at  which  this  electrical  disturbance  travels  along 
the  nerve  is  the  same  as  the  speed  of  the  nerve-impulse.  Such  a 
current  of  action  is,  indeed,  not  peculiar  to  the  nerve;  it  occurs  in 
muscles,  glands,  the  retina,  and  apparently  in  all  active  living  tis- 
sues. In  most  of  these  other  cases,  there  are  other  manifestations 
of  activity,  and  the  current  of  action  is  usually  regarded  as  a  mere 
by-product.  But  since  the  nerve  shows  no  other  manifestation, 
the  suggestion  has  been  made  by  several  physiologists  that  there  is 
no  other  process;  but  that  the  nerve-impulse  is  essentially  an  elec- 
trical disturbance  moving  along  the  nerve. 

Without  committing  ourselves  to  this  view  of  the  nerve-impulse 
further  than  to  recognize  it  as  a  respectable  hypothesis  with  a  con- 
siderable number  of  facts  in  its  favor,  we  may  employ  it  to  give 
point  to  our  further  consideration  of  the  electrical  phenomena  of 
the  nerve  in  action.  The  study  of  these  phenomena  has  engaged 
the  attention  of  many  able  investigators  and  has  proved  a  fruitful 
field.  To  present  the  subject  fully,  we  should  need  to  enter  much 
more  deeply  into  the  science  of  electricity  and  the  closely  related 
science  of  physical  chemistry  than  can  be  expected  within  the  lim- 
its of  a  brief  discussion.  A  rough  sketch  is  all  that  can  be  here 
attempted. 

§  13.  The  facts  bearing  on  this  subject  may  be  conveniently 
grouped  under  two  heads:  (1)  the  effects  of  the  electrical  current 
on  the  nerve;  and  (2)  the  electrical  phenomena  developed  by  the 
nerve  itself.  To  the  latter  class  belongs  the  current  of  action. 

Let  us  suppose  that  we  have  at  our  disposal,  for  examining  its 
effects  on  the  nerve,  a  current  from  a  zinc-copper  battery,  and  let 
us  follow  the  common  usage,  and  speak  of  the  copper  pole  as  posi- 
tive and  the  zinc  as  negative,  and  of  the  current  as  "flowing,"  out- 
side of  the  battery,  along  any  conductor  which  is  provided,  in  the 
direction  from  copper  to  zinc.  For  applying  the  current  to  the  nerve, 
we  shall  need  to  employ  "  non-polarizable  electrodes."  On  con- 
necting the  positive  or  copper  pole  with  one  point  of  the  nerve,  and 
the  negative  pole  with  another  point,  the  current  may  be  conceived 
of  as  entering  the  nerve  at  the  positive  pole,  or  anode,  and  leaving 
at  the  negative  pole  or  cathode.  A  key  is  provided  in  the  circuit, 
which  enables  us  to  start  and  stop  the  passage  of  the  current,  at 
will.  As  has  already  been  stated,  the  nerve  is  excited  by  the  start- 
ing or  stopping  of  the  current,  but  not,  ordinarily,  by  its  steady  flow. 
The  excitation  is  found  to  occur  at  the  cathode  when  the  current 
is  started,  and  at  the  anode  when  the  current  is  stopped.  In  other 
words,  the  nerve-impulse  starts  at  the  cathode  when  the  circuit  is 


ELECTROTONIC  CHANGES  IN  NERVES  139 

closed  and  at  the  anode  when  the  circuit  is  broken.  Further,  it 
was  shown  by  Pfliiger1  that  the  regions  of  the  anode  and  of  the 
cathode  are  each  in  a  condition  of  altered  excitability  during  the 
passage  of  the  current.  There  is  an  increase  of  excitability  at  and 
near  the  cathode,  and  a  decrease  at  and  near  the  anode.  On  the 
stopping  of  the  current,  a  back  swing  occurs,  making  the  anode 
for  a  time  over-excitable  and  the  cathode  under-excitable.  The 
changes  in  excitability  are  greatest  at  the  poles  and  decrease  gradu- 
ally as  we  move  into  the  extrapolar  regions  and  into  the  region  be- 
tween the  poles;  in  the  latter  region  there  is  an  indifference  point 
where  the  cathodic  effect  merges  into  the  opposite  anodic.  On 
increasing  the  strength  of  the  current,  the  extent  of  the  nerve  af- 
fected increases,  and,  between  the  poles,  the  anodic  depression  en- 
croaches more  and  more  on  the  cathodic  exaltation.  This  depress- 
ing effect  also  occurs  when  the  current  is  prolonged;  the  depression 
then  finally  invades  even  the  region  of  the  cathode.  The  lowering 
of  excitability  at  the  anode  during  the  passage  of  the  current  is 
attended  by  a  lowering  of  the  conductivity  of  the  nerve  for  nervous 
impulses;  the  conductivity  is  so  far  lost  at  the  anode  during  the  pass- 
age of  strong  currents  as  to  form  a  complete  block  to  the  passage 
of  the  impulses  generated  elsewhere.  These  changes  in  the  nerve 
during  the  passage  of  an  electric  current  are  grouped  under  the 
name  "  electrotonus " ;  and  the  anodic  depression  is  sometimes 
known  as  anelectrotonus,  while  the  cathodic  exaltation  is  called 
catelectrotonus. 

§  14.  Electrotonus  may  then  be  defined  as  an  altered  excita- 
bility and  conductivity  of  the  nerve  produced  by  the  passage  of  a 
current  through  some  part  of  its  extent.  But  there  are  also  certain 
purely  electrical  results  from  the  passage  of  the  current,  in  the  form 
of  additional  currents  which  appear  in  the  nerve,  or  on  its  surface; 
and  these  are  called  the  electrotonic  currents.  Such  currents  appear 
in  the  extrapolar  region,  and  their  direction  is  the  same  as  that  of  the 
current  that  is  led  into  the  nerve.  They  cannot,  therefore,  be  mere 
leakages  of  the  exciting  current;  they  are  new  currents  generated  in 
the  nerve  by  the  action  of  this  external  current. 

§  15.  These  electrotonic  currents  have  formed  the  starting-point 
for  an  interesting  line  of  experiments,  the  object  of  which  is  to  de- 
termine whether  the  phenomena  of  nerve  action  can  be  imitated  in 
a  physical  model.  Since  the  model  may  be  regarded  as  an  arti- 
ficial nerve,  just  as  an  arrangement  of  tubes  and  syringes  gives  a 
working  model  of  the  circulatory  system,  it  is  hoped  in  this  way  to 
throw  some  light  on  the  physical  structure  of  the  nerve.  Such  an 
instrument  of  investigation  is  called  a  "core  model"  (see  Fig.  60), 
1  Untersuchungen  iiber  Electrotonus,  Berlin,  1859. 


140  THE  NERVES  AS  CONDUCTORS 

or  a  "core  conductor."  It  consists  of  a  central  core  of  good  elec- 
trical conductivity,  surrounded  by  a  sheath  of  lower  conductivity. 
It  may  be  constructed  of  various  materials;  that  of  Hermann1  con- 
sisted of  a  platinum  wire  (the  core)  stretched  the  length  of  a  glass 
tube  filled  with  a  solution  of  some  salt.  The  glass  tube  was  em- 
ployed simply  for  the  purpose  of  holding  the  "sheath"  of  solution  in 
place,  and  was  provided  with  small  side  tubes  through  which  the 
exciting  current  could  be  led  in,  and  the  electrotonic  currents  (if  any) 
led  out  to  a  galvanometer.  Hermann  found,  in  fact,  that  currents 


B 


FIG.  60. — Hermann's  Core  Model.     AB,  glass  tube;  ab,  platinum  wire; 
c,  d,  e,  f,  g,  h,  side  tubes. 

equivalent  to  those  of  electrotonus  were  produced  in  the  extrapolar 
regions  of  the  core  model  as  in  the  nerve;  he,  accordingly,  con- 
cluded that  the  physical  structure  of  a  nerve  resembles  that  of  the 
core  conductor,  and  that  the  electrotonic  currents  were  purely 
physical  phenomena. 

The  same  line  of  investigation  has  been  carried  further  by  Bo- 
ruttau,2  who  has  found  that,  not  only  the  electrotonic  currents,  but 
all  the  electrical  phenomena  occurring  in  the  nerve  from  the  action 
of  a  current  on  it  can  be  imitated  in  the  core  model.  In  this  way, 
even  the  current  of  action  can  be  imitated;  and  as  this  is  closely 
associated,  in  a  genuine  nerve,  with  the  transmission  of  the  nerve 
impulse,  Boruttau  advances  the  view  that  the  core  conductor,  like 
the  nerve,  can  really  be  "excited";  and  that  the  activity  of  a  nerve 
is  imitated,  in  all  essential  respects,  by  the  action  of  the  core  con- 
ductor. According  to  this  view,  the  nerve  owes  its  functional  power 
to  its  physical  structure,  which  makes  of  it  an  electrical  conductor 
of  a  certain  type — namely,  a  so-called  "core  conductor."  Nerve 
action  would,  then,  not  be  a  catabolic  affair,  but  a  physical  rather 
than  a  chemical  action,  consisting  in  the  propagation  of  a  certain 
electrical  state.  The  electrical  state  may  be  generated  in  the  nerve 

1  Handbuch  der  Physiologic,  vol.  II,  i,  174. 

2  In  several  articles  in  Pfliiger's  Archiv  fur  die  gesammte  Physiologie,  from 
1894  on. 


CURRENT  OF  REST  OR  INJURY  141 

by  various  sorts  of  stimuli,  as  was  mentioned  in  a  preceding  page. 
In  the  core  model  also,  the  electric  wave  can  be  initiated  by  a  me- 
chanical "stimulus"  as  well  as  by  an  electrical;  and  this  fact  rather 
strengthens  the  analogy  between  the  nerve  and  the  core  conductor. 
The  rate  of  propagation  of  the  electric  wave,  in  a  core  conductor, 
varies  according  to  the  materials  of  which  it  is  constructed;  but  it 
may  be  as  low  as  100  metres  per  second,  and  thus  it  is  not  incom- 
parable with  the  speed  of  transmission  in  nerve.  The  fact  that 
electricity  travels  along  a  wire  between  the  poles  of  a  battery  or 
dynamo  at  an  infinitely  faster  rate  is  therefore  no  disproof,  as  was 
formerly  held,  of  the  view  that  the  nerve  impulse  is  essentially  elec- 
trical. 

The  actual  microscopic  structure  of  a  nerve-fibre  affords  some 
reason  for  believing  that  it  may  act  as  a  core  conductor.  There  is 
the  axon,  containing  its  fibrils;  and,  if  the  fibrils  are  better  electrical 
conductors  than  the  protoplasm  which  surrounds  them,  the  axon 
corresponds  in  structure  to  that  of  the  core  model.  Or,  again, 
and  less  hypothetically,  if  the  axon  as  a  whole  is  a  better  conductor 
than  the  myelin  sheath,  then  the  axon  represents  the  core,  and  the 
myelin  sheath  the  less  conductive  envelope. 

§  16.  A  further  set  of  facts  bearing  on  the  theory  of  the  nerve 
has  not  yet  been  touched  upon.  When  a  nerve  is  cut,  and  the  cut 
end  connected  with  one  pole  of  a  galvanometer,  while  the  other  pole 
is  connected  with  the  uninjured  convex  surface  of  the  nerve,  a  cur- 
rent flows  through  the  galvanometer  from  the  convex  surface  to 
the  cut  end.  The  cut  end  is  negative  with  respect  to  the  convex 
surface.  This  current  was  discovered  by  Du  Bois-Reymond,  who 
named  it  the  "  current  of  rest,"  thus  distinguishing  it  from  the  "  cur- 
rent of  action,"  and  indicating  also  his  view  that  the  current  so 
demonstrated  existed  in  the  resting  and  uninjured  nerve.  Her- 
mann showed  good  reason  for  doubting  this  interpretation,  and  for 
believing  that  the  negative  potential  of  the  cut  end  was  a  conse- 
quence of  the  injury  done  it  in  cutting.  A  controversy  waged  for 
years  regarding  this  matter,  one  of  the  most  bitter  controversies 
in  the  history  of  science;  but  the  evidence  adduced  by  Hermann 
gradually  won  the  assent  of  physiologists,  and  the  name  "current 
of  injury"  was  substituted  for  Du  Bois-Reymond's  "current  of 
rest."  Hermann's  conception  was  that  chemical  changes  occurred 
in  the  injured  and  therefore  dying  part  of  the  nerve,  and  that  these 
changes  were  the  cause  of  its  negative  potential.  The  phenomenon 
is  not  peculiar  to  the  nerve,  but  appears  in  all  dying  tissues.  Her- 
mann brought  this  negativity  of  dying  tissues  into  relation  with  the 
negativity  displayed  by  active  tissues  toward  tissues  when  resting. 
The  negative  potential  of  active  tissue  is  revealed  by  the  current  of 


142  THE  NERVES  AS  CONDUCTORS 

action;  for  the  active  portion  of  a  nerve  becomes  negative  to  the 
resting  portion.  Hermann's  views,  which  may  be  said  to  be  the 
views  now  generally  accepted,  are  summed  up  in  the  statements 
that  dying  tissue  is  electro-negative  with  respect  to  living;  and  act- 
ing tissue  is  also  electro-negative  with  respect  to  resting  tissue. 

§  17.  In  spite  of  the  wide-spread  acceptance  of  Hermann's  views, 
very  strong  evidence  has  recently  been  presented  in  favor  of  a  radi- 
cally different  conception  of  the  current  of  rest,  a  conception  which 
has  something  in  common  with  that  of  Du  Bois-Reymond,  but  which 
is  based  on  discoveries  in  physical  chemistry  which  have  been  made 
since  his  time. 

The  considerations  on  which  this  new  conception  is  based  are  the 
following:1  We  have  in  the  nerve-fibre  a  core  containing  a  solu- 
tion of  salts  (the  axon),  surrounded  by  the  myelin  and  primitive 
sheaths,  which  are  known  to  be  electrically  poor  conductors;  while 
outside  of  these  sheaths  is  also  a  solution  of  salts,  the  lymph  which 
bathes  the  nerve  as  it  bathes  the  outside  of  all  the  cells  of  the  body. 
The  sheaths,  therefore,  like  the  bounding  membrane  of  all  living 
cells,  interpose  resistance  to  the  free  diffusion  of  salts  between  the 
solutions  that  are  within  and  those  without.  If,  now,  the  internal 
and  external  solutions  were  different  in  strength  or  in  concentra- 
tion, we  should  have  here  the  making  of  a  "concentration  cell"; 
and,  further,  if  the  separating  membrane  were  ruptured  at  any 
point,  so  that  diffusion  occurred  between  the  external  and  internal 
solutions,  the  diffusion  would  be  attended  by  electric  currents. 
It  is  then  only  necessary  to  suppose  that  the  internal  solution  is 
more  concentrated  than  the  external,  and  the  former  would  be 
negative  with  respect  to  the  latter:  thus  we  should  have  the  exact 
conditions  necessary  to  give  rise  to  the  current  of  rest  or  of  injury, 
as  this  current  is  actually  observed. 

Macdonald2  has  given  definite  evidence  in  favor  of  this  theory 
by  showing  that  the  current  due  to  injury  of  a  nerve  is  increased  by 
weakening  the  external  solution,  and  diminished  or  even  reversed  by 
making  the  external  solution  sufficiently  concentrated.  The  re- 
sults come  out  as  they  would  on  the  supposition  that  the  nerve,  in 
case  of  the  current  of  injury,  acts  as  a  concentration  cell,  with  the 
more  concentrated  solution  normally  inside  the  sheaths.  Such  a 
current  can  be  imitated  by  a  core  model  constructed  of  two  solu- 
tions of  different  concentration  separated  by  a  membrane  (Borut- 
tau).  The  evidence  of  these  experiments  is  therefore  favorable 

1  See  J.  S.  Macdonald,  "The  Injury  Current  of  Nerve,"  in  reports  of  the 
Thompson  Yates  Laboratories,  Liverpool,  1902,  III,  213-347;  Proceedings  of 
the  Royal  Society  of  London,  1900,  LXVII,  310. 

3  Op.  cit.,  pp.  273,  288. 


THEORIES  OF  NERVOUS  FUNCTION  143 

to  the  conception  of  the  nerve  as  a  core  conductor  of  the  construc- 
tion already  described.  The  non-conducting  character  of  the 
myelin  sheath  is  indicated  by  direct  observation.1 

§  18.  According  to  this  view,  which  is  thought  to  explain  the 
electrical  phenomena  as  well  as  the  absence  of  signs  of  catabolism 
in  the  active  nerve,  the  nerve-impulse  is  a  special  sort  of  electrical 
wave  propagated  from  point  to  point  along  the  nerve-fibre,  and  capa- 
ble of  exciting  a  muscle-fibre  where  the  terminations  of  the  axon 
come  into  close  contact  with  the  muscular  substance.  The  ob- 
jection to  this  theory  which  is  derived  from  the  fact  that  a  dead 
nerve  will  not  conduct,  may  be  met  by  maintaining  that  a  physical 
structure  which  is  capable  of  transmitting  the  electrical  wave  de- 
pends upon  the  life  of  the  fibre. 

While,  then,  certain  competent  authorities  still  oppose  this  theory 
and  it  would  be  out  of  place  in  a  book  like  this  to  take  sides  in  a 
controversial  matter,  it  must  be  admitted  that  it  accounts  for  more 
of  the  very  puzzling  phenomena  than  do  any  other  of  the  present 
theories  of  conduction  in  the  nerves.  It  may,  then,  properly  serve  as 
a  hypothesis  about  which  to  gather  the  principal  facts  of  the  physi- 
ology of  nerves.  At  any  rate,  the  only  rival  view,  at  the  moment, 
seems  to  be  a  chemical  theory,  which  must  either  controvert  the 
doctrine  of  the  conservation  of  energy,  or  else  admit  such  an  amaz- 
ing ability  for  promptness  and  completeness  of  metabolism  as  it 
is  difficult  to  imagine.  To  suppose  the  nerve-fibre  capable  of  in- 
stantaneously recombining,  without  any  detectable  loss  of  energy, 
the  elements  which  have  been  separated  by  the  work  done  through 
hours  of  continuous  functioning,  is  to  convert  it  into  a  wonderful 
kind  of  laboratory.  But  it  can  scarcely  be  denied  that  nature  may 
impart  such  a  capacity  to  such  a  kind  of  living  tissue.  In  a  word, 
then,  the  physico-chemical  theory  of  the  process  in  which  consists 
the  impulse  passing  along  an  excited  nerve,  is  still  in  need  of  pro- 
longed and  careful  investigation.2  But  whatever,  more  precisely, 

1  Gothlin,  Upsala  Lakareforenings  Forhandlingar,  1902,  VIII,  156  ff. 

3  It  seems  to  us  that  there  is  no  incompatibility  between  the  two  theories 
which  have  been  presented  in  this  chapter  and  which  are  customarily  looked 
upon  as  rival  ways  of  explaining  the  phenomena.  On  the  contrary,  both  the 
classes  of  phenomena  to  be  explained,  and  also  the  theories  that  are  set  forth 
in  their  explanation,  are  supplementary  and  necessary  to  be  combined  in 
order  to  account  for  all  the  facts.  It  will  have  been  noticed  that  in  the  model 
core  conductor,  as  in  the  nerve  itself,  one  of  the  materials  used  in  its  con- 
struction is  a  saline  solution.  If  this  were  not  so,  the  phenomena  could  not 
be  obtained.  But  saline  solutions  are  electrolytic  conductors;  and  electrol- 
ysis is  the  chemical  change  which  is  analogous  to  catabolism  in  living  tissues. 
It  is  perfectly  possible,  however,  to  construct  an  electrolytic  conductor  which, 
on  passing  a  weak  current  through  it,  and  yet  a  current  far  within  the  limits 
of  detection  by  a  galvanometer,  will  give  no  detectable  evidence  of  any  chemi- 


144  THE  NERVES  AS  CONDUCTORS 

this  impulse  is  found  to  be,  there  is  no  doubt  that  conduction  is 
the  fundamental  function  of  nervous  tissue;  and  all  its  other  functions 
are  secondary  and  derived. 

cal  or  thermal  change  along  its  course — except  at  the  poles.  Even  here,  the 
chemical  changes  may  be  so  small  that  it  would  take  thousands  of  years  for  it 
to  disengage  from  one  pole,  and  transfer  to  the  other,  a  single  cubic  centimetre 
of  copper;  but  all  the  while  chemical  changes,  too  small  to  be  detected,  would 
be  taking  place  along  the  entire  stretch  of  the  conducting  medium. 

Still  further,  it  is  not  proper  to  speak  of  what  does  take  place  in  an  electro- 
lytic conductor  as  the  passage  of  a  mere  wave  of  electricity.  It  is  doubtful 
whether  there  can  be  any  such  "mere  wave"  in  any  material  structure. 

Now,  we  know  that  catabolism  does  take  place  in  the  muscle-fibre;  and 
there  is  sufficient  evidence  to  show  that  it  also  takes  place  in  the  nerve-cells 
of  the  central  organs.  These  are  the  terminals,  between  which  the  nerve- 
fibres  are  conductors.  The  analogy,  we  repeat,  seems  to  suggest  a  combination  of 
both  theories. 


CHAPTER  VII 
REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

§  1.  When  a  stimulus  acting  on  a  sense-organ  arouses  to  activity 
a  muscle,  by  the  medium  of  a  sensory  nerve,  a  nervous  centre,  and 
a  motor  nerve,  the  entire  transaction  is  said  to  be  reflex;  and  its 
paHTls"  called  a  "  reflex  arc."  In  the  case  of  the  lower  forms  of 
animal  life  we  have  already  seen  how  the  simplest  reactions  of  an 
organism  which  depend  on  nervous  action  are  of  the  reflex  type. 
The  reflex  is,  therefore,  the  unit  or  element  of  nervous  function. 
(Compare  Fig.  61). 

§  2.  In  the  case  of  the  higher  animals,  or  of  man,  there  are  many 
prompt  reactions  to  stimuli  which,  because  they  involve  either  voli- 
tion, or  previous  learning,  or  both,  are  not  usually  classed  as  re- 
flexes. In  reading  aloud,  for  example,  the  reaction  conforms  to 
the  neurological  type  of  a  reflex;  since  a  stimulus  to  the  eye  arouses 
promptly  a  response  of  the  vocal  muscles;  but  the  reaction  has  been 
learned.  A  true  reflex  should  be  not  learned,  but  innate.  A  good 
example  of  the  true  reflex  is  afforded  by  the  contraction  of  the  pupil 
in  response  to  bright  light  entering  the  eye.  This  is  not  an  ac- 
quired reaction,  nor  is  it  dependent  on  the  will.  Of  some  of  these 
true  reflexes  we  are  wholly  unconscious;  in  the  case  of  others,  such 
as  flushing,  shivering,  starting,  the  secretion  of  saliva,  we  are  aware 
of  their  occurrence,  but  have  no  voluntary  control  over  them;  in 
the  case  of  still  others,  such  as  coughing,  sneezing,  winking,  we  have 
some  degree  of  control,  and  yet  there  can  be  no  doubt  that  they  were 
never  learned  by  the  individual.  There  are  yet  other  reactions,  of 
which  a  good  example  is  afforded  by  the  turning  of  the  eyes  toward 
any  "attractive"  object,  which  appear  at  a  very  early  age  in  the 
infant,  and  without  any  evidence  that  they  are  learned;  but  which 
are  very  closely  interwoven  with  our  conscious  life,  and  which  are 
controllable,  to  a  large  extent,  by  the  adult  animal.  There  is  thus 
a  graded  series  of  reactions  to  stimuli,  which  are  alike  in  being  native 
to  the  individual,  but  which  vary  in  their  relations  to  consciousness. 
The  most  satisfactory  basis  for  classifying  reflexes  would  seem, 
then,  to  be  found  in  the  history  of  the  reaction  in  the  individual — 
according  as  it  is  innate  or  has  to  be  learned. 

145 


146    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

Another  graded  series  of  reflexes  can  be  arranged  on  the  basis 
of  their  complexity.  The  pupillary  reflex,  for  example,  is  about  as 
simple  and  strictly  local  as  any;  only  one  or  two  muscles  are  called 
into  action.  Much  the  same  is  true  of  winking  and  of  rotations  of 
the  eyes.  Coughing,,  by  comparison,  is  highly  complicated;  in 
this  reflex,  a  strong  inspiration  is  followed  by  forced  expiration,  at 
first  against  the  closed  glottis,  which  then  is  suddenly  opened,  al- 
lowing the  air  to  escape  with  a  rush.  Some  reflexes  call  into  action 
only  one  of  the  limbs,  whereas  others  involve  all  four  limbs,  the 


FIG.  61. — Diagram  of  a  Reflex  Arc. 

trunk,  and  perhaps  the  head  besides.  Since  it  is  impossible  to 
draw  any  sharp  line  between  the  most  simple  and  the  more  com- 
plex, the  degree  of  complexity  cannot  be  used  as  the  basis  for  sepa- 
rating reflexes  from  other  reactions. 

§  3.  Instinct,  also,  is  a  word  which  cannot  be  sharply  distin- 
guished from  the  type  of  reactions  called  reflex.  Both  have  in  com- 
mon the  notion  of  innate,  as  distinguished  from  learned,  reaction. 
"Instinct,"  however,  is  seldom  applied  to  such  simple  reactions  as 
that  of  the  pupil  to  light;  it  is  most  frequently  applied  to  complex 
series  of  reactions,  like  those  of  birds  in  nest-building,  or  of  frogs  in 
hibernating.  It  is  also  probable  that  such  typical  instincts  are  at- 
tended by  much  of  consciousness,  and  even  of  desire.  But,  as  we 
have  seen,  neither  in  terms  of  the  attendant  consciousness  nor  in 
terms  of  complexity,  is  it  anywhere  possible  to  draw  a  sharp  line; 
and  instinct  and  reflex  may  best  be  regarded  as  synonymous  terms, 
definable  as  innate  reactions  to  stimuli. 

The  conception  of  a  "simple  reflex,"  or  better,  perhaps,  of  an 
"isolated  reflex,"  should  also  be  considered  in  this  connection. 
The  point  here  is  not  that  the  movement  evoked  shall  be  as  lim- 
ited and  free  from  complexity  as  possible,  but  that  it  shall  run  its 
course  uncomplicated  with  other  reactions  that  may  be  occurring 
at  the  same  time.  In  any  complex  nervous  mechanism,  there  is 
much  going  on  simultaneously;  many  receptors  are  undergoing 


INSTINCTIVE  AND  SIMPLE  REFLEXES  147 

/ 

stimulation,  and  many  effectors  are  acting  at  once.  A  quite  simple 
reflex  would,  therefore,  be  one  which  went  through  without  inter- 
fering with  other  reactions,  or  being  interfered  with  by  them.  This 
requires  that  a  certain  reflex  centre,  starting  from  a  condition  of 
rest,  shall  be  acted  on  by  one  group  of  ^enspry  fibre.s,  and  shall  dis- 
charge into  a  group  of  motor  fibres,  the  centre  meanwhile  remaining 
unaffected  by  influences  from  any  other  parts  of  the  system.  But 
such  simplicity  is  seldom,  if  ever,  realized.  Every  centre  is  con- 
tinually subjected  to  some  degree  of  excitation  or  other  influence, 
from  the  periphery;  and  the  centres  are  so  richly  connected  by  fibres 
with  one  another,  that  mutual  influences  are  probably  always  in 
operation.  Hence  Sherrington1  has  called  the  "simple  reflex"  a 
convenient  but  artificial  abstraction,  and  has  emphasized  the  fact 
that  "the  nervous  system  functions  as  a  whole";  while  Dewey2  di- 
rects attention  to  the  psychological  errors  which  result  from  treating 
this  abstraction  as  an  actuality  and  building  a  psychology  on  it. 

In  distinguishing  purely  reflex  from  learned  and  from  voluntary 
reactions,  we  need  to  notice  that  many,  or  perhaps  all,  reactions 
of  the  higher  class  are  based  on  reflexes,  and  include  reflex  elements. 
It  is  within  the  power  of  an  adult  to  direct  his  eyes  on  an  object 
at  will;  but  in  so  doing  he  makes  use  of  the  same  movement  which 
appears  reflexly  in  the  infant.  A  similar  combination  of  volitional, 
habitual,  and  instinctive  factors  is  seen  in  speaking,  chewing,  or 
walking.  Reflex  elements  are  therefore  more  common  in  human 
action  than  would  at  first  appear;  and  a  knowledge  of  the  reflexes 
is  important  in  the  psychology  of  human  behavior,  as  well  as  in 
understanding  the  inner  mechanism  of  the  nervous  system. 

§  4.  The  reflex,  it  has  been  stated  above,  is  the  unit  of  ner- 
vous function.  There  is,  indeed,  another  possible  unit — another 
sort  of  functioning  of  the  nerve-centres,  which  is  known  by  the  name 
of  automatic  action.  In  this,  the  centre  discharges  into  its  motor 
nerves,  without  itself  receiving  any  stimulus,  either  from  a  sense- 
organ  or  from  any  other  part  of  the  nerve-centres.  Such  activity 
is  supposed  to  originate  within  the  centre.  Its  cause  may  be  sought 
either  in  the  inner  metabolism  of  the  centre;  or  in  stimuli  acting 
directly  on  the  centre,  as  for  example  from  the  chemical  action  of 
the  blood.  In  these  cases,  the  centre  itself  would  seem  to  play  the 
part  of  a  receptor.  For  a  time,  the  concept  of  "automatic"  action 
seemed  to  fall  into  discredit;  since  it  was  found  that  certain  reactions 
which  had  been  considered  such  were  really  reflex.  For  example, 
the  convulsions  which  result  from  strychnine  poisoning,  and  which 

1  The  Integrative  Action  of  the  Nervous  System,  1906,  p.  114. 

2  "The  Reflex  Arc  Concept  in  Psychology,"  Psychological  Review,  1896,  III, 
357. 


148    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

were  formerly  attributed  to  automatic  discharge  of  the  motor  cells 
of  the  spinal  cord,  do  not  take  place,  if  all  the  sensory  nerves  lead- 
ing to  the  spinal  cord  are  severed.1 

One  of  the  most  probable  instances  of  automatic  action  occurs 
in  the  respiratory  centre  in  the  bulb.  It  is  certain  that  venous 
blood — i.  e.,  blood  poor  in  oxygen  and  rich  in  carbon  dioxide — acts 
as  a  stimulant  to  this  centre,  raising  its  irritability  and  thus  increas- 
ing respiration.  When  the  blood  circulating  through  the  bulb  is 
abnormally  rich  in  carbon  dioxide,  the  rate  of  breathing  is  increased; 
and  when  the  blood  is  poor  in  carbon  dioxide,  the  rate  of  breathing  is 
slackened.  These  facts  have  led  to  the  view  that  the  normal  stimulus 
to  respiration  is  venous  blood,  and  especially  the  carbon  dioxide  in 
such  blood,  which,  by  circulating  through  the  medulla,  directly 
excites  the  respiratory  centre.  More  precisely,  the  centre  is  con- 
ceived as  automatic,  at  least  so  far  as  concerns  inspiration;  expi- 
ration, in  quiet  breathing,  is  mere  passive  relaxation;  and  the  cause 
of  the  relaxation  has  been  traced  to  sensory  impulses  reaching  the 
centre  through  the  tenth  or  vagus  nerve.  The  fibres  concerned  in 
expiration  come  from  the  lungs,  and  are  excited  by  the  distension 
of  the  lungs  which  occurs  in  inspiration.  The  effect  of  these  sen- 
sory impulses,  themselves  resulting  from  inspiration,  is  to  "inhibit'* 
the  centre,  and  check  inspiration,  thus  giving  rise  to  passive  ex- 
piration. According  to  this  view,  inspiration  would  be  an  auto- 
matic movement,  but  expiration  a  reflex.  But  even  inspiration 
can  be  shown  to  be  sometimes  the  result  of  sensory  stimuli,  origi- 
nating in  the  lungs  or  elsewhere;  so  that  the  possibility  remains 
that  inspiration  may  be,  in  part  at  least,  a  reflex  function.2 

§  5.  Very  recently,  a  strong  support  to  the  theory  of  automatic 
functions  has  come  from  the  discoveries  of  physiological  chemistry. 
It  is  found  that  certain  chemical  compounds,  called  "hormones" — 
especially  those  formed  by  the  internally  secreting  glands,  and  some 
which  can  even  be  made  synthetically — when  introduced  into  the 
blood,  have  the  power  to  select  definite  tissues  in  the  animal  organ- 
ism, and  produce  directly  in  them  specific  reactions  of  a  type  neces- 
sary (some  of  them,  absolutely)  to  the  maintenance  of  the  physio- 
logical system.  If  this  is  true  of  other  than  the  nerve  tissues,  it 

1  H.  E.  Hering,  in  Archiv  fur  experimentelle  Pathologic  und  Pharmakologie, 
1896,  XXXVIII,  276. 

2  For  a  fuller  discussion  of  this  question,  the  reader  may  be  referred  to  the 
text-books  of  physiology,  and  to  the  authorities,  especially  to  Rosenthal  (Die 
Athembewegungen,  1865),  who  supported  the  automatic  conception  by  isolating 
the  bulb — though  not  quite  completely — from  the  sensory  nerves,  -and  finding 
that  a  slow  and  imperfect  respiration  persisted  in  spite  of  this  operation;   and 
to  Head  (Journal  of  Physiology,  1889,  X,  1-70,  279-290),  whose  work  went 
far  toward  making  the  reflex  conception  seem  probable. 


THE  GANGLIONIC  REFLEXES  149 

would  seem  remarkable  that  the  capacity  for  such  special  automatic 
reactions  should  be  denied  to  the  tissues  of  the  central  nervous 
system.  All  our  theory  of  the  specialized  functions  of  this  system, 
especially  in  its  higher  forms  of  development  and  of  activity,  seems, 
therefore,  to  favor  the  opinion  that  it  is,  among  all  the  organic 
structures,  pre-eminently  automatic.  It  is,  therefore,  highly  proba- 
ble that  the  reflex  and  the  automatic  forms  of  its  functioning  are 
most  frequently,  if  not  uniformly,  combined  in  ever-varying  pro- 
portions. 

§  6.  From  these  preliminary  considerations,  we  may  turn,  first, 
to  a  brief  survey  of  the  reflexes  observed  in  mammals,  and,  after 
that,  to  a  discussion  of  the  laws  of  reflex  action  in  general. 

In  beginning  a  survey  or  inventory  of  reflexes,  attention  should 
first  be  given  to  a  class  of  actions  which  may  be  even  simpler  than 
reflexes.  An  example  is  the  beating  of  the  heart.  Since  the  heart 
is  supplied  with  nerves,  the  early  assumption  was  that  the  heart- 
beat is  a  reflex.  Later,  it  was  discovered  that  all  of  these  nerves 
could  be  cut,  or  the  heart  taken  entirely  out  of  the  body,  without 
destroying  the  power  of  the  heart  to  beat.  An  excised  heart,  if 
supplied  with  suitable  blood,  will  continue  to  beat,  of  itself,  for 
hours.  Since  the  walls  of  the  heart  contain  nerve-fibres  and  ganglia 
of  nerve-cells,  the  next  view  adopted  was  that  the  reflex  was  strictly 
local,  the  ganglia  acting  as  reflex  centres.  Another  theory  was  pro- 
posed by  Gaskell  and  by  Engelmann.1  According  to  this  theory, 
the  contractions  of  the  heart  muscle  are  essentially  independent 
of  any  nervous  influence;  or,  in  other  words,  the  heart  muscle  is 
automatic,  depending  only  on  chemical  or  other  stimuli  acting  di- 
rectly on  the  muscle  itself.  This  view  was  supported  by  the  ob- 
servation that  pieces  of  the  heart  muscle,  so  cut  as  to  contain  no 
nerve-cells,  showed  the  rhythmical  beat.  The  evidence  is  perhaps 
less  convincing  now  than  formerly,  since  the  advance  of  histological 
study  has  shown  the  existence  of  minute  nerve  ganglia  in  parts  of 
the  muscle  which  were  formerly  thought  to  be  free  from  them. 
There  is,  however,  no  inherent  improbability  in  the  view  that  the  heart 
muscle  has  the  power  of  reacting  directly — and  rhythmically — to 
chemical  stimuli  affecting  it;  and  it  is  said  that  the  heart  commences 
to  beat,  in  the  embryo,  before  the  growth  of  nervous  tissue  into  it. 
Essentially  the  same  things  may  be  said  in  regard  to  the  movements 
of  other  internal  organs,  such  as  the  stomach  and  intestines,  ureter, 
etc.  This  view  would  accord,  too,  with  the  facts  mentioned  in  the 
preceding  article. 

1  Gaskell,  Philosophical  Transactions,  1882,  p.  993;  Engelmann,  Pfluger's 
Archiv  fur  die  gesammte  Physiologic,  1882,  XXIX,  425;  and  many  other  papers 
by  each  of  these  authors. 


150    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

§  7.  The  appearance  of  the  chain  of  sympathetic  ganglia  is  such 
as  to  suggest  that  it  is  a  series  of  connected  reflex  centres,  much  like 
the  ganglionic  chain  of  a  worm.  It  was  long  accepted,  as  a  matter 
of  course,  that  this  appearance  furnished  a  correct  view  of  the  func- 
tions of  the  entire  sympathetic  system.  The  fibres  of  this  system 
are  ultimately  distributed  to  the  heart,  stomach,  intestines,  and  other 
internal  organs;  and  to  the  walls  of  the  blood-vessels,  the  iris  of  the 
eye,  the  sweat-glands,  and  the  cutaneous  muscles  which  erect  the 
hairs.  Associated  with  the  sympathetic  fibres  in  the  control  of 
these  organs  are  other  nerve-fibres  which  issue  from  the  brain  and 
cord,  but  which  do  not  pass  to  the  sympathetic  chain,  though 
they  do  make  connections  with  other  more  scattered  ganglia. 
This  whole  system  of  nerves  has  been  named  the  "autonomic 
system."1  It  is  characteristic  of  the  organs  supplied  by  the  fibres 
of  this  system  that  they  have  a  large  measure  of  local  automatism, 
as  was  noted  above  in  the  case  of  the  heart.  Yet  they  are  all  also 
subject  to  reflex  effects.  The  problem  is,  therefore,  to  determine 
the  reflex  centres  concerned.  Are  they  located  in  the  sympathetic 
ganglia  themselves?  Goltz  and  Ewald,2  in  some  remarkable  ex- 
periments in  which  they  removed  the  spinal  cord — leaving  the  upper 
part,  or  that  necessary  for  breathing — from  dogs,  which  were  kept 
alive  for  a  long  period  after  the  operation,  found  indeed  that  the 
actions  of  many  of  these  organs — for  example,  the  evacuation  of 
the  bladder  and  rectum,  and  the  bearing  and  nursing  of  young — 
were  retained.  But  they  found  also  that  the  action  of  these  organs 
was  no  longer  influenced  by  stimuli  acting  on  distant  parts  of  the 
body.  In  short,  there  was  evidence  of  the  automatic  action  of 
these  organs,  but  not  of  reflex  action  on  them  exerted  through  the 
sympathetic  ganglia.  More  minute  experiments3  have  shown  that 
it  is  impossible  to  secure  long-distance  reflexes  through  the  medium 
of  the  sympathetic  chain.  For  such  reflexes,  the  connections  of 
the  sympathetic  with  the  cord  are  essential.  It  is  evident,  accord- 
ingly, that  the  older  conception  of  the  sympathetic  as  a  relatively 
independent  reflex  centre,  charged  with  the  co-ordination  of  the 
more  "vegetative"  functions  of  the  organism,  must  be  abandoned. 
The  centre  for  such  co-ordination  is  to  be  sought  in  the  central 
nervous  system,  of  which  the  sympathetic  is  but  an  adjunct.  It 
is  even  doubtful  whether  limited  local  reflexes  occur  by  way  of  the 
sympathetic  ganglia.  The  latter  are,  more  probably,  not  reflex 


1  See  the  article  by  Langley  in  Schafer's  Textbook  of  Physiology,  1900,  vol. 
II,  pp.  616-696. 

2  Pfliiger's  Archiv  fur  die  gesammte  Physiologic,  1896,  LXIII,  362. 

3  See  Langley,  op.  cit. 


REFLEXES  OF  THE  SKELETAL  MUSCULATURE      151 

centres  at  all,  but  simply  relay  stations  in  the  path  of  the  outgoing 
fibres  to  the  organs  mentioned.1 

§  8.  In  contrast  to  the  musculature  of  the  internal  organs,  what 
is  called  the  skeletal  or  "voluntary"  musculature  does  not  have 
local  automatism  and  is  not  controlled  by  the  fibres  of  the  sympa- 
thetic system.  Its  nerve  supply  comes  directly  from  the  cord  and 
brain-stem,  on  which  it  is  dependent  for  all  normal  stimuli  to  ac- 
tivity. If  severed  from  the  central  nervous  system,  the  skeletal 
musculature  becomes  incapable  of  functioning,  and  even  degener- 
ates and  atrophies,  thus  losing  entirely  the  characteristics  peculiar 
to  it.  The  muscles  of  the  limbs  are  therefore  dependent  on  spinal 
reflex  action  for  their  very  existence.  They  are  not,  however,  de- 
pendent in  the  same  way  on  the  brain;  but  only  on  the  ventral  horn 
of  the  cord,  from  which  the  motor  nerve-fibres  for  these  muscles 
issue. 

For  an  exact  study  of  the  reflexes  of  this  order,  it  is  necessary 
to  exclude  the  action  of  the  brain  on  the  muscles;  and  for  studying 
the  reflex  powers  of  any  particular  portion  of  the  brain-stem  or 
cord,  it  is  necessary  to  isolate  this  portion  from  the  rest  of  the  cen- 
tral system.  Physiologists  who  investigate  the  laws  of  reflex  action 
begin,  therefore,  by  making  a  reflex  or,  usually,  a  spinal  prepara- 

1  The  nervous  system,  by  the  connections  which  it  establishes  between  dif- 
ferent parts  of  the  body,  is  the  supreme  factor  in  the  co-ordination  or  integra- 
tion that  is  necessary  if  the  body  is  to  maintain  life  and  efficiency.  But  not 
all  co-ordination  is  effected  through  the  nervous  system.  The  chemical  inte- 
gration of  the  body  is  very  largely  accomplished  by  transmission  of  chemical 
substances  from  one  organ  to  another,  through  the  circulation  (see  p.  148). 
Thus,  the  fuel  taken  in  at  a  meal  is,  to  a  large  extent,  stored  in  the  liver,  and 
doled  out  into  the  blood  as  muscular  activity  makes  demand  for  it.  The 
mechanism  by  which  the  demands  of  the  muscles  are  transmitted  to  the  liver 
is  supplied  by  the  circulation.  An  active  muscle  draws  fuel  from  the  blood, 
thus  lowering  the  proportion  of  sugar  in  the  circulation;  and  when  the  blood 
circulating  through  the  liver  has  less  than  a  certain  proportion  of  sugar,  the 
liver  gives  out  enough  to  bring  back  the  proportion  to  the  normal.  There 
are  many  "internal  secretions,"  produced  by  certain  organs,  as  the  adrenal 
bodies,  the  pancreas,  and  the  pituitary  body,  which  pass,  by  the  circulation, 
to  other  organs,  and  affect  their  activity.  One  of  the  best  examples  of  chemi- 
cal co-ordination  is  that  discovered  by  Bayliss  and  Starling  (Journal  of 
Physiology,  1902,  XXVIII,  325.  See  also  Starling,  Recent  Advances  in  the 
Physiology  of  Digestion,  1906),  through  which  the  secretion  of  the  pancreas 
is  excited.  The  hydrochloric  acid  which  forms  part  of  the  gastric  juice,  when 
it  passes  with  the  partly  digested  food  from  the  stomach  into  the  intestine, 
excites  cells  in  the  wall  of  the  intestine  to  secrete  into  the  blood  a  substance, 
called  secretin,  which,  being  carried  by  the  blood  to  the  pancreas,  excites  that 
gland  to  pour  its  secretion  into  the  intestine.  Thus  a  reaction  which  formerly 
was  taken  to  be  a  nervous  reflex  is  seen  to  be  of  quite  a  different  nature.  It 
is  probable  that  this  sort  of  chemical  action  of  one  organ  on  another  is  of  great 
importance  in  the  processes  of  growth  and  metabolism. 


152    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

tion.  For  this  purpose,  the  brain  need  not  actually  be  destroyed; 
but  it  must  be  disconnected  from  the  part  of  the  cord  to  be  exam- 
ined. Since  the  cord  and  brain-stem  afford  the  only  path  of  nervous 
connection  between  the  brain  and  the  cord,  or  between  the  brain 
and  the  muscles,  a  transection  of  the  brain-stem  or  cord,  at  any 
level,  isolates  the  part  below  the  cut  from  the  brain,  and  makes  of 
the  part  thus  severed  from  the  brain  a  "reflex  preparation."  Sev- 
eral varieties  of  such  preparations  have  been  employed.  The  "  spi- 
nal frog"  is  constituted  by  simply  decapitating  the  animal.  Since  the 
frog,  like  other  cold-blooded  animals,  can  maintain  life  without 
breathing,  such  a  spinal  preparation  can  be  kept  alive  for  days  and 
even  for  months.1  In  mammals,  however,  decapitation  is  promptly 
fatal,  unless  artificial  respiration  be  provided;  the  animal  must  also 
be  kept  warm  and  much  loss  of  blood  prevented,  in  order  to  secure 
the  best  results.  With  these  precautions,  a  decapitated  cat  can  be 
kept  alive  for  several  hours,  and  forms  one  of  the  most  useful  of 
preparations  for  the  study  of  reflex  action.2 

Another  very  useful  preparation  is  the  "  decerebrate "  animal, 
which  differs  from  the  spinal  animal  in  that  the  transection  is  made 
through,  or  just  above,  the  mid-brain — thus  leaving  the  cerebellum, 
pons,  bulb,  and  cord  in  continuity,  but  excluding  the  action  of  the 
cerebrum,  thalamus,  and  (usually)  part  of  the  mid-brain.  Goltz 
successfully  performed  decerebration  of  a  dog,  by  removing  the 
cerebrum  in  three  operations,  allowing  the  animal  to  recover,  as 
far  as  possible,  from  each  operation  before  resorting  to  the  next. 
In  this  way  he  kept  one  animal  alive  for  18  months.3 

Sherrington  has  found  that,  if  care  is  taken  to  prevent  loss  of 
blood  and  of  bodily  heat,  a  decerebrate  dog  or  cat  can  be  easily  pre- 
pared and  kept  alive  for  several  hours,  during  which  time  it  shows 
many  reflex  phenomena.4 

§  9.  The  different  separate  parts  of  the  cord  can  be  isolated  by 
similar  methods.  In  the  frog,  for  example,  a  transection  of  the  cord 
in  the  middle  of  the  back,  secures  a  reflex  preparation  consisting 
principally  of  the  hind-limbs  and  their  nerves,  together  with  the 
part  of  the  cord  which  receives  these  nerves.  Similarly,  two  cuts 
across  the  frog's  cord,  just  above  and  just  below  the  exit  of  the 
brachial  nerves,  isolate  a  fore-limb  preparation;  and  both  of  these 
fractions  of  the  animal  show  reflex  activity.  In  mammals,  it  is 
common  to  sever  the  cord  below  the  exit  of  the  phrenic  nerve,  a  pro- 

1  See  especially  the  work  of  Schrader,  Pfliiger's  ArcMv  fur  die  gesammte 
Physiologic,  1887,  XLI,  82,  and  1888,  XLIV,  175. 

2  Sherrington,  Journal  of  Physiology,  1909,  XXXVIII,  375. 

3  Goltz,  Pfliiger's  Archiv  fur  die  gesammte  Physiologic,  1892,  LI,  570. 

4  Sherrington,  Proceedings  of  the  Royal  Society,  1896,  LX,  411. 


LOCAL  REFLEXES  OF  THE  SPINAL  CORD       153 

cedure  which  allows  respiration  to  persist  and  the  animal  to  be  kept 
alive  for  months,  while  at  the  same  time  isolating  the  lower  two- 
thirds  of  the  cord  as  a  reflex  preparation.  Such  "spinal  dogs"  show 
a  variety  of  reflexes.  A  second  transection  can  be  made  at  some 
distance  below  the  first;  the  animal  then,  while  remaining  a  unit 
so  far  as  concerns  digestion  and  circulation,  is  divided  into  three, 
so  far  as  concerns  the  nervous  system.  The  "fore-dog"  includes 
the  head  and  the  fore  limbs,  and  retains  brain  action;  the  "mid- 
dog"  consists  of  the  trunk  (or  a  large  part  of  it)  with  its  nerves  and 
the  central  portion  of  the  cord;  while  the  "hind-dog"  consists  of  the 
hind-limbs,  tail,  and  pelvic  region,  with  its  nerves  and  the  lower  por- 
tion of  the  cord.  In  the  case  of  such  a  preparation,  the  "hind- 
dog"  shows  many  reflexes;  the  "mid-dog" — in  accordance  with 
the  small  variety  of  trunk  movements — shows  comparatively  few 
reflexes;  while  the  "fore-dog,"  retaining  as  it  does  the  brain  intact, 
is  a  "normal"  as  opposed  to  a  "reflex"  animal,  and  behaves  about 
as  dogs  usually  do,  except  that  it  receives  no  sensations  from  the 
middle  and  rear  portions  of  its  body,  and  has  no  control  over  the 
movements  of  those  parts.1 

In  the  human  subject,  accidents  to  the  spine  sometimes  sever 
the  cord,  leaving  the  lower  portion  isolated  from  the  brain,  as  in 
the  animal  preparations;  and  hemorrhage  more  frequently  inter- 
rupts the  motor  pathway  from  the  cerebrum  to  the  brain-stem,  leav- 
ing a  condition  of  the  cord  which  corresponds,  in  some  respects, 
to  that  of  the  decerebrate  animal.  The  discoveries  with  regard 
to  the  reflex  functions  of  the  nervous  system  have,  however,  been 
chiefly  obtained  from  the  animal  preparations. 

§  10.  One  of  the  most  striking  results  of  experiment  is  the  fact 
that  reflexes  are  elicited  from  very  short  lengths  of  the  cord,  pro- 
vided the  sensory  and  motor  nerves  attached  to  the  part  in  question 
are  intact.  In  the  frog,  for  example,  the  "flexion  reflex"  of  the 
hind  leg,  which  occurs  when  one  foot  is  gently  pinched,  consists  of  a 
drawing-up  of  the  whole  leg,  with  flexion  at  hip,  knee,  and  ankle. 
This  is  obviously  a  highly  co-ordinated  movement,  since  it  brings 
several  muscles  into  play;  it  has  also  the  character  of  "purpos- 
iveness"  or  utility,  being  in  fact  a  protective  reflex.  Another  ex- 
ample obtained  from  the  frog  preparation  is  the  so-called  "clasp 
reflex"  which  involves  both  of  the  fore-limbs.  This  reflex  is  ob- 
tained from  a  fore-limb  preparation  of  the  male  frog,  by  touching 
the  skin  of  the  chest,  and  consists  of  a  clasping  action.  In  the 
normal  frog,  this  action  is  elicited  only  by  the  presence  of  the  female 
frog  at  the  breeding  season;  but  in  the  reflex  preparation  it  can  be 

1  Goltz  and  Ewald,  Pftiiger's  Archiv  fur  die  gesammte  Physiologie,  1896,  LXIII, 
362. 


154    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

elicited  by  so  indifferent  a  stimulus  as  a  mere  touch  by  the  finger. 
Like  the  "flexion  reflex/'  it  is  clearly  co-ordinated  and  "purposive." 
The  characteristic  of  purposiveness  is  in  fact  true  of  reflexes  in 
general. 

The  best  interpretation  of  all  such  reflexes  assumes  that,  the 
sensory  and  motor  nerves  to  a  part  being  intact,  and  likewise  the 
central  connections  between  them,  the  reflex  is  to  be  expected 
as  a  matter  of  course.  The  results  show  that  the  necessary  connec- 
tion between  the  sensory  and  motor  nerves  to  a  given  part  is  estab- 
lished at,  or  very  near,  their  place  of  entrance  into  the  cord.  Such 
a  connection  is  what  would  be  anticipated  from  the  minute  anatomy 
of  the  sensory  and  motor  fibres  of  the  cord  (see  p.  89);  since  some 
branches  of  the  sensory  fibres  bend  immediately  into  the  ventral 
horn  of  the  gray  matter,  from  which,  in  turn,  issue  motor  fibres  that 
pass  directly  out  of  the  cord  into  the  motor  roots.  These  short  re- 
flex pathways  may  be  called  local  arcs,  and  their  reflexes  local  re- 
flexes. The  most  local  of  all  reflex  arcs  are  those  which  start  in 
a  given  muscle  and  lead  back  to  the  same  muscle.  The  receptors 
concerned  in  such  cases  are  sense-organs  situated  in  the  muscle  and 
its  tendon,  and  excited  by  movements,  active  or  passive,  of  the  mus- 
cle itself;  they  may  be  called  proprioceptors.  This  is  to  say  that 
movements  and  tensions  within  a  muscle  act  reflexly  on  the  muscle 
itself,  causing  it,  according  to  the  exact  nature  of  the  stimulus,  to 
contract,  or  to  relax,  or  to  maintain  the  degree  of  contraction  which 
it  already  has.1  Similar  reflexes  are  important  in  maintaining 
posture  against  the  action  of  gravity;  and  also  in  giving  steadiness 
and  persistence  to  muscular  action  in  general. 

§  11.  Among  other  local  reflexes  may  be  mentioned  the  "wiping" 
reflex  of  the  frog,  which  is  excited  by  placing  a  bit  of  paper  moistened 
in  weak  acid  on  the  skin  of  the  flank;  the  leg  of  the  same  side  is 
then  brought  up  and  brushed  across  the  place  stimulated;  and  this 
action  is  repeated  a  number  of  times.  If  the  leg  of  the  same  side 
is  held,  and  the  stimulus  continued,  the  leg  of  the  other  side  is 
brought  up  and  across,  to  perform  the  wiping  movement.  In  the 
spinal  dog  may  be  mentioned  the  "stepping"  or  "marking-time" 
reflex,  which  is  aroused  by  supporting  the  animal  under  the  arms 
and  letting  the  hind  legs  hang  down.  In  this  case,  the  weight  of 
the  legs  seems  to  constitute  the  stimulus;  and  the  reaction  is  an  al- 
ternate raising  and  lowering  of  the  two  hind  legs  as  in  normal  walk- 
ing. The  "  extensor  thrust "  (Sherrington)  is  a  brief  but  strong  push- 
ing down  of  the  foot  when  pressure  is  exerted  upward  on  the  sole. 

Sherrington,  "On  Plastic  Tonus  and  Proprioceptive  Reflexes,"  Quarterly 
Journal  of  Experimental  Physiology,  1909,  II,  108;  and  Integrative  Action  of 
the  Nervous  System,  1906,  pp.  129  ff. 


LOCAL  REFLEXES  OF  THE  SPINAL  CORD          155 

Other  reflex  arcs  pass  from  receptors  in  one  hind  leg  to  muscles  in 
the  other,  or  from  the  tail  to  the  hind  legs.  There  are  also  vaso- 
motor  and  visceral  reflexes,  obtainable  from  the  " hind-dog";  some 
of  them,  such  as  micturition  and  defecation,  are  accompanied  by 
suitable  movements  or  postures  of  the  limbs. 

From  a  " mid-dog"  preparation  can  be  obtained  such  reflexes 
as  shivering,  curving  of  the  trunk  toward  the  side  stimulated,  and 
shaking  of  the  trunk.  From  an  isolated  fore-limb  preparation  can 
be  obtained  reflexes  similar  to  those  of  the  hind-limbs.  In  case  of 
the  monkey,  even  more  varied  and  detailed  reflexes  can  be  ob- 
tained. In  man,  reflexes  are  obtained  which  are  on  the  whole 
similar  in  character  to  those  seen  in  the  lower  animals;  but  the 
reflex  activity  of  severed  portions  of  the  cord  is  usually  com- 
paratively small  in  man;  perhaps,  in  part,  because  of  the  greater 
roughness  and  severity  of  the  accidental,  as  compared  with  the  ex- 
perimental severing  of  the  cord;  but  more  probably,  because  of  the 
greater  importance,  in  man's  case,  of  the  dependence  of  the  cord 
on  the  brain  and  the  receptors  of  the  head. 

§  12.  If  longer  portions  of  the  cord,  or  if  the  whole  cord,  be  left 
in  continuity,  there  are  added  to  the  local  reflexes  others  involving 
longer  arcs,  and  the  co-ordination  of  more  muscles.  Thus,  if 
the  cord  is  transected  at  about  the  level  of  the  shoulders,  the 
"scratch  reflex"  of  the  hind  leg  can  be  aroused  by  tickling  or  prick- 
ing the  skin  as  far  forward  as  nearly  to  the  fore-limbs.  If  the  whole 
cord  is  left  in  continuity,  pinching  the  pinna  of  the  ear  may  evoke 
a  combination  of  movements  of  the  ear,  neck,  all  four  limbs,  trunk, 
and  tail.  These  larger  movements  are,  like  the  simpler  local  re- 
flexes, co-ordinated;  they  often  have  either  a  protective  or  a  loco- 
motor  character.  The  movements  of  the  four  limbs  are  frequently 
combined  as  in  the  trotting  common  to  quadrupeds.  Such  loco- 
motor  movements  are  not  very  efficient,  however,  when  the  cord  is 
isolated  from  the  brain;  one  important  difference  is  that  balance  is 
not  maintained.  In  the  decerebrate  animal  some  of  this  deficiency 
is  supplied;  and  a  decerebrate  frog  can  jump  and  swim  in  almost  a 
normal  manner.  The  decerebrate  dog  or  cat,  soon  after  the  opera- 
tion, usually  does  little  more  than  crawl;  but  the  animal  which  Goltz 
kept  alive  came  to  walk  normally.  The  contribution  of  the  brain- 
stem  to  the  function  of  locomotion  is  in  large  measure  to  be  explained 
by  reference  to  the  vestibular  branch  of  the  eighth  nerve,  with  its 
receptors  in  the  inner  ear,  and  its  central  connections  in  the  pons 
and  bulb.  These  receptors  are  located  in  the  vestibule  and  semi- 
circular canals;  they  are  excited  by  movements  and  positions  of 
the  head,  and  their  reflexes  consist  of  compensatory  or  corrective 
movements  and  postures,  and  also  result  in  maintaining  the  tonus  of 


156    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

many  muscles.  Such  reflexes  are  essential  for  the  maintenance  of 
the  erect  posture  and  for  well-directed  locomotion. 

§  13.  The  cerebellum  is  closely  related  to  the  reflex  functions 
just  mentioned.1  It  is  located,  as  an  outgrowth  of  the  brain-stem, 
near  the  level  of  the  eighth  nerve,  and  receives  many  fibres  from  its 
vestibular  branch.  The  disturbances  which  result  from  injuries 
to  the  cerebellum  resemble  those  which  result  from  injury  to  this 
nerve  or  to  its  receptors.  The  cerebellum  seems,  therefore,  to  be 
fundamentally  an  expansion  of  the  local  centre  of  the  vestibular 
branch  of  the  eighth  nerve.  But  it  receives  also  numerous  fibres 
from  other  sense-organs,  especially,  as  appears  probable,  from  those 
situated  in  and  about  the  muscles,  and  belonging  to  that  proprio- 
ceptive  system  which  was  mentioned  a  few  paragraphs  back  in 
connection  with  local  reflexes  (see  p.  154).  We  there  took  note  of 
the  action  reflexly  exerted  on  a  muscle  by  stimuli  arising  in  the  mus- 
cle itself  from  its  own  contractions  and  from  pulls  and  pushes  ex- 
erted on  it.  Such  local  reflexes  were  found  to  be  important  in 
efficient  muscular  action.  But  it  is  likely  that  these  same  stimuli 
act  on  the  cerebellum  through  the  cerebellar  tracts  (compare  p.  95) ; 
and  this  portion  of  the  brain  thus  becomes  an  organ  where  are 
gathered  together  sensory  impulses  from  all  the  muscles  of  the  body. 
In  this  way,  the  cerebellum  receives  information,  as  it  were,  regard- 
ing the  condition  of  every  muscle;  in  it  is  formed  a  sort  of  representa- 
tion, or  reproduction,  detailed  and  yet  comprehensive — though,  as 
far  as  known,  unattended  with  consciousness — of  the  dynamic  con- 
dition of  the  entire  musculature.  To  this  is  added  the  very  im- 
portant function,  provided  for  by  the  receptors  in  the  inner  ear, 
which  responds  to  the  position  and  movements  of  the  head  in  space. 
Thus  the  posture,  movements,  muscular  tensions,  and  external 
strains  exerted  on  the  body  at  every  moment,  act  on  the  cerebellum, 
and  through  it,  reflexly,  react  on  the  muscles. 

We  need  to  recall,  further,  that  in  mammals  the  cerebellum  re- 
ceives a  large  mass  of  fibres  which  apparently  bring  impulses  from 
the  cerebrum.  The  exact  function  of  this  connection  is  altogether 
unknown;  but  it  is  reasonable  enough  to  suppose  that,  if  the  cere- 
bellum is  to  preside  over  the  dynamic  condition  of  the  muscles,  it 
should  receive  advance  information  from  the  cerebrum  regarding 
what  movements  are  next  to  be  made;  and  in  the  higher  mammals, 
it  is  the  cerebrum  which  very  largely  determines  the  nature  of  the 
postures  assumed  and  the  general  course  of  bodily  movements. 

In  accordance  with  this  rather  vague  conception  of  the  function 

1  See  Luciani,  II  cerveletto,  1891;  Sherrington,  in  Schafer's  Textbook  of 
Physiology,  1900,  vol.  II,  pp.  903-910;  and  Integrative  Action  of  the  Nervous 
System,  1906,  p.  347. 


REFLEXES  OF  THE  CEREBELLUM       157 

of  the  cerebellum,  we  find  that  the  results  of  injury  to  it  are  seen 
principally  in  defects  of  balancing  power  or  of  co-ordinated  move- 
ment. The  muscles  that  maintain  the  posture  of  the  animal,  in 
which  this  organ  has  been  injured,  do  not  maintain  their  "tone,"  or 
mild  steady  state  of  contraction,  so  well  as  do  the  muscles  of  a  nor- 
mal animal.  The  movements  are  also  lacking  in  force  and  are 
affected  with  a  tendency  to  tremor.  There  is  uncertainty  in  main- 
taining equilibrium  against  gravity,  and  in  keeping  to  a  straight 
line  in  locomotion.  If  the  injury  to  the  cerebellum  is  one-sided,  the 
symptoms  are  even  more  striking;  because  there  is  a  lack  of  sym- 
metry or  balance  between  the  movements  of  the  two  sides  of  the 
body:  "forced  movements"  occur,  such  as  an  uncontrollable  turning, 
or  rolling,  to  one  side  (the  "circus  movement").  It  is  indeed  re- 
markable to  what  an  extent  the  early  symptoms  of  cerebellar 
injury  disappear  with  time.  Sometimes,  in  the  case  of  man,  very 
extensive  destructions  of  cerebellar  substance,  if  they  come  on 
gradually,  betray  themselves  scarcely  at  all  to  ordinary  observa- 
tion. It  should  be  said,  however,  that  such  cases  have  not  yet  been 
studied  with  the  minuteness  which  the  subject  demands.  Part  of 
the  difficulty  of  the  study,  and  part  of  the  vagueness  to  our  minds 
of  the  resulting  conception  of  the  reflex  functions  of  the  cerebellum, 
may  be  due  to  the  fact  that  the  matters  over  which  this  organ  pre- 
sides are  not  customarily  dependent  on  conscious  control,  and  so 
do  not  arouse  our  attention. 

§  14.  The  completeness  with  which  locomotion  occurs  in  de- 
cerebrate  animals  makes  it  probable  that  the  cerebrum  is  not 
fundamentally  concerned  in  this  form  of  co-ordination.  That  loco- 
motion is  reflex,  in  the  sense  of  not  needing  to  be  learned,  is  clear 
in  the  case  of  those  animals  which  walk,  run,  crawl,  swim,  or  fly, 
at  birth  or  on  emerging  from  the  egg.  In  animals  which  pass 
through  a  period  of  helpless  infancy,  the  case  is  not  so  clear;  but  it 
was  proved  by  Spalding1  that  birds,  at  the  proper  age,  fly  perfectly, 
even  when  they  have  been  prevented  from  seeing  old  birds  fly  and 
from  exercising  their  own  wings.  In  the  human  infant,  walking 
seems,  from  such  observations  as  have  been  collected,2  to  occur  at 
the  proper  age  without  training  or  unsuccessful  efforts. 

§  15.  The  bulb,  or  medulla,  from  its  being  the  place  of  entry  of 
the  vagus  nerve,  is  the  local  reflex  centre  for  the  receptors,  supplied 
by  that  nerve,  and  located  in  the  lungs,  heart,  stomach,  etc.  (compare 
p.  148).  It  contains  the  chief  centres  for  respiration,  for  regulating 
the  rate  and  force  of  the  heart-beat,  for  controlling  the  diameter  of 
the  blood-vessels  and  so  the  distribution  of  the  blood,  for  swallow- 

1  Nature,  1875,  XII,  507. 

2  See  Woodworth,  Le  Mouvement,  1903,  p.  315. 


158    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

ing  and  for  regulating  the  movements  of  the  stomach.  These,  and 
other  related  functions,  are  not  fundamentally  interfered  with  by 
severing  the  brain-stem  close  above  the  bulb.  Other  visceral  re- 
flexes, as  previously  mentioned,  have  centres  in  the  cord,  especially 
in  its  lower  portion,  where  also  are  the  centres  of  the  fundamental 
sexual  functions,  these  also  being  reflex. 

It  appears,  accordingly,  that  visceral  and  locomotor  reactions 
are  fundamentally  reflex;  and  to  these  should  be  added  the  protective 
reactions  aroused  by  "painful"  or  injurious  stimuli;  of  the  latter 
class  of  reflexes,  the  flexion  and  scratch  reflexes  are  examples;  and 
so  also  is  the  pupillary  reflex,  whose  centre  is  in  the  mid-brain. 
With  the  protective  reflexes  may  perhaps  be  classed  cries  of  pain 
and  anger,  and  some  movements  of  facial  expression,  which  appear 
in  the  decerebrate  mammal.  All  in  all,  it  would  seem  that  the  funda- 
mental co-ordinations  of  movement  are,  generally  speaking,  of  a 
reflex  nature.  It  is  as  provided  with  such  materials  that  the  proc- 
esses of  learning  movements  and  of  gaining  voluntary  control  take 
their  start. 

§  16.  As  to  the  influence  of  the  cerebrum  on  the  reflex  activity 
of  the  lower  centres,  it  was  formerly  held1  that  reflexes  to  present 
stimuli  are  more  regularly  and  easily  elicited,  but  that  anything  like 
spontaneous  movement  is  absent,  in  the  decerebrate  animal.  This 
statement  of  the  case  is,  however,  too  simple  to  cover  all  the  facts. 
It  is  indeed  true  that  many  reflexes  are  more  certainly  evoked  in  a 
spinal  or  a  decerebrate  animal  than  in  a  normal  animal;  among  such 
are  especially  the  protective  reflexes.  But  in  the  higher  mammals, 
separation  from  the  brain  seems  on  the  whole  to  depress  the  reflex 
activity.  Much  depends,  too,  on  the  level  at  which  the  transection 
occurs.  If  special  care  is  taken  not  to  injure  the  mid-brain,  thai- 
ami,  and  optic  nerves,  removal  of  the  cerebrum  is  followed  by  much 
less  of  apparent  loss  of  "spontaneity."2  The  difference  in  such 
cases  must,  therefore,  be  largely  due  to  the  retention  of  the  connec- 
tions with  the  organ  for  vision,  and  so  of  visual  stimuli.  Cutting 
off  so  important  a  class  of  stimuli  necessarily  reduces  the  animal's 
activity.  It  is  unavoidable,  in  all  removals  of  the  cerebrum,  that 
the  olfactory  lobe,  and  the  central  connections  of  the  sense  of  smell, 
should  be  destroyed;  the  loss  of  this  class  of  stimuli,  also,  lowers  the 
activity  of  the  animal — particularly  in  the  case  of  those  animals 
which  depend  greatly  on  the  sense  of  smell.  In  mammals,  most  of 
the  central  connections  of  sight  and  hearing,  as  well  as  of  smell, 
run  through  the  cerebrum;  loss  of  the  hemispheres  therefore  renders 

1  See  Ferrier,  Functions  of  the  Brain,  1886,  p.  109. 

*  Schrader,  Pfluger's  Archiv  filr  die  gesammte  Physiologie,  1887,  XLI,  75; 
and  1888,  XLIV,  175. 


NERVOUS  MECHANISM  OF  REFLEXES  159 

the  mammal  practically  blind  and  deaf.  Accordingly,  the  loss  of 
all  these  senses  needs  to  be  considered,  whenever  the  behavior  of 
a  decerebrate  mammal  is  examined.  There  is  no  doubt  that  loss 
of  the  cerebrum  means  loss  of  learned  movements;  and  it  also  in- 
volves the  loss  of  both  the  inhibitory  and  the  tonic  influences  which 
are  exerted  normally  by  the  cerebrum  on  spinal  and  brain-stem  re- 
flexes. 

Regarding  the  general  characteristics  of  the  reflex  functions  of  the 
nervous  system,  much  may  be  gleaned  from  the  incomplete  inventory 
of  reflexes  which  has  been  given  in  the  preceding  paragraphs. 
Much,  however,  still  demands  more  special  and  detailed  considera- 
tion. Probably  the  greatest  authority  on  reflex  action  in  general 
is  Sherrington;  and  in  what  follows  reliance  will  be  placed  chiefly 
on  his  numerous  special  studies,  and  especially  on  his  philosophical 
presentation  of  the  whole  matter  in  his  book  with  the  title  "Inte- 
grative  Action  of  the  Nervous  System."1 

§  17.  Returning  to  the  conception  of  the  reflex  arc,  we  see  that 
the  particular  muscular  movement  which  is  to  follow  any  stimulus 
is  dependent  on  the  nervous  paths  that  lead  from  the  receptor  which 
has  been  stimulated.  Since  a  most  general  characteristic  of  re- 
flexes is  to  bring  into  play  a  considerable  amount  of  musculature 
in  response  to  the  excitation  of  even  a  very  small  group  of  receptors, 
it  follows  that  the  reflex  arc  must  undergo  more  or  less  of  branching, 
so  as  to  distribute  the  excitation  sufficiently  widely.  Such  distri- 
bution (compare  Fig.  62)  is  provided  for,  to  a  limited  extent,  by  the 
branching  of  motor  nerve-fibres,  each  of  which  may  innervate  sev- 
eral muscle-fibres.  But  the  required  distribution  is  much  more  the 
result  of  the  branching  which  takes  place  within  the  nervous  cen- 
tres themselves.  As  has  already  been  shown  (see  p.  47),  the  sen- 
sory fibres,  when  they  enter  the  cord,  branch  widely,  by  means  of 
collaterals;  and  it  is  highly  probable  that  central  neurones,  or  what 
von  Monakow  has  called  "interpolated  cells/52  intervene  between 
the  sensory  and  the  motor  cells,  and  act  as  still  further  distributing 
agents.  Such  central  cells  are  shown  by  histology  to  exist  in  abun- 
dance, and  part  of  their  function  is,  probably,  the  distribution  of 
impulses.  The  character  of  any  reflex  is  dependent,  then,  first  of 
all,  on  a  large  amount  of  branching  in  the  pathway  which  extends 
from  any  given  receptor  to  many  effector  units.  How  wide  this 
distribution  can  become,  is  seen  most  clearly  when  the  stimulus 
is  very  intense;  for  then  the  reflex  may  even  spread  over  a  large 
share  of  the  muscles  of  the  entire  body.  In  certain  abnormal  con- 
ditions of  the  nerve-centres,  such  as,  especially,  the  condition  brought 

1  New  York,  1906. 

2  Ergebnisse  der  Physiologic,  1902,  I,  part  ii,  563. 


160    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 


about  by  strychnine  poisoning,  a  stimulus  to  any  receptor  calls  into 
action  almost  the  whole  musculature;  and  this  result  shows  that,  po- 
tentially at  least,  paths  exist  from  each  receptor  to  every  effector  unit. 
But  the  distribution  of  nervous  impulses  over  large  areas  of  the 
musculature  is  by  no  means  at  random;  for,  in  normal  conditions 


Muscle 


Motor 
neurones 


Central 
neurones 


A 


B 


FIG.  62.— The  Distribution  of  Sensory  Impulses  in  the  Cord.  In  A,  the  distribution  is 
accomplished  by  the  branching  of  the  sensory  axon;  in  B,  by  the  branching  of  an 
interpolated  central  axon. 

of  the  nervous  centres,  the  reflex  is  not  a  general  convulsion,  but  a 
co-ordinated  movement.  The  distribution,  in  other  words,  is 
highly  selective:  the  excitation  is  carried  to  muscles  which  work  in 
harmony;  while  other  muscles,  which  would  work  against  those 
employed,  are  passed  over  in  the  distribution  of  excitation.  The 
distribution  is  different,  also,  according  as  it  comes  from  one  par- 
ticular receptor  or  from  some  other:  one  receptor  calls  into  play  a 


NERVOUS  MECHANISM  OF  REFLEXES  161 

certain  combination  of  muscles;  another  receptor,  a  very  different 
combination. 

§  18.  Distribution  of  excitation,  by  means  of  branching  path- 
ways, is  therefore  a  primary  characteristic  of  reflex  action.  An- 
other equally  salient  feature  of  this  class  of  nervous  functions  is 
almost  the  reverse  of  this.  There  is  a  convergence  of  pathways, 
so  that  the  same  effector  organ  can  be  excited  from  any  one  of 
many  receptors.  This  is  made  evident,  first  of  all,  by  the  fact  that 
the  same  reflex  can  be  excited  from  any  one  of  many  different  points. 
The  scratch  reflex,  for  example,  can  be  evoked  by  suitable  stimula- 
tion applied  anywhere  within  a  large  area  of  the  skin  of  the  back 
and  sides.  The  flexion  reflex  of  a  limb  can  be  evoked  by  a  stimulus 
applied  almost  anywhere  on  the  same  limb.  The  pupillary  re- 
flex can  be  aroused  by  a  beam  of  light  falling  anywhere  on  the  ret- 
ina. In  general,  the  "receptive  field"  of  a  reflex  is  often  wide;  and 
from  anywhere  within  this  field  the  same  muscles  are  thrown  into 
action,  though  not  always  to  the  same  degree,  or  in  the  same  pro- 
portion. It  is  made  obvious,  from  such  facts  as  these,  that  the 
paths  from  the  numerous  receptors  within  any  receptive  field  must 
converge  upon  the  same  muscles.  And,  plainly,  such  convergence 
does  not  occur  within  the  sensory  or  the  motor  nerves  themselves; 
for  the  fibres  in  these  nerves  run  their  courses  parallel  and  inde- 
pendent, with  no  chance  for  communication  from  one  to  another. 
The  convergence  must,  therefore,  occur  within  the  nervous  centres; 
and  it  seems  to  be  provided  for,  in  part,  by  the  spreading  dendrites, 
which  are  capable  of  receiving  excitation  from  many  axons,  and  which 
converge  upon  their  own  cell-body  and  axon.  It  is  probable,  also, 
that  central  or  interpolated  cells  have  a  share  in  the  convergence 
of  excitation,  as  well  as  in  its  distribution. 

The  convergence  of  reflex  paths  from  all  parts  of  the  receptive 
field  of  a  single  reflex  is  only  one  case  of  convergence;  and  it  is  the 
simplest  case.  But  the  same  muscle  may  be  employed  in  different 
reflexes.  For  example,  the  muscles  which  bend  and  which  extend 
the  knee  are  active  in  the  scratch  reflex,  and  also  in  the  stepping 
reflex.  The  receptive  field  for  the  latter  lies  within  the  muscles 
themselves,  far  distant  from  the  receptive  field  of  the  scratch  re- 
flex. Moreover,  though  the  two  reflexes  employ  the  same  muscles, 
they  employ  them  differently;  the  action  of  the  knee  muscles  is  dif- 
ferently combined  with  the  action  of  other  muscles,  and  besides, 
the  rhythm  of  the  scratch  reflex  is  faster  than  that  of  the  stepping 
reflex.  Since  the  same  muscles  act  differently  in  different  reflexes, 
the  difference  cannot  be  attributed  to  the  muscles  themselves;  they 
are  merely  obedient  to  the  different  excitations  which  they  receive. 
Nor  can  the  difference  lie  in  the  peculiarities  of  the  receptors,  as 


162    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

can  be  proved  by  suitable  experiments.1  Such  differences,  on  the 
contrary,  must  be  located  in  the  nerve-centres;  and  the  converging 
paths  that  lead  within  the  centres  toward  the  motor  cells  govern- 
ing a  given  muscle  must,  therefore,  not  simply  converge,  but  dis- 
charge their  impulses  with  differing  force  and  rhythm.  Among 
the  central  cells  of  the  cord  there  must  apparently  be,  not  only  col- 
lectors of  sensory  impulses,  but  different  collectors  for  the  several 
reflexes  which  employ  the  same  muscle;  and  these  different  collec- 
tors must  have  differing  rates  of  discharge,  etc.  The  collectors  them- 
selves have  also  differing  distributions  to  the  motor  cells  of  the  cord. 

If  these  inferences  make  the  connections  within  the  cord  seem  high- 
ly complicated,  we  must  remember  that  the  complexity  of  connec- 
tions revealed  by  histology  is  as  ample  as  could  be  desired  by  the 
physiologist. 

§  19.  There  is  a  yet  further  complication  to  be  noted.  The  ef- 
fect exerted  reflexly  on  a  muscle  is  not  always  that  of  arousing  it 
to  activity;  it  may  be  the  exact  opposite,  that  of  suppressing  what- 
ever activity  is  going  on  in  a  muscle.  This  process  of  checking  ac- 
tivity is  called  inhibition,  and  the  process  of  inhibition  seems  to  be 
scarcely  less  important  than  that  of  excitation,  in  securing  harmoni- 
ous action  by  the  muscles.  It  is  seldom,  indeed,  that  a  stimulus 
to  a  sense-organ  finds  the  system  in  a  completely  resting  condition; 
almost  always,  some  action  is  already  going  on  within  the  system. 
In  particular,  the  muscles  which  maintain  the  posture  of  the  animal 
are  usually  in  activity;  such  are,  especially,  the  extensor  muscles 
of  the  limbs  and  neck,  and  the  muscle  which  supports  the  jaw  against 
gravity. 

Now,  whatever  movement  is  called  for  by  any  stimulus  is  pretty 
sure  to  require  the  temporary  abandonment  of  the  existing  posture; 
and  if  the  muscles  which  maintain  this  posture  were  left  in  their 
active  state,  they  would  hinder  the  quick  and  powerful  execution 
of  the  newly  required  reflex  movement.  It  is  found,  as  a  matter  of 
fact,  that  the  contraction  existing  in  such  muscles  is  inhibited  by 
the  reflex  action  of  the  new  stimulus,  simultaneously  with  the  excita- 
tion of  the  muscles  which  execute  the  reflex  movement.2  This  has 
been  demonstrated  by  Sherrington  in  the  case  of  the  extensor  mus- 
cles of  the  knee,  which  lose  their  tonic  contraction  simultaneously 
with  any  reflex  action  of  the  flexors  of  the  knee;  he  has  also  demon- 

1  Sherrington    (op.    cit.,  pp.  55-61)  finds  that  the  rhythm  of  the  scratch 
reflex   is   not  interfered  with  by  simultaneously  or  alternately  exciting  two 
points  on  the  dog's  back,  and  argues  that,  if  the  rhythm  were  determined  in 
the  receptors,  the  rhythmic  impulses  sent  in  from  one  point  of  stimulation 
would  combine  with  those  sent  in  from  the  other  point,  and  produce  a  rhythm 
of  twice  the  frequency,  or  at  least  change  the  rhythm,  as  it  fails  to  do. 

2  Sherrington,  op.  cit.,  83-101. 


PHENOMENA  OF  INHIBITION  163 

strated  the  same  phenomena  in  the  muscles  of  the  eye  and  in  other 
pairs  of  antagonistic  muscles.  When  one  muscle  of  a  pair  of  an- 
tagonists is  excited  reflexly,  the  opposing  muscle  is  simultaneously 
deprived  of  whatever  contraction  it  may  have.1  There  are  no  doubt 
exceptions,  of  a  sort,  to  this  rule,  since  in  voluntary  action  it  is  possi- 
ble to  fix  any  joint  in  a  rigid  position  by  contracting  at  the  same 
time  the  antagonistic  muscles  that  act  on  the  joint.  But  in  move- 
ments, the  inhibition  of  antagonists  seems  to  be  a  very  general 
principle. 

Inhibition  is  a  frequent  phenomenon  in  some  of  the  internal 
organs.  The  movements  of  the  stomach  and  intestines,  which, 
as  noted  above  (p.  149),  go  on  without  the  action  of  the  central 
nervous  system,  can  yet  be  checked  by  outgoing  impulses  from  the 
nervous  centres.  The  muscles  of  the  arteries,  similarly,  are  made  to 
relax  by  the  action  of  the  "vaso-dilator"  nerves.  But  the  most  in- 
teresting case  is  that  of  the  heart.  The  vagus  nerve,  which  sends  a 
branch  to  the  heart,  acts  to  check  the  heart-beat.  When  this  nerve 
is  stimulated,  the  heart-beat  is  slowed  down  or  even  stopped  for  a 
time.  Since  this  result  is  obtained  by  stimulating  the  outgoing 
nerve  to  the  heart,  it  is  obvious  that  the  inhibition  operates  within 
the  heart  itself;  and  strong  evidence  has  been  offered  to  show  that 
it  operates  within  the  heart  muscle.  But  such  inhibition  as  we  are 
here  considering — namely,  the  inhibition  of  the  skeletal  muscles — 
does  not  operate  within  those  muscles;  for  no  good  evidence  ex- 
ists that  there  are  any  specifically  inhibitory  fibres  running  to  the 
muscles,  as  there  are  running  to  the  heart  by  the  vagus.  The  in- 
hibition of  the  skeletal  muscles  operates  within  the  spinal  cord. 
The  postural  activity  of  these  muscles  is  itself  of  a  reflex  nature, 
being  maintained  through  reflex  centres;  and  the  inhibition  works 
on  those  centres,  stopping  them  from  sending  out  excitatory  im- 
pulses to  the  muscles. 

When  inhibition  as  well  as  excitation  is  taken  into  account,  the 
breadth  of  a  reflex  action,  or  the  extent  of  its  distribution,  is  seen  to 
be  twice  as  great  as  at  first  appears.  For  the  influence  extends  not 
only  to  the  muscles  which  become  active,  but  also  to  those  muscles 
which  are  inhibited.  A  small  group  of  sensory  fibres  may  thus  exert 
a  wide  influence  on  the  motor  cells  of  the  cord;  but  they  may  excite 
some  of  these  cells  and  others  they  may  inhibit  or  depress. 

§  20.  It  is  not  easy  to  form  a  complete  conception  of  the  mechan- 
ism of  inhibition,2  but  there  is  one  significant  fact  about  it  which 

1  In  the  case  of  movements  which  amount  to  changes  of  posture,  the  in- 
hibited muscle  often  loses  only  a  part  of  its  contraction  (Sherrington,  Quar- 
terly Journal  of  Experimental  Physiology,  1909,  II,  pp.  109  ff.). 

3  See,  however,  pp.  289  ff. 


164    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

is  of  no  small  assistance  to  this  end.  Inhibition  itself  is  not  merely 
an  interruption  of  activity;  for  it  has  an  after-effect  which  is  the 
opposite  of  depression.  When  a  muscle  has  undergone  inhibi- 
tion, it  becomes  at  once  readier  for  a  new  phase  of  activity.  It  is 
more  easily  aroused  than  it  was  before,  and  it  is  likely  to  show  more 
force  in  its  next  contraction.  That  is,  the  phase  of  inhibition  is 
followed  by  a  rebound  to  greater  activity;1  and  the  rebound,  like 
the  inhibition,  is  primarily  a  central,  and  not  a  muscular  affair. 
This  after-effect  of  inhibition  is  probably  important  in  the  numer- 
ous alternating  movements  which  occur  in  locomotion,  breathing, 
chewing,  pounding,  etc.;  the  muscles  (or  their  controlling  nerve- 
cells)  which  are  inhibited  in  one  phase  of  the  movement  are  thereby 
made  ready  for  the  succeeding,  opposite  phase. 

§  21.  Closely  related  to  inhibition  is  the  phenomenon  of  the  "re- 
fractory period,"  already  mentioned  (p.  131)  in  the  case  of  nerve- 
fibres.  Immediately  after  acting,  or  starting  to  act,  any  excitable 
organ  loses  its  excitability  and  becomes  refractory  or  unrespon- 
sive for  a  brief  period.  The  duration  of  the  refractory  period  differs 
in  different  organs;  it  is  shortest  in  the  case  of  nerve-fibres,  where  it 
does  not  exceed  .002  sec.  It  is  much  longer  than  this  in  some  of 
the  reflexes,  but  varies  greatly  from  one  reflex  to  another.  The 
"extensor  thrust,"  for  example,  has  a  long  refractory  period,  which 
may  reach  a  full  second;  this  means  that  repeating  the  stimulus 
within  a  second  after  a  thrust  has  been  evoked  does  not  evoke  a 
second  thrust.  More  concretely  stated  and  illustrated:  If  a  gentle 
upward  pressure  is  exerted  on  the  "pads"  of  a  spinal-dog's  hind 
foot,  the  leg  responds  by  a  vigorous  downward  thrust  (as  if  in 
jumping).  Now,  though  this  thrust  lasts  for  only  a  fifth  of  a  sec- 
ond, yet  repeating  the  upward  pressure  on  the  pads  does  not  evoke 
another  thrust,  unless  an  interval  of  a  full  second  is  allowed  to 
elapse  between  the  two  stimuli.  The  duration  of  the  refractory 
period  in  this,  as  in  other  reflexes,  is,  however,  somewhat  vari- 
able. 

The  length  of  the  refractory  period  in  the  winking  reflex  is  about 
the  same  as  that  in  the  extensor  thrust;  and  in  the  swal.lowing  re- 
flex it  is  half  a  second  or  longer.  In  the  "stepping  reflex"  it  is  about 
two-fifths  of  a  second,  and  in  the  "scratch  reflex,"  about  one-fifth. 
In  other  movements,  which  are  not  obviously  rhythmical,  and  which 
show  a  prolonged  contraction  of  the  muscles  (such  as  the  flexion 
reflex),  it  is  found,  on  careful  examination,  that  the  apparently 
steady  contraction  of  the  muscle  includes  a  series  of  waves,  follow- 
ing one  another  at  the  rate  of  eight  to  twelve  per  second.  These 
waves  represent  discharges  from  the  cord,  which  are,  in  effect, 
1  Sherrington,  op.  cit.,  p.  206. 


THE  REFRACTORY  PERIOD  165 

fused  into  the  steady  prolonged  contraction.  The  refractory  phase, 
in  such  cases,  varies  from  an  eighth  to  a  twelfth  of  a  second;  as  we 
know  by  the  fact  that  the  waves  are  not  increased  in  frequency  by 
stimulating  the  receptor  at  a  faster  rate,  as,  for  example,  by  elec- 
tric shocks  at  an  interval  of  from  twenty  to  fifty  per  second. 

§  22.  Like  those  other  peculiarities  of  reflex  action  that  have  al- 
ready been  mentioned,  the  refractory  phase  is  not  a  peripheral 
phenomenon,  the  cause  'of  which  resides  either  in  the  muscles  or 
in  the  receptors.  That  it  does  not  reside  in  the  muscles  is  evident 
from  the  fact  that  the  same  muscle  shows  refractory  periods  of 
different  duration  according  to  the  reflex  combinations  into  which 
it  enters.  The  extensor  muscle  of  the  knee,  for  example,  takes 
part  in  both  the  scratch  reflex  and  the  extensor  thrust;  but  in  the 
first  reflex,  its  refractory  period  is  only  a  fifth  of  a  second,  while  in 
the  extensor  thrust  this  period  lasts  for  a  full  second.  That  the 
refractory  phase  cannot  be  attributed  to  the  receptors  is  evident 
from  the  fact  that  a  reflex  evoked  by  stimulating  one  receptor  is 
refractory,  for  the  usual  time,  to  stimuli  applied  to  any  other  re- 
ceptor which  normally  evokes  the  same  reflex.  Accordingly,  the 
refractory  period  must  be  considered  as  pre-eminently  a  central 

Ehenomenon;  and  it  probably  belongs  to  those  central  or  interpo- 
ited  neurones  which  have  been  referred  to  in  previous  paragraphs, 
as  taking  an  important  part  in  the  reflex  functions  of  the  nervous 
system.  Different  central  mechanisms  have  refractory  periods  of 
different  duration;  and  the  duration  is  in  each  case  adapted  to  se- 
cure the  final  purpose,  or  greatest  utility,  of  the  particular  reflex. 
The  general  utility  of  a  refractory  phase  is  clearest  in  the  case  of 
rhythmical  or  alternating  movements,  such  as  scratching,  walking, 
etc.  The  stimulus  is  here  continuous;  but  a  single  prolonged  con- 
traction of  the  muscles  would  not  be  an  efficient  response.  The 
refractory  phase,  on  the  contrary,  secures  a  rhythmical  response  to 
a  continuous  stimulus. 

§  23.  It  seems  obvious,  accordingly,  that  the  "impulses"  which 
pass  along  the  fibres  in  the  nerve-centres  are,  very  often  at  least, 
diphasic.  An  excitatory  phase  is  followed  at  once  by  an  inhibi- 
tory phase.  In  other  cases  the  inhibitory  phase  precedes,  and  is 
followed  by  a  phase  of  rebound  to  a  condition  of  heightened  ex- 
citability. In  general,  the  duration  of  the  phases  varies  in  differ- 
ent reflexes — i.  e.,  in  different  fibres  or  neurones,  or  at  different 
synapses. 

We  have,  therefore,  not  exhausted  our  knowledge  of  reflex  action, 
and  of  the  central  mechanisms  which  control  it,  until  we  have  taken 
note  of  all  the  facts  regarding  the  time,  extent,  and  intensity  of  the 
different  reactions. 


166    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

§  24.  As  to  the  time,  the  rhythmic  character  of  reflex  action  has 
already  been  sufficiently  considered.  The  duration  of  the  reflex 
contraction  depends  to  some  extent  on  the  duration  of  the  stimulus; 
but  such  dependence  is  fa?  less  close  in  the  case  of  reflex  action  than 
when  a  muscle  is  directly  excited  by  an  artificial  stimulus,  or  when 
it  is  excited  by  stimulating  artificially  its  motor  nerve.  In  these 
cases  of  direct  stimulation,  the  response  lasts  as  long  as  the  stimu- 
lus, and  stops  when  the  stimulus  stops — or  nearly  so — subject  of 
course  to  modification  by  fatigue.  But  in  reflex  action,  the  move- 
ment in  response  has  in  many  cases  a  fixed  duration  that  is  more  or 
less  independent  of  the  duration  of  the  stimulus.  This  is  true,  for 
example,  of  the  winking  reflex  and  of  the  extensor  thrust.  In  the 
scratch  reflex,  also,  the  alternating  movement  of  the  leg  is  likely  to 
stop  during  the  continuation  of  the  stimulus.  On  the  other  hand, 
if  the  stimulus  is  brief,  but  sufficiently  intense,  the  scratching  con- 
siderably outlasts  it.  Other  reflexes,  too,  outlast  a  brief  stimulus; 
and,  in  general,  the  duration  of  this  "after-discharge"1  is  greater 
for  a  strong  stimulus  than  for  a  weak. 

Between  the  beginning  of  the  stimulus  and  the  beginning  of  the 
muscular  response  there  is  always  an  interval,  which  is  called  the 
latent  time  of  the  reflex,  or,  more  briefly,  the  re-flex  time.  This  is 
analogous,  on  the  one  hand,  to  the  "reaction  time"  of  voluntary 
movements,  and,  on  the  other,  to  the  "latent  time"  of  muscular 
contraction.  When  a  muscle  is  directly  excited  by  an  artificial 
stimulus,  a  brief  interval  elapses  between  the  stimulus  and  the  com- 
mencement of  visible  movement  in  the  muscle.  This  latent  time 
usually  appears  as  about  .01  sec.  It  is  probable  that  each  recep- 
tor, likewise,  has  a  latent  period,  i.  e.,  an  interval  between  the  ap- 
plication of  the  stimulus  and  the  starting  of  the  nerve  impulse  along 
the  sensory  nerve.  The  latent  time  is  probably  different  for  the 
different  kinds  of  receptors;  but,  in  general,  it  is  fully  as  brief  as 
that  given  above  for  the  muscle.  The  total  Beflex  time  includes 
the  latent  periods  of  both  receptors  and  muscles;  in  addition,  it 
includes  the  time  consumed  in  nerve  transmission  to  and  from  the 
centres;  and,  finally,  it  includes  whatever  time  is  consumed  within 
the  gray  matter  of  the  centres.  It  is,  therefore,  a  highly  composite 
affair;  but  its  most  interesting  and  important  factor,  as  bearing 
upon  our  knowledge  of  the  reflex  functions  of  the  nervous  system, 
concerns  the  problem  of  how  much  of  this  time  is  lost,  or  absorbed, 
in  the  nerve-centres.  This  problem  may  be  approximately  solved, 
in  the  following  way:  If  we  take  the  speed  of  nerve  conduction 
to  be  30  metres  per  second,  and  assume  the  receptor  and  muscular 
latent  periods  to  be  each  .01  second,  then,  knowing  the  length  of 
1  Sherrington,  Integrative  Action  of  the  Nervous  System,  p.  26  ff. 


LATENT  TIME  OF  NERVOUS  REFLEXES  167 

nerve  traversed  in  the  reflex  arc,  we  can  subtract  the  time  con- 
sumed in  the  nerves,  and  in  the  receptors  and  muscles,  and  have 
left  the  so-called  "reduced  reflex  time."1  Some  of  the  uncertain- 
ties of  this  reduction  can  be  avoided  by  stimulating  the  sensory 
nerve  close  to  the  spinal  cord.  The  shortest  reduced  reflex  times 
seem  to  amount  to  about  .01  second. 

We  have  already  seen,  however,  that  the  reflex  time  is  far  from 
constant.  It  varies,  first,  with  the  intensity  of  the  stimulus,  becom- 
ing shorter  with  increasing  stimulus.  Thus,  the  latent  time  for 
the  flexion  reflex  of  the  dog  may  rise  to  .20  second  with  weak  stim- 
uli, and  sink  to  .02  second  with  strong  stimuli;  and  the  time  for  the 
scratch  reflex  varies,  similarly,  between  .14  and  .50  second.2  These 
figures  show  that  the  reflex  time  varies,  not  only  with  the  intensity 
of  the  stimulus,  but  also  with  the  character  of  the  particular  reflex. 
The  flexion  reflex  is  about  the  quickest  of  all; — and  here  the  final 
purpose  of  protecting  the  organism  applies;  for  this  reflex  needs  to 
be  prompt,  since  its  normal  stimulus  is  a  harmful  agent  from  which 
the  muscular  reaction  must  snatch  the  limb  away.  The  scratch 
reflex  is  much  less  prompt.  The  winking  reflex  is  also  relatively 
slow,  giving  times  of  about  .05  of  a  second.  These  differences  can 
not  be  due  to  the  muscles;  and  not  entirely,  at  least,  to  the  receptors; 
they  must  be  attributed,  principally,  to  differences  in  the  character 
of  the  different  central  neurones  and  of  the  central  connections  in- 
volved.3 

§  25.  The  intensity,  or  muscular  force  of  a  reflex  varies  in  about 
the  same  ways  as  the  latent  time.  In  many  reflexes,  the  response 
increases  in  force  with  increasing  intensity  of  the  stimulus.  But 

1  Exner,  Pfluger's  Archiv  filr  die  gesammte  Physiologic,  1874,  VIII,  526. 

2  Sherrington,  op.  cit.,  p.  21. 

3  The  "  knee  jerk,"  or  kick  of  the  lower  leg  aroused  by  a  blow  on  the  patellar 
tendon  just  below  the  knee-cap,  has  a  very  brief  latent  time — as  short,  in  some 
measurements,    as    .02   second.      This  extreme  shortness  leads  to  a  doubt 
whether  this  movement  is  properly  a  reflex  at  all;    though  it  is  often  called 
the  "patellar  reflex"  and  is  regarded  as  a  true  reflex  by  many  authorities. 
The  difficulty  with  this  view  is  that  the  length  of  nerve  which  must  be  trav- 
ersed, from  the  knee  to  the  spinal  cord,  and  back  to  the  quadriceps  muscle 
of  the  thigh,  which  performs  the  movement,  is  so  great  that,  at  the  accepted 
rate  of  nerve  transmission,  a  time  of  .03  would  be  consumed  in  the  nerve;  and, 
of  course,  some  time  must  also  be  allowed  for  the  latent  period  of  the  muscle 
and  of  the  receptors.     These  times  add  up  to  considerably  more  than   the 
total  latent  time  of  the  knee  jerk;   and  this  fact  has  led  to  the  conclusion  that 
this  jerk  is  a  direct  response  of  the  muscle  to  the  mechanical  stimulus.     On 
the  contrary,  it  is  found  that  the  knee  jerk  cannot  be  got  unless  the  sensory 
and  motor  nerves  of  the  muscle  are  both  intact;   it  is  dependent  on  the  exist- 
ence of  reflex  "tone"  in  the  muscle;   therefore,  whether  it  is  itself  a  reflex  or 
not,  it  serves  admirably  as  a  test  of  the  reflex  condition  of  the  muscle,  and  so 
of  the  condition  of  the  cord. 


168    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

there  are  exceptions  to  this  rule,  in  which  the  reflex  shows  about 
the  same  force  whatever  the  intensity  of  the  stimulus,  provided 
only  that  the  latter  is  strong  enough  to  arouse  any  reflex  at  all. 
The  extensor  thrust,  for  example,  is  found  by  Sherrington  to  show 
this  peculiarity. 

The  reflex  movement  in  response  to  stimulation  also  differs  in 
strength  according  to  the  receptor  stimulated.  Within  the  re- 
ceptive field  of  a  reflex,  some  points  yield  a  stronger  effect  than 
others.  To  give  an  example:  the  pupillary  reflex  is  stronger,  i.  e., 
the  pupil  is  more  narrowed,  when  a  beam  of  light  falls  on  the  centre 
of  the  retina  than  when  it  falls  toward  the  side  of  the  retina.  A 
comparison  of  different  reflexes  shows  that  there  is  little  general 
correspondence  between  the  energy  of  the  stimulus  and  that  of  the 
response.  Some  reflexes  show  a  powerful  response  to  a  weak 
stimulus,  while  others  give  a  comparatively  feeble  response  to  a 
strong  stimulus.  A  clear  example  of  the  former  class  is  seen  in  the 
violent  reaction  which  is  made  to  a  tickling  stimulus;  in  this  case, 
slight  brushing  of  the  skin  evokes  a  much  stronger  reaction  than 
does  a  firm  pressure.  Or,  again,  a  feeble  force  exerted  on  a  point 
of  the  skin,  so  as  to  prick  it,  gives  a  strong  reaction;  while  a  much 
greater  force  exerted  on  a  large  area  of  the  skin  results  in  a  compara- 
tively feeble  reaction.  The  same  muscle,  too,  may  contract  strongly 
in  one  reflex  and  much  less  strongly  in  another.  From  such  phe- 
nomena as  these  it  is  safe  to  conclude. that  the  central  mechanisms 
of  the  different  reflexes  differ  in  the  intensity  with  which  they  ex- 
cite the  motor  cells  of  the  cord  and  brain-stem. 

§  26.  The  extent  of  the  reflex  movement,  or  the  amount  of  the 
musculature  which  it  brings  into  play,  differs  greatly  in  different 
reflexes;  since  some  of  them,  as,  for  example,  winking  or  the  con- 
traction of  the  pupil  of  the  eye,  involve  only  a  small  mass  of  muscle, 
whereas  others,  as  locomotion,  bring  into  play  a  large  share  of 
the  whole  musculature.  In  other  words,  the  pathways  of  some  re- 
flex arcs  are  much  branched  and  widely  distributory — the  pathways 
of  others  much  less  so.  But  to  speak  of  this  difference  tells  only 
half  the  story.  The  distribution  is  not  fixed  in  extent  for  each  re- 
flex, but,  as  we  have  seen,  increases  with  the  strength  of  the  stimulus. 
Even  in  case  of  the  pupillary  reflex,  any  considerable  increase  of 
the  intensity  of  the  light  entering  the  eye  causes  an  extension  of 
this  reflex  beyond  the  little  muscle  of  the  iris,  to  which  it  may  be 
confined  when  the  light  is  of  ordinary  intensities.  A  very  intense 
light  even  causes  the  eyelids  to  close,  the  head  to  be  turned  away, 
the  hand  to  be  brought  up  before  the  eyes,  or  a  general  movement 
of  flight  to  be  begun.  Other  similar  examples  can  be  obtained 
from  the  spinal  or  the  decerebrate  animal.  Pinching  the  pinna 


INTENSITY  AND  EXTENT  OF  REFLEXES  169 

of  a  spinal-cat's  ear  will,  if  the  stimulus  is  weak,  arouse  nothing 
more  than  a  twitch  of  the  pinna.  If  the  stimulus  is  increased,  the 
head  may  be  turned  to  the  other  side,  and  the  fore  limb  of  the  same 
side  brought  forward  toward  the  ear.  Still  stronger  stimulation 
arouses  movements  of  all  four  limbs.  Again,  pinching  gently  the 
forefoot  of  a  decerebrate  cat  evokes  only  the  flexion  reflex  in  that 
limb;  but  a  firmer  or  more  persistent  pinch  causes  the  head  to  turn 
toward  the  point  stimulated,  and,  perhaps,  the  jaw  to  snap;  loco- 
motion may  also  be  evoked  in  this  way,  and  strong  stimuli  may 
bring  out  a  snarl  or  whine.  Sometimes,  as  the  stimulus  is  increased, 
the  original  local  response  is  abruptly  abandoned,  and  another  re- 
action of  a  different,  more  efficient  nature  is  substituted.  But,  in 
many  cases,  the  original  local  reflex  is  maintained,  and  other  move- 
ments are  added  to  it. 

The  wider  distribution  of  the  response  with  increasing  stimulus 
is  often  called  by  such  names  as  "irradiation"  or  "spread"  of  re- 
flexes. Neither  of  these  names,  however,  is  entirely  suitable,  since 
both  seem  to  present  a  picture  of  a  general  diffusion  of  the  same 
effect.  The  fact  is,  on  the  contrary,  that  the  effect  is  far  from  be- 
ing diffused  indiscriminately  to  neighboring  regions  of  the  cord, 
or  of  the  body.  It  is  indeed  true  that  the  reflex  often  spreads  to 
neighboring  regions — as  from  the  pinna  to  the  neck — before  it 
reaches  more  distant  members.  But  it  always  spreads  only  to 
muscles  which  give  a  harmonious  total  result.  Or,  more  precisely, 
the  stimulus  is  "selective,"  and  excites  only  muscles  which  combine 
harmoniously;  when  it  spreads  to  their  antagonists,  also,  its  effect  is 
to  serve  the  same  purpose  of  the  organism  by  inhibiting  them.  Still 
more  precise  would  probably  be  the  statement  that  the  reflex,  in 
spreading  beyond  its  local  area,  excites  the  central  mechanisms  of 
other  reflexes,  which  belong  more  intimately  to  other  stimuli  oc- 
curring in  their  own  locality,  but  which  are  allied  to  the  reflex  which 
in  the  given  case  is  primary.  Other  reflexes,  however,  which  are 
antagonistic  to  the  primary  reflex  of  the  moment,  are  not  excited 
in  this  spread.  This  "alliance"  is,  beyond  all  doubt,  pre-eminently 
a  central  and  not  a  peripheral  phenomenon;  it  must  be  an  "alli- 
ance" amongst  the  central  mechanisms  of  different  reflexes;  and  it 
therefore  depends  on  the  particular  distribution  of  the  neurones  in 
the  centres. 

These  facts,  regarding  the  distribution,  convergence,  time,  and 
intensity  relations  of  reflex  action,  show  the  extreme  nicety  with 
which  the  reflexes  are,  in  general,  adapted  to  their  several  uses. 
When  taken  neurologically,  they  convey  some  conception  of  the 
complex,  co-operative  workings  of  the  spinal  cord  and  the  other  re- 
flex centres. 


170    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

§  27.  It  remains  to  take  due  account  of  the  fact  stated  near  the 
beginning  of  this  chapter; — the  fact,  namely,  that  the  "simple" 
or  isolated  reflex,  which  has  virtually  been  the  theme  of  the  last 
few  pages,  is  an  abstraction.  This  appears  true  for  the  following 
reasons:  first,  that  more  than  one  reflex,  and  indeed  several  reflexes, 
are  simultaneously  in  progress,  interacting  as  a  rule  with  each  other; 
and,  second,  that  any  new  stimulus  breaks  in  upon  a  condition  which 
is  not  one  of  rest  and  neutrality,  but  upon  an  existing  condition  of 
reflex  activity  already  established.  We  have,  therefore,  always  to 
consider  how  the  reflex  effect  of  a  given  stimulus  is  modified  by 
the  action  of  other  stimuli,  both  simultaneous  and  preceding. 

Light  has  been  thrown  on  this  rather  complicated  problem  by 
the  studies  of  physiologists,  among  whom  Exner1  and  Sherrington2 
deserve  special  mention.  The  two  notions  of  " facilitation"  and 
"  inhibition/'  first  clearly  formulated  by  Exner,  seem  to  give  the  key 
to  the  true  explanation  of  the  phenomena.  Exner  used  the  German 
word,  Bahnung;  which  cannot  easily  be  rendered  into  English. 
Its  meaning  is,  however,  that  one  stimulation  of  any  part  of  the  ner- 
vous mechanism  may  prepare  a  path,  or  break  open  a  way,  and  so 
increase  the  effect  of  other  and  subsequent  stimulations.  The  same 
meaning  is  fairly  well  expressed  by  saying  that  one  stimulus  "fa- 
cilitates" the  action  of  another.  "Reinforcement"  is  a  fact  closely 
related  to  facilitation;  and  "inhibition"  is  the  opposite  of  facilita- 
tion. The  application  of  these  conceptions  to  reflex  action  will 
now  be  illustrated  by  a  few  examples. 

Suppose  that  two  stimuli  act  simultaneously  on  a  spinal  animal, 
and  we  observe  carefully  their  combined  effect.  The  simplest  case  is 
that  in  which  both  stimuli  are  of  the  same  nature,  and  applied  within 
the  receptive  field  of  the  same  reflex.  In  this  case,  each  of  them, 
taken  by  itself,  would  tend  to  evoke  a  merely  local  response;  and 
this  response  would  spread  if  the  stimulus  were  made  more  intense. 
But  taken  together,  they  facilitate  or  reinforce  each  other's  effect. 
If  then  each  of  them,  taken  alone,  is  just  too  weak  to  arouse  any  re- 
flex whatever,  the  two  acting  simultaneously,  or  nearly  so,  will  call 
out  the  customary,  normal  reflex.  And  if  each  of  them,  taken  alone, 
is  of  such  an  intensity  as  only  to  evoke  a  feeble  or  moderate  response, 
the  two  acting  together  reinforce  each  other's  effect,  so  that  the  re- 
sponse becomes  relatively  strong.  This  phenomenon  can  be  easi- 
ly obtained  in  the  case  of  the  scratch  reflex.  Feeble  irritation  of 
two  points  of  the  spinal-dog's  back  may  evoke  the  scratching,  though 
neither  of  the  two  irritations  does  so  by  itself;  and  the  strength  of 
the  scratching  movement  evoked  by  moderate  irritation  at  one  point 

1  Pfliiger's  Archiv  fur  die  gesammte  Physiologie,  1882,  XXVIII,  487. 

2  Integrative  Action  of  the  Nervous  System,  pp.  114-234. 


FACILITATION  OF  REFLEXES 


171 


40- 
30- 
20- 
10- 


10- 
80- 

30- 


mnt 


NORMAL 


TIME 


is  increased  by  moderately  irritating  another  point.  The  nearer 
together  the  two  points  are  on  the  skin,  the  more  powerfully  do 
they  act  in  this  co-operative  way. 

The  more  interesting  cases  of  facilitation  are  those  in  which  the 
two  points  of  stimulation  do  not  lie  in  the  same  receptive  field;  and, 
therefore,  do  not  tend  to  call  out  the  same  local  reflex.  It  was 
noticed  above  that  a  reflex,  in  spreading,  takes  up  local  reflexes 
from  other  parts  and,  as  it  were,  incorporates  them  into  itself.  For 
example,  pinching 
either  hind  leg 
causes,  as  its  local 
response,  a  pulling- 
up  of  the  same  leg; 
but  if  this  reflex 
spreads,  it  results 
in  a  similar  move- 
ment in  the  oppo- 
site fore  limb.  This 
movement  of  the 
fore  limb  is  the 
local  flexion  reflex 
to  pinching  the 
forefoot;  and  it 
can,  accordingly, 
be  more  readily 
aroused  in  this  lat- 
ter way  than  by 
stimulating  the 
hind  limb.  Now 
if  both  of  these 

points — for  example,  the  left  forefoot  and  the  right  hindfoot — are 
stimulated  at  the  same  time,  but  so  feebly  that  neither  stimulus, 
taken  alone,  would  arouse  the  movement  of  the  fore  limb,  this  limb 
may,  nevertheless,  be  made  to  move  through  the  combined  effect  of 
the  two  stimuli.  In  general,  distant  stimuli  may  facilitate  each  other's 
action.  The  impulses  started  at  two  widely  separated  receptors 
may  converge  upon  the  same  central  cells,  and  so  produce  move- 
ment of  the  same  muscles.  Even  an  auditory  stimulus  may  facili- 
tate a  reflex  in  the  hind  limb.  In  man,  the  knee  jerk  is  strengthened 
by  a  sudden  noise  or  other  stimulus  which  has  the  effect  of  startling 
the  subject  of  experiment.1 

Mental  states  may  exert  a  similar  reinforcement  on  various  re- 
flex functions  of  the  nervous  system;  and  artificial  stimuli  applied 
1  See  .Lombard,  American  Journal  of  Physiology,  1887,  I,  1. 


QT02*    0.4* 


10" 


IT' 


FIG.  63. — Reinforcement  of  the  Knee  Jerk  Giving  Way  to 
Inhibition.  ( Bo wditch  and  Warren.)  Distances  along  the 
horizontal  line  represent  the  time  elapsing  between  the 
clenching  of  the  fist  and  the  tap  on  the  tendon  which 
evoked  the  jerk.  Distances  above  the  "normal"  line  rep- 
resent the  amount  by  which  clenching  the  fist  increased 
the  movement  of  the  foot,  and  distances  below  the  line 
represent  the  amount  of  decrease.  The  increase  gives 
way  to  decrease  at  about  0.4  sec. 


172    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

to  the  cortex  of  the  brain  may  facilitate  the  reflex  effect  of  stimula- 
tion to  the  skin. 

§  28.  This  facilitating  effect  is,  however,  customarily  one  of 
brief  duration;  and  it  probably  is  subject  to  the  diphasic  law  which 
was  noted  above  (compare  p.  165  and  see  Fig.  63).  That  is  to  say, 
the  reinforcement  soon  gives  way  to  its  opposite,  inhibition,  which 
latter  then  fades  away  more  gradually.  In  some  experiments  on  the 
knee  jerk,  when  reinforced  by  a  vigorous  voluntary  clenching  of  the 
fist,  it  appeared  that  the  phase  of  reinforcement  yielded  to  the  phase 
of  inhibition,  after  about  half  a  second,  and  that  no  effect  of  any 
sort  could  be  discovered  after  a  period  much  exceeding  two  seconds.1 

§  29.  Not  every  two  stimuli,  however,  facilitate  each  other's 
action;  but  only  two  stimuli  which,  when  taken  singly,  lead  to  the 
same  reaction  or  to  "allied"  reactions.  If  the  two  stimuli,  singly, 
lead  to  opposed  or  inconsistent  reactions,  the  one  does  not  facili- 
tate the  other,  but  rather  tends  to  inhibit  it.  For  example,  the  ex- 
ecution of  the  ^scratch  reflex  with  one  hindleg  of  the  animal  is 
inconsistent  with  the  attempt  to  execute  the  same  reflex  with  the 
other  hindleg;  because,  when  one  limb  is  engaged  in  scratching, 
the  other  must  be  used  to  support  and  brace  the  body.  Accord- 
ingly, if  the  skin  of  the  back  is  irritated  in  two  spots  at  the  same 
time,  one  on  each  side  of  the  mid-line,  two  stimuli  are  acting  simul- 
taneously, which  would  tend  to  call  out  inconsistent  results.  One 
of  these  stimulations  must  inhibit  the  other. 

But  it  is  important  to  inquire  into  the  exact  character  of  such 
cases  of  inhibition.  The  law  of  the  results  obtained  from  apply- 
ing two  stimulations  simultaneously  does  not  follow  the  parallelo- 
gram of  forces,  nor  any  sort  of  algebraic  addition;  neither  does  it 
give  an  average  or  compromise  of  the  two  reflexes.  The  actual  re- 
sult is  that  either  one  leg  or  the  other  scratches;  that  is  to  say,  one 
stimulus  gains  the  right  of  way,  and  the  other  is  excluded.  By 
nicely  balancing  the  stimuli,  it  may  be  possible  to  obtain  a  mutual 
inhibition  of  both  reflexes,  and  it  is  often  possible  to  delay  the  onset 
of  either;  but,  finally,  one  or  the  other  is  pretty  sure  to  break  through. 
In  a  normal  state  of  the  nervous  centres,  it  is  probable  that  the  mus- 
cles are  never  simultaneously  excited  for  antagonistic  reflexes;  and 
this  is  true,  although  the  sensory  stimuli  for  both  are  acting  at  the 
same  time.  However  mixed  the  stimuli  at  any  moment — and  they 
are  often  much  mixed — the  reaction  itself  is  never  a  mixture  of 
opposing  movements,  but  is  always  a  harmonious  whole,  consisting 
of  allied  reflexes,  with  their  antagonists  shut  out.  Nothing  could 
be  more  essential  to  well-directed  and  efficient  reaction  than  this 
principle.  And  no  more  conclusive  proof  can  be  demanded  of  the 
1  Bowditch  and  Warrent  Journal  of  Physiology,  1890,  XI,  25. 


INTERFERENCE  OF  REFLEXES  173 

selective  and  purposeful  character  of  the  functions  of  the  nervous 
mechanism — at  least  so  far  as  its  normal  reflex  activities  are  con- 
cerned. 

The  suppression  of  any  given  reflex  by  an  opposing  reflex  is 
not,  however,  the  end  of  the  matter.  Inhibition,  we  saw  (p.  164), 
is  followed  by  a  rebound  effect;  and  a  reflex  which  has  been  inhib- 
ited is,  the  next  moment,  specially  sensitive  to  a  new  stimulus. 
The  scratch  reflex,  in  the  experiment  mentioned  in  the  last  para- 
graph, begins  on  one  side;  but,  if  the  stimuli  continue  to  act,  one 
on  each  side  of  the  animal,  the  scratching  may  shift  from  one  leg  to 
the  other.  Each  of  the  opposed  reflexes  has  its  turn.  This  is 
true,  at  least,  when  the  stimuli  are  about  evenly  balanced.  And, 
in  general,  a  reflex  which  has  just  been  inhibited  is  for  some  time 
afterward  much  more  readily  excited. 

§  30.  The  situation  in  the  spinal  cord  and  other  reflex  centres 
at  any  time  is  accordingly  one  of  competition  between  different 
stimuli  for  the  control  of  the  muscles.  Some  of  the  stimuli  are  al- 
lied, in  the  sense  that  their  action  on  the  muscles  would  be  the  same ; 
but  other  stimuli  are  antagonistic.  There  are  not  simply  two  par- 
ties, since  the  same  muscle  may  be  used  in  several  different  ways 
by  as  many  different  reflexes.  The  scratch  reflex  of  one  leg  is  not 
only  inconsistent  with  that  of  the  other  leg;  but  it  is  also  incompati- 
ble with  the  flexion  reflex  in  either  leg,  with  the  extensor  thrust,  and 
with  the  tonic  postural  reflex  which  would  maintain  the  standing 
position.  However  keen  and  many-sided  the  competition  for  the 
control  of  a  muscle  may  be,  the  control  goes,  at  any  moment,  to 
one  reflex;  and  this  same  reflex  and  its  allies  have  control  of  all 
the  muscles,  either  to  excite  them  or  to  inhibit  their  action.  "All 
the  muscles"  makes  the  preceding  statement  pretty  strong;  it  is 
doubtless  true  in  the  case  of  intense  stimuli  and  widely  spreading 
reflex  effects;  but  in  the  case  of  weak  stimuli,  some  reflexes  may  be 
so  neutral  to  others  that  each  may  go  on  without  interfering  with 
the  other. 

In  the  competition  of  reflexes,  certain  kinds  usually  have  more  or 
less  advantage  over  others.  As  a  rule,  protective  reflexes  have  the 
advantage  over  all  others.  Postural  reflexes,  on  the  other  hand,  are 
usually  the  most  at  a  disadvantage,  and  are  liable  to  be  thrust  aside 
by  almost  any  stimulus  which  calls  for  a  movement. 

The  fatality  and  predictability  of  reflex  action  have  sometimes 
been  overstated.  In  the  case  of  protective  reflexes,  which  have 
the  right  of  way  over  anything  else  in  the  cord,  prediction  is  easy; 
but  in  the  case  of  many  others,  prediction  is  by  no  means  sure,  be- 
cause of  the  diversity  of  the  stimuli  which  are  likely  to  be  acting 
at  the  same  time.  In  general,  reflex  action  is  much  less  predictable 


174    REFLEX  FUNCTIONS  OF  THE  NERVOUS  SYSTEM 

than  the  response  of  a  muscle  to  direct  excitation;  and  the  reflexes 
obtained  from  abbreviated  nervous  centres,  such  as  the  "mid- 
dog"  and  "hind-dog"  mentioned  earlier  in  the  chapter,  are  more 
predictable  than  those  obtained  from  an  entire  and  uninjured  spinal 
cord.  The  rule  seems  to  be  that  the  greater  the  number  of  influ- 
ences to  which  any  organ  is  exposed,  the  greater  will  be  the  varia- 
bility of  its  action,  and  the  less  of  regularity  and  fatality  will  appear 
in  its  responses  to  stimuli. 

§  31.  Many  of  the  characteristics  of  reflex  action — such  as  dis- 
tribution and  convergence,  facilitation  and  inhibition — are  to  be 
regarded  as  fundamental  properties  of  the  action  of  nerve-centres, 
and  as  applicable  to  the  brain,  in  all  probability,  as  well  as  to  the 
cord.  There  are  psychological  facts  (to  be  brought  forward  later) 
which  are  closely  analogous  to  these  facts  of  reflex  action;  and  the 
analogy  is  of  great  importance  in  any  attempt  to  comprehend  the 
action  of  the  brain  in  its  relation  to  these  psychological  phenom- 
ena. The  cord  offers  a  simpler  field  for  the  unravelling  of  nervous 
function,  and  the  results  obtained  are  also  of  great  importance  in  a 
physiological  psychology. 


CHAPTER  VIII 

END-ORGANS,  OR  RECEPTORS,  OF  THE  NERVOUS  SYSTEM 

§  1.  In  order  to  understand  the  end-organs,  or  receptors,  it  is 
necessary  to  refer  again  to  the  place  which  they  hold  in  the  threefold 
arrangement  of  the  nervous  mechanism.  In  the  general  division 
of  labor,  the  function  of  certain  cells  situated  at  the  surface  of  the 
body  becomes  that  of  receiving  the  action  of  the  stimuli,  of  modify- 
ing this  action,  and  thus  of  setting  up  in  the  conducting  nerves  the 
neural  process  which  is  propagated  to  the  central  organs.  It  is 
obvious,  then,  that  the  structure  and  grouping  of  such  superficial 
cells  must  bear  some  definite  relation  both  to  the  external  stimulus 
and  also  to  the  nerve-fibres  which  convey  inward  the  nervous  im- 
pulse occasioned  by  it.  The  end-organs  of  sense  may  then  all  be 
described  as  special  adaptations  of  the  superficial  cells  to  different 
kinds  of  stimuli.  Even  undifferentiated  living  matter  is  chiefly 
sensitive  to  certain  stimuli,  such  as  mechanical  jar,  heat,  chemical 
agents,  and  electricity.  But  in  the  process  of  differentiation,  cer- 
tain cells  become  further  specialized  in  the  direction  of  their  sensi- 
tivity, so  that  very  slight  stimuli  of  a  particular  sort  are  capable 
of  arousing  them.  This  increase  of  sensitivity  is  "specific"  in 
the  sense  that  each  receptor  is  thus  made  highly  sensitive  to  one 
kind  of  physical  agent;  whereas  it  loses  rather  than  gains  in  its  re- 
ceptiveness  toward  other  agents.  The  eye,  for  example — or  more 
precisely  the  retina — combines  with  its  very  delicate  sensitivity 
to  ether  vibrations  of  certain  frequencies  relative  insensitivity  to 
mechanical  jar  and  even  to  other  vibrations  of  the  low  frequency 
which  affect  the  sense  of  temperature. 

It  is  such  specialization  of  receptors  that  gives  precision  and  detail 
to  the  deliverances  of  the  senses.  If  the  eye  were  as  sensitive  to 
sound  as  it  is  to  light,  we  should  see  so  much  that  we  should  get 
little  definite  information  regarding  surrounding  objects.  That  is, 
there  could  be  no  "  apperceptive "  vision.  The  particular  agent  to 
which  a  receptor  is  adapted  is  called  its  "adequate  stimulus." 

§  2.  In  the  end-organs  of  the  special  senses  the  fibrils  of  the 
sensory  nerves,  as  a  rule,  terminate  in  cellular  structures  which  have 
the  morphological  significance  of  metamorphosed  epithelial  cells. 
The  end-organs  of  smell  show  this  characteristic  development 

175 


END-ORGANS  OF  SMELL 


177 


Most  physiologists  follow  Schultze  in  holding  that  the  two  kinds 
of  cells  are  distinct  both  in  form  and  in  function,  and  that  only  the 
"olfactory"  cells  are  connected  with  the  end-fibrils  of  the  nerve  of 
smell,  the  others  serving  the  purpose  of  supporting  the  olfactory 
cells.  The  internal  or  proximal  process  of  the  olfactory  cell  is  a 
nerve-fibre,  which  passes  back  through  the  cribriform  or  sieve-like 
plate  of  the  ethmoid  bone  into  the  brain  cavity;  and  these  fibres 
from  the  numerous  olfactory  cells  are  made  up  into  small  bundles 
which,  taken  together,  are  called  the  olfactory  nerve.  Arrived 
within  the  brain  cavity,  the  fibres  penetrate  the 
olfactory  bulb,  and  there  end,  forming  synapses 
with  other  fibres  which  conduct  further  back 
into  the  brain  (compare  p.  107). 

§  5.  The  contrivance  for  applying  the  stimulus 
to  the  end-organs  of  smell  is  very  simple;  in 
general  it  is  only  necessary  that  a  current  of  air, 
in  which  the  stimulating  particles  float,  shall  be 
drawn  through  the  nasal  passages  over  the 
mucous  membrane  of  the  regio  olfactoria.  Even 
ammonia  and  camphor,  when  placed  under  the 
nostrils,  have  no  smell  so  long  as  the  breath  is 
held  or  drawn  through  the  mouth.  In  quiet  inr 
spiration  much  the  greater  part  of  the  current 
of  air  is  conducted  to  the  pharynx  directly,  and 
comparatively  little  reaches  the  ridge  situated 
above  the  nasal  dam  at  the  back  of  the  nose, 
where  the  end-organs  of  smell  are  placed.  In 
full  inspiration,  and  still  more  when  short  and 
deep  draughts  are  drawn  through  the  nasal 
passages,  a  considerable  amount  of  the  air  is 
forced  over  the  sensory  parts.  By  snuffing  we 
increase  the  amount  of  air  drawn  into  the  region  by  first  creating 
a  partial  vacuum  in  its  cavity,  and  also  by  creating  eddies  in  the 
air  current,  which  carry  the  odoriferous  substance  out  of  the  main 
stream  and  into  the  olfactory  recess.  In  expiration  the  breathing 
passage  is  so  located  as  to  carry  nearly  all  the  air  past  the  sensory 
parts  without  striking  them.  For  this  reason  smelling  is  almost 
exclusively  confined  to  inspiration;  it  has  been  disputed  whether  the 
current  of  expiration  can  be  smelled  at  all.  But  Debrou  showed 
that  the  odor  of  orange  blossoms,  when  water  tinctured  with  them 
has  been  drunk,  can  be  detected  in  the  expired  air.  The  current 
which  passes  through  the  anterior  part  of  the  nasal  passages  seems  to 
be  the  more  important.  This  is  probably  the  reason  why  the  loss 
of  the  nose  is  so  frequently  attended  with  loss  of  the  sense  of  smell. 


FIG.  64.— Olfactory 
Cells  and  Epithelial 
Cells  from  the  Mucous 
Membrane  of  the 
Nose.  «>%.  (After 
Schultze.) 


178  END-ORGANS  OF  THE  NERVOUS  SYSTEM 

§  6.  The  end-organs  of  taste,  called  gustatory  bulbs  or  flasks, 
or  more  commonly  taste-buds,  lie,  for  the  most  part,  on  the  upper 
surface  and  edges  of  the  tongue;  though  some  occur  on  the  soft 
palate,  and  even  on  the  epiglottis  and  in  the  larynx.  More  precisely, 
they  are  situated  in  the  papillae  or  projections  which  give  a  rough 
surface  to  the  tongue.  The  roughness  is,  however,  chiefly  due  to 
the  filiform  papillae,  which  do  not  contain  taste-buds;  the  latter 
being  found  in  the  circumvallate  papillae  (Fig.  65)  at  the  back  of 
the  tongue,  and  in  the  fungiform,  scattered  over  its  upper  surface. 
The  circumvallate,  few  in  number  but  large  in  size,  are  circular 

and  surrounded  by  a  trench,  in 
the  walls  of  which  are  embedded 
the  taste-buds.  The  fungiform 
papillae  are  comparatively  small 
structures;  the  taste-buds  lie  in 
their  tops  and  sides. 

The  extent  of  distribution  of 
the  taste-buds,  and  of  the  sense 

FIG.   65.— Transverse    Section   through    a      of  taste,  On  the  upper  Surface  of 
Papilla  Circumvallata  of  a  Calf.     Show-       ,1        ,  •  u«*    •„ 

ing  the  arrangement  and  distribution  of       the    tongue    Varies    Somewhat    in 

the  gustatory  bulbs.  *£.  (Engeimann).  different  individuals,  and  con- 
siderably as  between  children 

and  adults.  In  young  children  all  of  the  upper  surface  of  the 
tongue  has  taste-buds,  but  in  adults  the  front  third  is  free  from 
them,  except  at  the  edges;  and  there  is  no  sense  of  taste  in  this 
part  of  the  tongue. 

§  7.  Taste-buds  resemble  minute  bulbs  (less  than  one-tenth  of 
a  millimetre  in  length)  growing  in  the  mucous  membrane.  In 
shape  they  are  flask-like,  narrowing  to  a  neck  and  opening  to  the 
surface  by  a  little  pore  (see  Fig.  65).  They  are  composed  of  cells 
which  are  arranged  like  the  leaves  of  a  bud  in  closely  compressed 
rows  around  the  axis.  As  in  the  olfactory  end-organ,  there  are  here 
two  sorts  of  cells  (Figs.  66  and  67),  the  supporting  and  the  gustatory. 
The  latter,  also  called  taste-cells,  are  slender  with  a  central  enlarge- 
ment containing  the  nucleus,  and  two  processes.  One  process  ex- 
tends toward  the  mouth  or  pore  of  the  taste-bud,  and  terminates 
in  a  short  hair-like  projection,  which  enters  into  the  pore.  The  other 
process  extends  away  from  the  pore,  and  is  often  branched. 

Sensory  nerve-fibres  penetrate  the  taste-bud,  and  branch  among 
and  around  the  taste-cells.  These  fibres  are  derived  from  three  of 
the  cranial  nerves.  The  glosso-pharyngeal  supplies  the  rear  of 
the  tongue,  and  the  lingual  branch  of  the  trigeminus  the  front  of 
the  tongue;  while  the  vagus  supplies  the  few  taste-buds  in  the  phar- 
ynx and  larynx.  The  course  of  the  fibres  from  the  front  of  the 


END-ORGANS  OF  TOUCH 


179 


tongue  is  curiously  intricate  and  apparently  varies  in  different 
individuals.  From  the  lingual  nerve  they  pass  to  the  chorda 
tympani,  which  crosses  the  cavity  of  the  middle  ear;  in  some  cases 
they  enter  the  medulla  by  way  of  the  intermediate  nerve. 

§  8.  An  interesting  example  of  the  far-reaching  results  of  that 
more  discriminating  and  thorough  analysis  in  which  modern  science 
delights,  is  afforded  by  the  case  of  the  so-called  Sense  of  Touch. 
It  was  formerly  customary  to  lump  together  all  forms  of  sensation 
caused  by  irritating  any  area  of  skin,  and  to  classify  them  all  as 
one  of  the  five  senses  with  which  the  human  animal  was  endowed. 


FIG.  66. — Isolated    Gus-     FIG.  67. — a,  Isolated  Gustatory  Cells,  from  the  Lateral 
tatory  Bulb,  from  the         Organ  of  the  Rabbit;    6,  an  Investing  and  Two  Gus- 
tatory  Cells,   isolated  but  still  in  connection.    ^^. 
(Engelmann.) 


Lateral  Gustatory  Or- 
gan of  the  Rabbit. 
(Engelmann.) 


It  is  now  recognized,  however,  that  the  conscious  states  which  re- 
sult from  applying  different  stimuli  to  the  superficial  area  of  the 
body  resemble  one  another  scarcely  more  closely  than  do  sight  and 
hearing,  and  no  more  closely  than  do  smell  and  taste. 

In  considering  the  sensory  end-organs  in  the  skin,  we  must  there- 
fore anticipate  what  will  be  said  in  the  chapter  on  qualities  of  sen- 
sation regarding  the  division  of  the  old  "sense  of  touch"  into  several 
senses,  namely,  those  for  contact,  for  temperature,  whether  warm 
or  cold,  and  for  pain.  The  problem  which  psychology  proposes 
to  physiology  is,  accordingly,  that  of  discovering,  if  possible,  the 
minute  structures  in  the  skin  which  serve  as  receptors  for  these 
different  forms  of  sensation.  It  may  be  said  at  once,  however, 
that  this  is  possible  only  to  a  limited  extent  from  present  knowl- 
edge. 

§  9.  The  short  hairs  on  hairy  surfaces,  which  comprise  ninety- 
five  per  cent,  of  the  area  of  the  skin,  are  to  be  regarded  as  specific 
organs  of  the  tactile  sense.  About  the  root  of  the  hair  is  coiled 
the  termination  of  a  sensory  nerve-fibre.  The  nerve-ending  is 
thus  excited  by  touching  the  hair,  or  by  touching  the  skin  on  the 
"windward"  side  of  the  hair. 


180 


END-ORGANS  OF  THE  NERVOUS  SYSTEM 


FIG.   68. — Corpuscle    of    Touch 
from  the  Palm  of  the  Human 


On  hairless  surfaces  there  occurs  in 
great  numbers  a  form  of  nerve-ending 
which  apparently  takes  the  place  of  the 
hair-receptor.  This  form  is  called  the 
"touch-corpuscle"  (see  Fig.  68).  These 
corpuscles  occur  in  some  of  the  papillae 
of  the  skin  of  the  palm  and  sole.  They 
are  composed  of  a  capsule  of  connective 
tissue  and  a  core  of  cells  among  which 
winds  the  branched  termination  of  a 
nerve-fibre. 

These  two  forms  of  end-organ  may 
with  probability  be  assigned  to  the  sense 
of  touch  proper;  but  it  is  not  certain 
that  none  of  the  other  forms  of  nerve- 
ending  met  with  in  the  skin  are  con- 
nected with  this  sense. 

"End-bulbs"  of  various  shapes  also 
Forefinger.  (Ranvier.)  n,  sen-  occur  jn  the  skin,  some  cylindrical,  some 

sory    axon  ;    a,    its    branching  ,          ,       .      ,    '          .       /  .  , 

termination  within  the  corpuscle,    nearly  spherical.    1  heir  structure  is  much 

like  that  of  the  touch-corpuscles:  a  cap- 

sule encloses  a  core,  into  which  penetrates  a  sensory  nerve-fibre. 
The  nerve-fibre  loses  its  sheaths,  and  coils  and  ramifies  as  a 
naked  axon  in  the  interior  of  the  bulb  (see  Fig.  69). 

The  most  highly  developed  in  structure  of  the  cutaneous  end- 
organs  are  the  Pacinian  corpuscles  (compare 
Fig.  70).  They  are  larger  than  the  other  forms, 
and  their  capsule  is  composed  of  concentric 
plates  of  connective  tissue  like  the  layers  of  an 
onion.  Each  Pacinian  corpuscle  is  entered  at 
one  end  by  a  nerve-fibre,  which  loses  its  myelin 
sheath  in  the  interior  of  the  corpuscle,  and,  pass- 
ing along  the  axis,  terminates  near  the  other  end. 
The  Pacinian  corpuscles  lie  just  beneath  the 
skin,  or  deeper  in;  and  also  in  other  situations, 
as  in  the  muscles,  in  the  neighborhood  of 
tendons,  ligaments,  and  bones,  and  in  the 
mesentery. 

There  is  still  another  mode  of  nerve-ending  on 
the  skin,  quite  different  from  the  forms  already    Fig 
described,  in  that  the  nerve-fibre  branches  freely 
among  the  cells  of  the  skin,  being  unprovided 
with  a  capsule   (Fig.  71).     This  form  may  be      its  branching  termi- 

,11     i          ((  e  jj.        „  rf  nation  within  c,  the 

called  a  "free  nerve-ending.  capsule. 


_  End.Bulb 
from    the  Human 


THE  PACINIAN  CORPUSCLES 


181 


§  10.  As  to  the  specific  function  of  the  end-bulbs,  Pacinian 
corpuscles,  and  free  nerve-ends,  there  is  nothing  of  a  conclusive 
nature  to  bring  forward.  The  distribution  of  the  various  forms 
over  the  skin  is  unequal,  and,  since  the  distribution  of  the  four 
cutaneous  senses  is  also  unequal,  attempts  have  been  made1  to 
determine  the  function  of  the  end-organs  by  correlating  their  dis- 
tribution with  that  of  the  senses.  Thus,  spherical  end-bulbs  occur 
in  the  conjunctiva  of  the  eye,  which,  to  many  observers,  lacks  tac- 
tile sensation,  though  possessing  that  of 
temperature  and  especially  that  of  cold; 
this  form  of  end-organ  may  very  well, 
then,  be  the  receptor  for  cold.  On  ac- 
count of  the  longer  reaction-time  to 
warmth  than  to  cold,  and  of  the  less 
sharp  localization  of  the  "warmth- 
spots,"  it  is  believed  that  the  end- 
organs  for  this  sense  probably  lie  in 
the  deeper  layers  of  the  skin,  and  this 
fact  leads  to  the  supposition  that  a 
cylindrical  form  of  end-bulb,  which 
occurs  deep  in  the  skin,  may  be  the 
warmth-receptor. 

The  pain  sense  is  perhaps  served  by 
the  free  nerve-ends.     A  fact  supporting 
this  view  is  the  absence  of  other  sense 
qualities  from  the  cornea,  the  nerve-    FIG.  70.— Corpuscle  of  Pacini  (or 
ends  in  which  are  of  the  free-branching 

type.       The    Pacinian    Corpuscles,  from 

their  location,  must  serve  a  subcutane- 
ous form  of  sensibility — perhaps  what  is       thYnerve-fibre  ends. 
known  as  "deep  sensibility"  to  pressure. 

Sensory  endings  of  complicated  structure  are  found  in  muscles 
and  their  tendons,  and  are  of  interest  in  connection  with  the  "mus- 
cle sense."  The  "muscle  spindle"  (Fig.  72)  occurs  embedded  in 
the  substance  of  the  muscle,  and  is  composed  of  several  modified 
muscle  fibres  bound  together  by  a  capsule  of  connective  tissue,  and 
supplied  by  one  or  more  sensory  nerve-fibres,  which,  losing  their 
myelin  sheath,  break  up  into  fine  branches  that  coil  around  the 
muscle-fibres  within  the  spindle.  That  these  are  sense-organs  has 

1  Especially  by  von  Frey,  Beitrdge  zur  Sinnesphysiologie  der  Haut,  in  Be- 
richte  d.  k.  sacks.  Gesellsch.  d.  Wissensch.  zu  Leipzig,  math.-phys.  Klasse,  1894, 
1895,  1897;  see  also  later  discussions  by  Sherrington,  in  Schafer's  Textbook  of 
Physiology,  1900,  II,  920  ff.,  and  by  Thunberg  in  Nagel's  Handbuch  der  Physi- 
ologic, 1905,  III,  654. 


its  sheaths;  6,  system  of  tunics 
constituting  the  capsule  of  the 
corpuscle;  c,  axial  canal,  in  which 


182 


END-ORGANS  OF  THE  NERVOUS  SYSTEM 


been  proved  by  showing  that  the  nerve-fibres  which  enter  them  do 
not  degenerate  after  section  of  the  ventral  spinal  roots  supplying 
the  muscle  in  question;1  these  nerve-fibres,  therefore,  come  from  the 
dorsal  roots  and  are  sensory.  Tendon  spindles  occur  in  the  part 
of  a  tendon  near  to  its  muscle,  and  are  of  very  similar  structure  to 
the  muscle  spindles.  Other  simpler  sensory  end-organs — Pacinian 
corpuscles  and  end-bulbs — are  found  in  muscles,  tendons,  the  cap- 
sules of  joints,  the  periosteum  and  interior  of  bones;  and  these  all 
may  be  connected  with  that  complex  of  sensory  apparatus  which 
goes  by  the  inexact  name  of  "muscle-sense." 

§  11.  With  the  exception  perhaps  of  the  ear,  the  eye  is  by  far 
the  most  elaborate  and  complicated  of  the  end-organs  of  sense. 
This  is  true  of  those  portions  of  it  which  are  designed  merely  to 


\~? 


Fig.  71. — Free  Branching  Sensory  Axons  from  the  Larynx.     (Retzius.) 
n,  axons. 

bring  the  external  stimulus  to  bear  upon  the  nervous  structure,  as 
well  as  of  this  structure  itself.  Considering  it  as  a  whole,  we 
may  say  that  the  peripheral  organ  of  sensations  of  light  and  color 
is  an  optical  instrument  constructed  on  the  plan  of  a  water  camera 
obscura,  with  a  self-adjusting  lens,  and  a  concave,  sensitive,  nervous 
membrane,  as  a  screen  on  which  the  image  is  formed. 

§  12.  The  eyeball  consists  of  three  coats  or  tunics  enclosing  three 
translucent  refracting  media.  Since,  however,  the  front  part  of 
the  outer  one  of  these  coats  is  itself  translucent  and  refracting, 
the  number  of  refracting  media  in  the  eye  is  really  four.  (l)The 
first  or  external  coat  consists  of  two  parts:  (a)  the  Sclerotic  or 
posterior  five-sixths  part  ("white  of  the  eye"),  which  is  a  firm, 
fibrous  membrane  formed  of  connective  tissue  intermingled  with 
elastic  fibres;  and  (6)  the  Cornea,  or  translucent  anterior  one-sixth 
part,  which  is  circular  and  convex  in  form,  and  covered  with  con- 

1  Sherrington,  Journal  of  Physiology,  1894,  XVII,  211  ff.  Sensory  endings 
are  found  also  in  the  mucous  membrane  and  quite  widely  distributed  through 
the  viscera. 


STRUCTURE  OF  THE  EYEBALL 


183 


J'unctival  epithelium.  The.  cornea  rises  and  bulges  in  the  middle 
ike  a  watch-glass.  (2)  The  second  coat,  or  tunic  of  the  eye,  also 
consists  of  two  parts:  these  are  (a)  the  Choroid  coat,  which  com- 
prises much  its  larger  portion,  is  of  a  dark  brown  color,  due  to  its 
pigment  cells  (except  in  the  case  of 
albinos),  and  is  abundantly  provided 
with  nerves  and  blood-vessels;  and 
(6)  the  Iris,  a  circular,  flattened, 
disk-shaped  diaphragm  in  front  of 
the  lens  (the  colored  part  of  the 
visible  eyeball),  bathed  with  aqueous 
humor,  and  having  in  its  centre  a 
circular  aperture  called  the  " pupil" 
of  the  eye.  The  anterior  border 
(corpus  ciliare)  around  the  iris  con- 
sists of  the  ciliary  muscle  and  the 
ciliary  processes.  (3)  The  Retina  is 
the  third  or  inner  coat  of  the  eye. 
It  is  a  delicate  membrane  of  exqui- 
site transparency  and  almost  perfect 
optical  homogeneity;  it  has  a  highly 
complex  structure,  consisting  of  nine 
or  ten  layers,  the  truly  nervous  por- 
tions of  which  contain  nerve-fibres, 
nerve-cells,  and  special  end-organs,  , 
together  with  supporting  tissue  and 
blood-vessels.  The  inner  surface  of 
the  retina  is  moulded  on  the  vitreous 
body,  and  it  extends  from  the  en- 
trance of  the  optic  nerve  nearly  as 
far  forward  as  the  ciliary  processes. 
§  13.  The  eyeball  has  four  trans- 
lucent refracting  media.  The  first  of 
these — enumerating  inward  from  the 
outside  front — is  (1)  the  Cornea,  Fig.  72.— A  Muscle  Spindle.  (Ciaccio.) 
already  spoken  of  as  the  anterior 

one-sixth  of  the  outer  coat  of  the  eye.  (2)  The  Aqueous  Humor 
fills  the  space  between  the  cornea  and  the  lens,  and  is  divided  by 
the  iris  into  two  chambers,  of  which  the  front  one  is  much  the 
larger.  It  is  limpid  and  watery;  it  holds  in  solution  the  salts  of 
the  blood-serum,  with  traces  of  organic  substances.  (3)  The  Crys- 
talline Lens  is  situated  between  the  iris  and  the  vitreous  body.  It  is 
a  transparent  biconvex  lens,  with  its  antero-posterior  diameter  about 
one-third  less  than  the  transverse  diameter.  It  consists  of  a  capsule 


rfnc 


184 


END-ORGANS  OF  THE  NERVOUS  SYSTEM 


and  enclosed  body.  It  is  of  "buttery  consistency,"  composed,  like 
an  onion,  of  a  number  of  easily  separable  layers.  Each  layer  consists 
of  fibres,  which  within  the  layer  are,  as  a  rule,  radial.  Between 
the  entire  ciliary  part  of  the  retina  and  the  corresponding  part  of 
the  vitreous  humor  is  interposed  a  structureless  membranous  body, 


FIG.  73.— Horizontal  Section  through  the  Left  Eye. 
from  Gegenbaur.) 


(Schematic, 


to  which  the  edge  of  the  lens  is  attached,  and  which  radiates  out- 
ward and  maintains  the  lens  in  tension.  It  is  called  the  suspen- 
sory ligament  (or  Zonida  of  Zinn),  and  its  office  is  very  important 
in  the  accommodating  of  the  eye  to  different  distances.  (4)  The 
Vitreous  Humor  consists  of  a  number  of  firm  sheets  or  layers 
(lamellae),  between  which  fluid  is  contained,  built  into  a  body  that 
is,  optically  considered,  transparent  and  homogeneous.  It  occupies 


MUSCLES  OF  THE  EYEBALL 


185 


most  of  the  space  enclosed  by  the  tunics  of  the  eye.  It  is  thought 
to  be  a  gelatinous  form  of  connective  tissue,  and  is  composed  most- 
ly of  water  with  salts  in  solution,  of  proteids  and  mucin,  fats  and 
extractive  matters — especially  urea.  Its  peculiar  structure  is  of 
little  significance  for  the  physiology  of  the  eye. 

§  14.  Of  the  appendages  or  accessory  parts  of  the  eye — such  as 
the  eyebrows,  the  eyelids,  lachrymal  apparatus,  muscles  of  the 
eyeball — only  the  mechanism  by  which  the  eye  is  moved  in  its 


fr 


o.r 


Fia.  74.— Muscles  of  the  Left  Hu- 
man Eye,  seen  from  above,  rs, 
rectus  superior;  re,  rectus  ex- 
ternus;  and  rit,  rectus  internus; 
os,  superior  oblique,  with  its 
tendon,  t,  which  runs  through  the 
membranous  pulley,  u,  at  the 
inner  wall  of  the  cavity  of  the 
eyeball. 


FIG.  75.— Muscles  of  the  Left  Human  Eye, 
seen  from  the  outside.  Ir,  levator  of  the  up- 
per eyelid,  which  covers  the  rectus  superior, 
rs,  re,  os,  as  in  the  preceding  figure;  rif, 
rectus  inferior;  oi,  inferior  oblique. 


orbit  has  any  special  significance  for  physiological  psychology. 
The  building-up  of  a  world  of  visible  objects,  and  even  the  forma- 
tion of  a  so-called  "field  of  vision,"  is  dependent  upon  the  great 
mobility  of  the  eye.  The  eyeball  is  moved  in  its  bony  socket, 
where  it  is  embedded  in  a  mass  of  fat  as  in  a  socket-joint,  by  six 
muscles,  which  are  attached  to  it  somewhat  like  the  bridle  to  the 
horse's  head.  Four  of  these  muscles  spring  from  the  bony  wall 
near  the  point  where  the  optic  nerve  enters,  extend  through  the 
length  of  the  socket,  and  pass  directly  to  the  eyeball,  where  they 
are  attached  to  it,  one  above,  one  below,  one  on  the  outer,  and  one 
on  the  inner  side  (the  recti  internus  and  ext-ernus,  superior  and  infe- 
rior). In  moving  both  eyes  up  or  down,  the  same  muscles  in  both 
contract  simultaneously;  in  moving  the  eyes  to  the  right,  the  outer 
muscle  of  the  right  eye  and  the  inner  of  the  left  contract  simul- 
taneously (and  vice  versa) ;  in  turning  both  eyes  inward  to  converge 
them  upon  a  near  object,  the  two  inner  muscles  contract  together. 


tK, 

** 


186  END-ORGANS  OF  THE  NERVOUS  SYSTEM 

The  other  two  of  the  six  muscles  of  the  eye  are  called  oblique.  Of 
these  one  is  superior  and  internal;  it  does  not  pass  directly  forward 
from  its  place  of  origin,  at  the  posterior  aperture  through  which 
the  optic  nerve  enters  to  the  eye,  but  first  runs  through  a  ring,  then 
turns  around,  and  is  attached  obliquely  to  the  upper  surface  of  the 
eyeball.  The  other  oblique  muscle  begins  at  the  inner  wall  in  the 
socket,  passes  under  the  eyeball,  and  is  attached  to  it  opposite  to 
the  superior  oblique  muscle.  The  two  oblique  muscles  combine 
with  the  four  recti  to  move  the  eyes  in  various  directions  which 
would  be  impossible  for  the  latter  alone. 

§  15.  The  problem  which  is  to  be  solved  by  the  end-organ  of 
vision  may  be  stated  in  a  general  form  as  follows:  A  mosaic  of 
localized  sensations  must  be  so  constructed  that  changes  in  the 
quantity,  quality,  local  relation,  and  sequence  of  these  sensations 
shall  be  quickly  interpreted  as  indicative  of  the  color,  shade,  size, 
shape,  locality,  and  motion  of  external  visible  objects.  The  most 
important  part  of  the  solution  of  this  problem  falls  upon  the  nervous 
structure  of  the  retina.  It  is  itself  a  mosaic  of  nervous  elements,  the 
excitation  of  which  may  vary  in  quality,  quantity,  local  coloring, 
and  sequence  of  the  different  elements  excited.  But  in  order  that 
the  retina  may  exercise  its  function  with  the  precision  and  delicacy 
of  detail  for  which  its  structure  fits  it,  the  rays  of  light  reflected 
from  a  single  point  of  the  surface  of  the  visible  object  must  excite  a 
single  one,  or  at  most  a  small  and  definite  group,  of  the  retinal 
nervous  elements.  The  sensations  thus  occasioned  can  then  un- 

rgo  a  systematic  arrangement  by  the  mind.  It  is  the  work  of 
the  translucent  refracting  media  of  the  eye  to  apply  the  stimulus  to 
retinal  elements  exactly  discriminated,  and  in  an  order  correspond- 
ing to  the  object;  that  is  to  say,  the  cornea,  the  humors  of  the  eye, 
and  the  lens  must  form  an  image  on  the  retina. 

§  16.  Light  entering  the  eye  passes  successively  through  the  cor- 
nea, the  aqueous  humor,  the  lens,  and  the  vitreous  humor.  At 
each  of  the  surfaces  between  these  media  the  light  suffers  refrac- 
tion. Since  the  surfaces  at  which  the  refraction  occurs  are  approxi- 
mately spherical,  and  the  centres  of  the  spherical  surfaces  all  lie  in 
one  straight  line — the  so-called  "optic  axis" — the  eye  is,  optically 
considered,  a  centred  system.  An  important  result  of  this  arrange- 
ment is  that  it  avoids  distortion  in  the  image.  For  example,  the  pen- 
cils of  rays  issuing  from  the  various  points  of  any  plane  perpendicular 
to  the  axis  will  be  finally  focussed  in  points  which  lie  in  a  plane  hav- 
ing the  same  direction;. and  the  image  formed  of  these  latter  points 
will,  therefore,  be  in  approximately  true  proportion  to  the  seen  object. 

To  determine  the  refractive  power  of  the  eye,  we  need  to  know 
(1)  the  index  of  refraction  of  each  of  the  media  through  which  the 


INDICES  OF  REFRACTION  187 

light  passes,  (2)  the  radius  of  curvature  of  each  of  the  surfaces  at 
which  refraction  occurs,  and  (3)  the  distance  apart  of  these  sur- 
faces. 

The  indices  of  refraction  which  have  been  worked  out  by  vari- 
ous ingenious  methods  are  in  round  numbers  the  following:  for 
aqueous  and  vitreous  humors,  1.34,  or  very  nearly  the  same  as  for 
water;  for  the  cornea,  1.38;  for  the  lens,  increasing  from  1.39  in  the 
outer  layers  to  1.41  at  the  centre.  Since  the  curvature  of  the  layers 
of  the  lens  becomes  sharper  from  the  outside  inward,  the  refract- 
ive power  of  the  lens  is  increased;  taken  as  a  whole,  it  is  equiva- 
lent to  a  lens  of  the  same  size  and  shape  with  an  index  of  about 
1.44.  But  since  the  cornea  is  thin  and  differs  but  little,  in  its 
index  of  refraction,  from  the  adjacent  aqueous  humor,  it  may  be 
considered  as  a  part  of  the  latter.  The  only  surfaces  at  which  in- 
coming rays  are  effectively  bent  are,  therefore,  the  outer  surface  of 
the  cornea,  and  the  front  and  rear  surfaces  of  the  lens.  The  radii 
of  these  surfaces  are  as  follows:  of  the  cornea,  about  8  millimetres; 
front  of  lens,  about  10  millimetres;  back  of  lens,  about  6  milli- 
metres. It  should  be  understood,  however,  that  these  measure- 
ments differ  considerably  in  different  individuals. 

The  distance  from  the  corneal  surface  to  the  front  of  the  lens 
is  about  3.6  millimetres,  and  the  thickness  of  the  lens  about  the 
same.  The  data  are  thus  provided  for  calculating  the  strength  of 
the  eye  as  an  optical  instrument,  and  the  calculation  gives  it  a 
strength  of  about  67  diopters.  Its  power  is  such  that  parallel 
rays — rays  from  a  distant  object — entering  the  eye  are  brought  to  a 
focus  at  a  distance  of  about  20  millimetres  behind  the  cornea — 
a  distance  which  in  the  normal  eye  corresponds  to  the  actual  lo- 
cation of  the  retina.1  If,  however,  the  axis  of  the  eye,  from  the 
cornea  to  the  retina,  is  too  long  for  the  refractive  power,  the  image 
of  distant  objects  is  formed  in  front  of  the  retina,  and  only  near 
objects  can  be  clearly  seen  (near-sightedness  or  myopia);  whereas 
if  the  axis  of  the  eyeball  is  too  short,  the  image  of  distant  objects 
will  be  formed  behind  the  retina,  and  the  refractive  power  of  the 
eye  must  be  increased  to  permit  of  clear  vision  (hypermetropia).  If, 
as  is  common,  the  surface  of  the  cornea  is  not  truly  spherical,  the 
rays  of  light  are  brought  to  a  line  rather  than  a  point  on  the  retina 
(astigmatism). 

§  17.  The  preceding  remarks  apply  to  what  is  called  the  "rest- 
ing eye."  The  eye,  when  its  internal  mechanism  is  at  rest,  is  fo- 
cussed  on  distance,  and  does  not  form  clear  images  of  near  objects. 

1  These  measurements  and  calculations  are  from  several  investigators,  and 
are  here  cited  after  Schenck,  in  NagePs  Handbuch  der  Physiologie,  1905,  III, 
38  ff. 


188  END-ORGANS  OF  THE  NERVOUS  SYSTEM 

To  obtain  clear  vision  for  near  objects,  the  optical  power  of  the  eye 
must  be  increased.  The  process  by  which  this  is  accomplished  is 
called  "accommodation."  The  mechanism  of  accommodation 
differs  in  different  orders  of  animals;1  in  amphibia  and  reptiles 
the  lens  is  moved  forward  as  in  the  camera.  In  mammals  this  is 
not  the  case,  but  the  power  of  the  eye  is  increased  by  increasing 
the  convexity  of  the  lens;  its  front  surface  bulges  forward,  becoming 
more  curved,  while  its  rear  surface  remains  in  position. 

There  are  several  methods  of  experiment  which  demonstrate 
that  in  accommodation  for  near  distances  the  front  of  the  lens  be- 
comes more  strongly  arched.  When  accommodation  is  taking 
place,  the  pupil  may  be  seen  not  only  to  contract,  but  also  to  draw 
its  edge  forward.  Helmholtz  calculated  the  amount  of  this  forward 
movement  for  two  cases  at  about  -^  and  -fa  of  an  inch,  respectively. 
Moreover,  by  an  ingenious  contrivance  the  image  reflected  from  the 
anterior  surface  of  the  lens  may  be  watched  as  it  becomes  smaller 
and  more  distinct  on  adjustment  for  near  distances,  thus  showing 
that  the  surface  from  which  it  is  reflected  has  increased  its  curva- 
ture. 

It  is  obvious  that  the  mechanism  for  adjusting  the  eye  must  be 
under  the  brain's  control,  since  adjustment  is  voluntary;  and  that 
it  must  consist  of  muscles  which  lie  within  the  eyeball.  The  ac- 
cepted hypothesis  concerning  the  nature  and  action  of  this  mechan- 
ism was  first  proposed  by  Helmholtz.  This  investigator  assumes 
that  the  lens,  when  the  eye  is  at  rest,  does  not  have  the  form  which 
corresponds  to  a  condition  of  equilibrium  in  its  own  elastic  power. 
If  it  were  not  held  in  by  its  surroundings,  it  would  be  more  arched 
than  it  is  both  before  and  behind.  But  it  is  kept  flattened  by  the 
radial  tension  of  the  suspensory  ligament;  when  this  tension  is  with- 
drawn the  lens  becomes  curved  by  the  action  of  its  own  elasticity. 
The  withdrawal  of  the  tension  is  accomplished  by  the  action  of  the 
ciliary  muscle,  the  fibres  of  which  have  their  point  of  fixation  at  the 
edge  of  the  cornea,  and  run  from  here  in  the  direction  of  a  merid- 
ian toward  the  equator  of  the  eye.  When  the  ciliary  muscle  con- 
tracts, the  free  ends  of  its  fibres  are  drawn  toward  its  fixed  ends 
on  the  edge  of  the  cornea;  the  radial  tension  of  the  suspensory  lig- 
ament is  thus  relaxed,  and  the  lens  is  allowed  to  assume  its  natural 
form  under  the  equipoise  of  its  own  elastic  forces  (Fig.  76). 

The  iris  of  the  eye  corresponds  to  the  diaphragm  of  the  camera. 
It  contains  two  sets  of  contractile  fibres,  one  circular,  the  effect  of 
which  is  to  diminish  the  size  of  the  pupil;  the  other  radial  and  en- 
larging the  pupil.  Contraction  of  the  pupil  occurs  under  two  con- 
ditions: When  bright  light  (or,  more  exactly,  light  brighter  than  that 
1  Beer,  Wiener  klin.  Wochenschrift,  1898,  XI,  942. 


MECHANISM  OF  ACCOMMODATION 


189 


to  which  the  retina  is  at  the  time  adapted)  enters  the  eye;  and  when 
the  eye  is  accommodated  for  a  near  object.  In  both  cases  the  effect 
of  narrowing  the  pupil  is  to  increase  the  clearness  of  vision.  A  sud- 
den bright  light  has  a  dazzling  effect,  and  this  is  lessened  by  di- 
minishing the  quantity  of  light  entering  the  eye.  The  divergent 
rays  from  a  near  object  are  not  so  well  focussed  as  are  the  more 
nearly  parallel  rays,  on  account  of  spherical  aberration;  but  this  bad 
effect  is  diminished  by  cutting  off  the  more  oblique  rays  which 
enter  through  the  periphery  of  the  cornea  and  lens. 

The  nerves  which  supply  the  muscles  of  the  eye  are  the  third, 
fourth,  and  sixth  of  the  cranial  nerves,  and  the  sympathetic.  Of 
these,  the  third  or  oculomotor  is  the  largest;  in  it  are  contained  the 
fibres  which  supply 
the  ciliary  muscle, 
and  those  which 
supply  the  circular 
muscle  of  the  iris. 
The  fibres  which 
supply  the  radial 
muscle  of  the  iris 
are  from  the  sym- 
pathetic, and  arise  Fig.  76.— The  Change  of  the  Lens  in  Accommodation, 
from  a  ppntrp  in  thp  (Helmholtz.)  The  left  half  of  the  figure  shows  the  lens 
focussed  on  a  distant  object,  the  right  half  on  a  very  near 
Cervical  COrd.  The  object,  c,  the  ciliary  muscle;  s,  the  suspensory  ligament. 

fibres  of  the  oculo- 
motor nerve  originate  in  the  mid-brain,  in  the  floor  of  the  third 
ventricle.  Convergence,  accommodation,  and  constriction  of  the 
pupil  can  all  and  severally  be  excited  by  stimulating  special  por- 
tions of  this  region.  The  pupillary  reflex  to  light  has  as  its  sensory 
nerve  the  optic;  the  fibres  concerned  are  those  which  pass  to  the 
anterior  quadrigemina,  whence,  apparently,  other  fibres  pass  to  the 
nucleus  of  the  oculomotor  nerve  and  so  excite  the  motor  fibres  to 
the  iris. 

§  18.  Given  the  formation  of  the  image  upon  the  retina,  it  is  fur- 
ther required  in  order  to  vision  that  this  physical  process  shall 
be  changed  into  a  physiological  process.  We  now  examine  briefly 
the  mechanism  by  which  such  a  change  is  accomplished.  The 
retina,  or  inner  tunic  of  the  eye,  contains  the  nervous  elements  by 
whose  action  the  system  of  refracted  rays  is  changed  into  a  mosaic 
of  nerve  commotions.  But  light  does  not  act  as  a  stimulus  to  the 
nervous  substance,  either  fibres  or  cells,  unless  it  have  an  intensity 
which  is  nearly  deadly  to  that  substance.  Since  we  are  able  to 
see  the  feeblest  rays  of  the  moon  as  reflected  from  white  paper,  the 
nervous  excitation  which  is  the  condition  of  vision  cannot  be  pro- 


190 


END-ORGANS  OF  THE  NERVOUS  SYSTEM 


duced  by  the  direct  action  of  light  on  the  nerve-fibres  or  nerve-cells 
of  the  eye.  A  photo-chemical  substance  and  process,  as  well  as 
a  special  end-apparatus,  seems  therefore  to  be  necessarily  involved 
in  the  problem  which  is  given  to  the  retina  to  solve. 

§  19.  The  nervous  and  other  elements  of  the  retina  are  arranged 


OUTER  SURFACE. 


10    ....  Layer  of  pigment  cells. 


Layer  of  rods  and  cones. 


,  Mernbrana  limitans  externa. 


7    ....  Outer  nuclear  layer. 


....  Outer  molecular  layer. 


....  Inner  nuclear  layer. 


....  Inner  molecular  layer. 


....  Layer  of  nerve-cells. 

....  Layer  of  nerve-fibres. 

. .  Mernbrana  limitans  interna. 


INNER  SURFACE. 

FIG.  77. — Diagrammatic  Section  of  the  Human  Retina. 


(Schultze.) 


(see  Figs.  77  and  78)  in  the  following  ten  layers,  counting  from  within 
outward  and  backward:  (1)  the  membrana  limitans  interna,  which  is 
the  retinal  border  toward  the  vitreous  body;  (2)  the  layer  of  optic 
nerve-fibres  distributed  from  the  papilla  where  this  nerve  breaks  in 
through  the  tunics  of  the  eye;  (3)  the  ganglion  cell-layer;  (4)  the 
inner  molecular  layer;  (5)  the  inner  nuclear  layer;  (6)  the  outer 
molecular  layer;  (7)  the  outer  nuclear  layer;  (8)  the  membrana  limi- 


NERVOUS  ELEMENTS  OF  THE  RETINA 


191 


tans   externa;    (9)   the   bacillary  layer,   or  »»   10 

layer  of  rods  and  cones;  (10)  the  pigment- 
epithelium  layer.  The  membranes  (Nos. 
(1)  and  (8) )  are  not  really  uninterrupted 
layers,  but  an  extremely  fine  network. 

By  no  means  all  the  retinal  substance 
is  nervous.  Indeed,  the  numerous  radial 
fibres  (fibres  of  Midler)  which  seem  to 
penetrate  its  entire  thickness  are  now  held 
to  be  in  great  part  elements  of  the  support- 
ing tissue;  moreover,  the  whole  connective 
substance  is  a  kind  of  sponge-like  tissue, 
in  the  gaps  of  which  the  true  nervous  ele- 
ments lie  embedded.  The  gaps  thus  filled 
are  especially  large  in  the  second,  third, 
fifth,  and  seventh  layers. 

The  principal  nervous  elements  of  the 
retina  can  be  arranged  in  three  parallel 
sets,  and  may,  therefore,  be  spoken  of  as 
lying  in  three  layers,  each  of  which  in- 
cludes more  than  one  of  the  layers  just 
mentioned.  These  three  sets  of  nerve-cells 
f  are  (1)  the  ganglion  or  optic  nerve-cells,  (2) 
the  bipolar  cells,  and  (3)  the  rod  and  cone 
cells.  The  fibres  of  the  optic  nerve  arise 
in  the  main  from  the  ganglion  cells;  they 
are,  indeed,  the  axons  of  these  cells,  and 
have  no  medullary  sheaths  in  their  course 
over  the  retina  to  the  point  of  exit  of  the 
nerve.  They  converge  from  all  parts  of 
the  retina  to  the  nerve,  thus  forming  the 
second  of  the  ten  layers  mentioned  above. 
Their  arrangement  is  special  at  the  yellow 
spot,  so  as  to  surround  and  not  cover  it. 
The  dendrites  of  the  ganglion  cells  branch 
in  the  fourth  layer,  the  "molecular"  ap- 
pearance of  which,  in  cross  section,  is  due 
to  the  cut  ends  of  numerous  fine  dendritic 
branches.  This  molecular  layer  may  also 
be  called  a  synapse  layer,  since  it  contains 
the  synapses  between  the  ganglion  cells  and  

the    bipolar    Cells.       The    Cell-bodies    of    the  FIG.  78.— Diagrammatic  Repre- 

latter  lie  in  the  fifth  or  inner  nuclear  layer;  £^N^|£^^ 
their  short  axons  branch  and  terminate  Retina.  (Schuitze.)  The 
among  the  dendrites  of  the  ganglion  cells.  SS^ta'Sfi*^  ref' 


B 


192 


END-ORGANS  OF  THE  NERVOUS  SYSTEM 


The  dendrites  of  the  bipolar  cells  extend  toward  the  layer  of  rods 
and  cones,  branching  in  the  outer  molecular  layer,  which  is,  again, 
a  synapse  layer  between  the  bipolar  cells  and  the  rods  and  cones. 
The  cell-bodies  of  the  rod-and-cone-cells  lie  in  the  outer  nuclear 
layer;  their  axons  run  inward  to  form  synapses  with  the  dendrites 


FIG.  79. — Rods  and  Cones  of  the  Human  Retina. 
(Schultze.)  A,  showing  inner  segments  of  the 
rods,  s  s  s,  and  of  the  cones,  z  z\  the  latter  in 
connection  with  the  cone-nuclei  and  fibres  as  far 
as  the  outer  molecular  layer.  &-\Q-.  B,  inner 
segment  of  a  cone  with  a  cone-nucleus. 
C,  isolated  interior  portion  of  a  cone. 


FIG.  80.— Rod  and  Cone 
from  the  Human  Reti- 
na, preserved  in  peros- 
mic  acid,  showing  the 
fine  fibres  of  the  sur- 
face and  the  different 
lengths  of  the  inter- 
nal segment.  JL^°-^. 
(Schultze.)  The  outer 
segment  of  the  cone  is 
broken  into  disks  which 
are  still  adherent. 


of  the  bipolar  cells,  while  what  correspond  to  their  dendrites  are 
the  specialized  branches  called  rods  and  cones.  These  branches 
are  the  real  receptors  of  the  retina;  it  is  they  which  are  sensi- 
tive to  light;  and  the  nerve  impulses  set  up  in  them  by  the  action 
of  light  pass  from  them  to  the  bipolar  cells;  from  the  bipolar  to  the 
ganglion  cells,  and  so,  by  the  axons  of  the  latter  cells  to  the  optic 
nerve  and  its  terminations  in  the  brain  (compare  p.  91). 


NERVOUS  ELEMENTS  OF  THE  RETINA  193 

The  researches  of  Cajal  and  other  recent  histologists  have  un- 
earthed a  wealth  of  further  facts  regarding  the  internal  structure 
of  the  retina,  to  only  a  few  of  which  brief  reference  can  now  be 
made.  It  is  found  that  some  of  the  fibres  of  the  optic  nerve  orig- 
inate within  the  brain,  and  terminate  in  the  retina;  these  probably 
conduct  outward  from  the  brain,  but  their  exact  function  is  not 
clearly  made  out.  There  are  also  cells  in  the  retina  which  do  not 
conduct  in  the  direction  of  the  rod-and-cone-bipolar-ganglion  cells, 
but  spread  out  horizontally,  as  if  to  associate  cells  of  the  same  layer. 
Among  the  bipolar  cells,  two  classes  are  recognized,  linked  respec- 
tively to  the  rods  and  to  the  cones.  Several  rods  are  linked  to  the 
same  bipolar  cell;  whereas  it  appears  that  each  cone  may  have  an 
individual  bipolar  cell  connected  with  it. 

§  20.  To  this  description  of  the  minute  nervous  elements  of  the 
retina,  a  brief  notice  of  some  of  the  more  distinctively  physical 
characteristics  of  certain  of  its  parts  may  now  be  added.  As  its 
very  name  suggests:  The  layer  of  rods  and  cones  (No.  9)  consists  of 
a  multitude  of  elongated  bodies  arranged  side  by  side,  like  rows  of 
palisades,  with  their  largest  extension  in  the  radial  direction.  These 
bodies  are  of  two  kinds — one  cylindrical,  and  called  "rods  of  the 
retina,"  the  other  rather  flask-shaped,  and  called  "cones  of  the 
retina"  (compare  Figs.  79  and  80).  The^rods  extend  the  entire 
thickness  of  the  layer,  and  are  about  -^  inch  in  length,  but  the  cones 
are" shorter;  the  roctslire  about  1 4 1 0  o  inch  in  diameter,  the  smallest 
cones  of  the  central  depression  Triinr  inch.  Each  rod  or  cone  is 
composed  of  an  inner  and  an  outer  segment  or  limb;  the  latter  is 
highly  refractile,  the  former  only  feebly  so.  The  inner  limbs  ap- 
pear under  the  microscope  like  a  mass  of  protoplasm. 

In  general,  the  rods  are  more  numerous  than  the  cones.  The 
distribution  of  the  two~elements~is  different  for  different  parts  of 
the  retina.  In  the  yellow  spot  only  cones  appear,  but  these  are  of 
more  slender  form,  and  of  increased  length,  so  that  not  less  than  one 
million  are  supposed  to  be  set  in  a  square  TV  inch;  while  not  far  from 
this  spot  each  cone  is  surrounded  by  a  crown-shaped  border  of 
rods.  Toward  the  ora  serrata  the  cones  become  continually  rarer. 
In  close  connection  with  the  rods  and  cones  stand  the  cells  of  the 
pigment-epithelium.  These  cells  form  a  regular  mosaic  of  flat, 
six-sided  cells,  which  send  out  pigmented  processes  between  the 
outer  limbs  of  the  rods  and  cones  (see  Figs.  81  and  82). 

§  21.  Two  minute  portions  of  the  inner  surface  of  the  retina  re- 
quire to  be  distinguished  from  the  rest  of  its  area:  the  yellow  spot 
(macula  lutea)  and  the  "blind  spot"  (papilla  opticd).  The  yellow 
spot  is  of  oval  shape,  about  ^  of  an  inch  in  its  long  diameter,  and 
has  in  the  centre  a  depression  called  the  fovea  centralis.  It  is  the 


194  END-ORGANS  OF  THE  NERVOUS  SYSTEM 

place  of  clearest  vision,  and  the  physiological  centre  of  the  eye. 
About  J  of  an  inch  inside  the  eye,  or  15,°  from  the  middle  of  the 
yellow  spot,  is  the  middle  of  the  papilla,  or  place  where  the  optic 
nerve  breaks  into  the  retina  (compare  Fig.  83).  The  blind  spot, 

or  portion  of  the  retina  which 
can  be  experimentally  shown 
to  be  inoperative  in  vision, 
has  been  proved  by  Helm- 
holtz  to  correspond  in  both 
size  and  shape  to  that  covered 
by  this  papilla.  Its  diameter 
is  about  ^  or  TV  of  an  inch, 

FIGS.  81  and  82.— Superficial  Aspect  of  the  Ar-  .                   .1        U1       £         ,.- 

rangement  of  the  Rods  and  Cones  in  the  Ret-  Varying  considerably    for  dlf- 

ina.    *££.     (Schultze.)     The   former  is  from  ferent  eyes.      It  is  Wanting  in 

the  region  of  the  macula   lutea;    the   latter  11,1                              i 

from  the  peripheral  region.  all  the  nervous  elements,  ex- 

cept, of  course,  thenerve-fibres. 

§  22.  In  answer  to  the  question,  What  elements  of  the  retina  are 
directly  affected  by  the  light  ?  both  anatomy  and  physiology  refer  to 
the  layer  of  rods  and  cones.  This  layer  alone  possesses  that  mosaic 
nervous  structure  which  appears  to  correspond  to  the  demands 
made  upon  the  end-apparatus  of 
vision.  It  can  be  demonstrated  that 
the  waves  of  light  pass  through  the 
structure  of  the  retina,  and  that 
the  nervous  process  must  begin  in 
the  back  part  of  this  structure.  In- 
deed, it  is  possible,  by  an  experi- 
ment (devised  by  Purkinje),  to  per- 
ceive with  one's  own  retina  the 
figure  formed  by  the  shadow  of  the 
blood-vessels  expanded  upon  its 
front  part. 

§  23.  Certain  changes  which  oc- 
cur in  the  retina  under  the  stimulus 

of   light    are   of   interest    from   their    FIG.  83.— Equatorial  Section  of  the  Right 

S?i       i  ,1  IP       Eye,  showing  the  Papilla  of  the  optic 

possible    bearing    On    the    mode    of       nejrvet     the    Blood-vessels    radiating 

this  organ's  action.     Electrical  cur-      f™"1/1'  a£d  the  Macula  lutea.    |. 

&        .         ,  1111  (Henle.)     S,    sclerotic;    Ch,   choroid; 

rents  can  be  detected,  both  when      and  R,  retina. 

the  eye  is  kept  in  the  dark  and 

when  it  is  suddenly  exposed  to  light;  these  currents  are  analogous, 

in  a  general  way,  to  the  currents   which    appear  in   nerves  and 

in  muscles;   they  show  certain  peculiarities,  but  not  much  use  can 

as  yet  be  made  of  them  for  an  explanation  of  the  function  of  the 

retina. 


CHEMICAL  CHANGES  IN  THE  RETINA  195 

Of  the  chemical  changes  in  the  retina,  the  most  striking  is  con- 
nected with  the  famous  visual  purple.  When  an  eye  has  been  kept 
for  some  minutes  in  the  dark,  the  outer  limbs  of  the  rods  become 
tinged  with  purple  or  rose-color.  This  color  is  quickly  bleached 
by  exposure  to  light,  and  reappears  again  in  the  dark.  The  bleach- 
ing effect  of  different  parts  of  the  spectrum  is  of  very  different 
strength,  being  greatest  for  a  yellowish  green.  Along  with  the 
changes  in  the  visual  purple  go  movements  of  the  cones  and  of  the 
pigment  cells  of  the  tenth  layer.  The  reaction  to  light  consists  in 
a  shortening  of  the  cones  by  drawing  back  toward  the  membrana 
limitans  externa;  and,  on  the  part  of  the  pigment  cells,  by  a  pushing 
forward  of  their  processes  between  the  rods.  When  exposed  to  the 
dark,  the  processes  of  the  pigment  cells  are  withdrawn,  and  the 
cones  extend  backward. 

It  has  long  appeared  probable  that  the  first  effect  of  light  on  the 
retina  must  be  of  a  chemical  character,  and  therefore  the  discovery 
of  the  visual  purple,  and  of  its  reaction  to  light,  was  hailed  as  open- 
ing the  way  to  a  more  complete  understanding  of  retinal  function.1 
It  soon  appeared,  however,  that  the  visual  purple  was  not  essential 
to  sight.  Kiihne  showed  that  a  frog,  with  the  purple  bleached  by 
exposure  to  light,  still  saw.  It  is  now  clear  that  the  changes  pro- 
duced by  light  in  the  visual  purple  are  not  the  photo-chemical  proc- 
esses which  were  looked  for  as  the  intermediary  between  light  and 
nervous  activity.  At  the  same  time,  it  is  not  impossible  to  see  a 
relation  between  the  visual  purple  and  retinal  function.  One  of 
the  most  remarkable  facts  in  the  physiology  of  the  retina  is  its  power 
of  adaptation  to  different  degrees  of  illumination.  Every  one  is 
familiar  with  the  fact,  that  on  passing  from  light  to  dark  the  eye 
seems  at  first  nearly  blind,  but  soon  becomes  "used  to  the  dark,"  or 
dark-adapted.  Also,  in  passing  from  dark  to  light,  the  first  effect 
is  that  of  being  dazzled;  but  soon  the  eye  grows  used  to  the  light, 
or  light-adapted.  Adaptation  to  the  dark  goes  on  rapidly  within 
the  first  ten  minutes  after  passing  into  the  dark,  and  thereafter  more 
slowly  for  half  an  hour  or  more.  Adaptation  to  the  light  occurs 
much  more  swiftly.  These  times  of  adaptation  correspond  rather 
closely  with  the  times  for  bleaching  and  regeneration  of  the  visual 
purple.  It  is  probable,  from  this  and  other  correspondences,  that 
the  changes  in  the  color  of  the  rods  have  something  to  do  with  the 
processes  of  adaptation  to  light  and  dark. 

The  fact  that  the  centre  of  clear  vision,  corresponding  to  the  fovea, 
is  blind  in  dim  light,  and  does  not  become  adapted  to  it,  combined 
with  the  fact  that  the  fovea  contains  only  cones,  in  connection  with 

1  The  important  names  in  connection  with  this  discovery  are  those  of  Boll 
and  Kuhne. 


196  END-ORGANS  OF  THE  NERVOUS  SYSTEM 

other  similar  facts,  has  led  to  the  view  that  the  cones  are  insensitive 
to  very  faint  light,  and  have  less  power  of  dark-adaptation  than  the 
rods.  The  further  fact  that  clear  vision,  and  vision  for  colors, 
diminish  from  the  fovea  toward  the  periphery  of  the  retina — along 
with  the  diminution  of  cones — has  led  to  the  view  that  color  vision, 
and  distinctness  of  vision,  are  properties  of  the  cones.  There  is 
thus  some  evidence  of  a  difference  in  function  between  the  rods  and 
the  cones.1 

In  very  dim  light,  all  colored  objects  appear  gray,  and  the  rela- 
tive brightness  of  different  colors  is  much  different  from  what  it  is 
under  good  illumination.  Thus,  a  red  and  a  green  which  appear 
equally  bright  in  good  light  are  seen  in  very  dim  light  as  if  the  green 
were  much  brighter  than  the  red  (the  Purkinje  phenomenon). 
The  point  of  greatest  brightness  in  the  spectrum,  which  is  in  the 
yellow  under  good  illumination,  moves  into  the  green  in  very  dim 
light.  These  changes  do  not  occur  in  central  or  foveal  vision. 

§  24.  In  a  word — to  repeat  our  summary:  The  human  eye  is  a 
camera,  which — if  one  were  advertising  it — might  be  described  as 
"a  wonderfully  compact  little  instrument,  capable  of  being  focussed 
on  any  distance  from  five  inches  upward,  provided  with  the  only 
original  iris  diaphragm,  and  having  the  special  feature  of  a  self- 
renewing  plate,  which  automatically  alters  its  sensitivity  to  suit 
the  illumination,  and  also  gives  colored  photographs.  The  camera 
cannot,  however,  be  guaranteed,  as  some  specimens  are  defective, 
and  even  the  best  are  liable  to  be  injured  by  hard  usage;  none  will 
be  replaced,  though  some  of  the  defects  can  be  partially  corrected." 

§  25.  The  end-organ  of  hearing  is  the  Ear.  But  in  this  case,  as 
in  that  of  the  eye,  a  very  large  part  of  the  apparatus  of  sense  is  sig- 
nificant simply  as  a  contrivance  for  applying  the  stimulus  to  the 
true  end-organ,  to  the  differentiations  of  epithelial  cells  and  nervous 
cells  connected  with  the  terminal  fibrils  of  the  sensory  nerve.  The 
entire  human  ear  consists  of  three  parts,  or  ears:  namely,  the  ex- 
ternal ear,  the  middle  ear,  or  tympanum,  and  the  inner  ear,  which 
is  also  called  the  "labyrinth/'  from  its  complex  construction. 

I.  The  External  Ear — exclusive  of  the  cartilaginous  plate  which 
is  extended  from  the  side  of  the  head — consists  of  (a)  the  concha,  a 
deep  hollow,  and  (b)  the  external  meatus,  or  passage  leading  from 
the  bottom  of  this  hollow  to  the  drum  of  the  ear.  The  concha  is 
probably  of  only  slight  service  in  sharpening  and  defining  our  per- 
ceptions of  sound.  Its  position,  however,  favors  the  reception  of 
sound-waves  which  come  from  in  front  rather  than  behind;  it  may, 

1  The  principal  champion  of  this  "duplex  theory"  of  vision  is  von  Kries. 
See  his  papers  in  Zeitschrift  f.  PsychoL,  IX,  82;  XII,  1;  XIII,  242;  and  his 
article  in  Nagel's  Handbuch  der  Physiologic,  1905,  III,  184. 


THE  EXTERNAL  EAR 


197 


therefore,  be  of  some  service  in  enabling  us  to  determine  more  read- 
ily and  accurately  the  direction  from  which  the  sound-waves  come. 
The  most  patent  office  of  the  external  meatus  is  the  protection 
of  the  ear-drum;  the  passage  is  so  curved  that  the  drum  cannot  be 
reached  from  the  outside  in  a  straight  line.  Helmholtz  called  at- 
tention to  the  fact  that  certain  tones  of  a  high  pitch  resound  strong- 
ly in  the  ear  when  the  meatus  is  of  normal  length,  but  cease  so  to 
resound  when  its  length  is  increased  artificially.  The  meatus 


Liy.  mallei 

8Uf. 

Processtis 

/of. 

Insert io  muse, 
tens. 


FIG.  84. — Drum  of  the  Right  Ear  with 
the  Hammer,  seen  from  the  inside, 
f .  (Henle.)  1,  chorda  tympani;  2, 
Eustachian  tube;  *,  tendon  of  the 
tensor  tympani  muscle  cut  off  close 
to  its  insertion;  ma,  anterior  ligament 
of  the  malleus;  Mcp,  its  head;  and 
Ml,  its  long  process.  Stp,  Spina 
tympanica  posterior. 


Chorda 


FIG.  85.— Side  Wall  of  the  Cavity  of 
the  Tympanum,  with  the  Hammer 
(M)  and  the  Anvil  (J).  The  former 
shows  the  connection  of  its  handle 
with  the  drum.  T,  Eustachian  tube, 
f.  (Gegenbaur.) 


probably,  therefore,  modifies  certain  tones  by  its  own  resonant 
action — strengthening  the  high  ones,  and  deadening  the  low,  in 
some  degree. 

Simple  experiments — such  as  placing  a  resounding  body  in  con- 
tact with  the  teeth — prove  that  the  surrounding  cranial  bones  con- 
duct sound  to  the  ear.  Various  paths  for  this  conduction,  both 
direct  by  way  of  the  cranial  and  petrous  bones  to  the  inner  ear, 
and  indirect  by  way  of  the  ear-drum  and  bones  of  the  middle  ear 
to  the  fenestra  ovalis,  are  theoretically  possible.  But  the  amount  of 
conduction  to  be  assigned  to  each  is  difficult  of  precise  determina- 
tion. 

§  26.  II.  The  Middle  Ear,  or  Tympanum  (Figs.  84  and  85),  is  a 
chamber  irregularly  cuboidal  in  form,  and  situated  in  the  temporal 
bone,  between  the  bottom  of  the  meatus  and  the  inner  ear.  Its 
outer  wall  is  (a)  the  membrana  tympani,  which  consists  of  three  lay- 
ers— an  external  tegumentary,  an  internal  mucous,  and  the  inter- 


* 


198 


END-ORGANS  OF  THE  NERVOUS  SYSTEM 


Hep 


mediate  membrana  propria,  composed  of  unyielding  fibres  arranged 
both  radially  and  circularly.  In  the  inner  wall,  which  separates 
the  tympanum  from  the  labyrinth,  are  two  openings  or  windows — 
the  fenestra  ovalis,  which  corresponds  to  the  vestibule  of  the  laby- 
rinth, and  the  fenestra  rotunda,  which  corresponds  to  the  tympanic 
passage  in  the  cochlea.  Near  its  anterior  part  the  tympanum  opens 

into  (b)  the  Eustachian  tube,  a  canal 
which  communicates  with  the  nasal 
compartment  of  the  pharynx. 

(c)  The  auditory  bones  are  three 
in  number,  called  Malleus,  Incus,  and 
Stapes,  and  arranged  so  as  to  form 
an  irregular  chain  stretched  across 
the  cavity  from  the  outer  to  the 
inner  wall  of  the  tympanum  (see  Fig. 
86).  The  malleus  has  a  head,  sepa- 
rated by  a  constricted  neck  from  an 
elongated  handle;  its  handle  is  con- 
nected with  the  centre  of  the  mem- 
brana tympani;  its  head  articulates 
with  the  incus.  The  incus  has  a 
body  and  two  processes.  On  the 
front  surface  of  the  body  is  a  saddle- 


film 


FIQ.  86.— Bones  of  the  Ear,  as  seen 
in  their  connection  from  in  front. 
f.  (Henle.)  /,  Incus  (anvil),  of 
which  76  is  the  short,  and  II  the 
long,  process;  c,  its  body,  and  pi, 
the  process  for  articulation  with  the 

stapes  (processus  orbicuiaris)     M,    shaped  hollow,  in  which  the  head  of 

Malleus  (hammer),  of  which  Me  is      .     r       ,,  n  *          i 

the  malleus  fits;  the  short  process  is 
bound  by  a  ligament  to  the  posterior 
wall  of  the 


the  neck,  Mcp  the  head,  Ml  the  long 
process,  and  Mm  the  manubrium; 
S,  stapes  (stirrup),  with  its  capitu- 
lum,  cp. 


ie  tympanum;  the  long  proc- 
ess ends  in  a  rounded  projection  (os 

orbiculare)  through  which  it  articulates  with  the  stapes.  The 
stapes,  or  stirrup-shaped  bone,  has  a  head  and  neck,  a  base  and 
two  crura.  The  head  articulates  with  the  incus;  from  the  con- 
stricted neck  the  two  crura  curve  inward  to  the  base,  which  is 
attached  to  the  fenestra  ovalis.  These  bones  are  moved  on  each 
other  at  their  joints  by  (d)  two  small  muscles — the  tensor  tympani 
and  the  stapedius.  The  first  of  these  is  inserted  into  the  malleus, 
near  the  root,  and  serves  to  tighten  the  tympanic  membrane  by 
drawing  the  handle  of  the  malleus  inward;  the  stapedius  is  inserted 
into  the  neck  of  the  stapes,  and  draws  the  stapes  from  the  fenestra 
ovalis,  thus  diminishing  the  pressure  of  the  chain  of  bones  and 
lessening  the  tension  of  the  tympanic  membrane;  it  therefore  acts 
as  the  antagonist  of  the  tensor  tympani. 

§  27.  The  general  office  of  the  tympanum  may  be  described  as 
that  of  transmitting  the  acoustic  waves  to  the  inner  ear,  while  at  the 
same  time  modifying  their  character.  Some  modification  is  neces- 


OFFICE  OF  THE  TYMPANUM  199 

sary  in  order  that  these  waves  may  occasion  such  vibrations  in  the 
elements  of  the  inner  ear  as  shall  be  adapted  for  the  excitation  of 
its  end-organs.  The  acoustic  motion  of  the  molecules  of  air,  in  the 
form  in  which  it  reaches  the  ear-drum,  has  a  large  amplitude,  but 
a  small  degree  of  intensity.  This  motion  must  be  changed  into  one 
of  smaller  amplitude  and  greater  intensity;  and  it  must  be  trans- 
mitted, with  as  little  loss  as  possible,  to  the  fluids  of  the  labyrinth. 
The  transmitting  vibrating  media  must  also  have  the  power  of  an- 
swering to  the  different  tones  of  any  pitch  perceptible  by  the  ear. 
The  description  of  the  manner  in  which  this  apparatus  of  membrane 
and  bones  solves  so  complicated  a  mechanical  problem  belongs  to 
the  physics  of  anatomy;  it  has  been  worked  out  with  great  detail 
by  Helmholtz  and  others,  although  certain  questions  still  remain  un- 
solved. We  can  here  only  indicate  one  or  two  particulars. 

A  flat  membrane,  evenly  stretched,  whose  mass  is  small  in  pro- 
portion to  the  size  of  its  superficies,  is  easily  thrown  into  vibration 
by  the  impact  of  acoustic  waves  upon  one  of  its  sides.  Such  a 
membrane  responds  readily  to  tones  which  approach  its  own  funda- 
mental tone;  but  if  divergent  tones  are  sounded  the  membrane  is 
unaffected.  A  motion  which  consists  of  a  series  of  harmonious 
partial  tones  cannot  then  be  repeated  by  such  a  membrane  in  the 
form  in  which  the  air  brings  it.  If,  then,  the  membrane  of  the 
tympanum  were  not  so  arranged  and  connected  as  to  have  no  pre- 
ponderating tone  of  its  own,  it  could  not  be  the  medium  of  our 
hearing  a  great  variety  of  tones.  The  property  of  taking  up  the 
vibrations  of  a  large  scale  of  tones  is  secured  for  the  tympanum  by 
its  funnel-shaped  form  and  by  its  being  loaded.  It  is  contracted 
inward  into  a  depression  of  the  right  shape  by  means  of  the  handle 
of  the  hammer;  it  is  therefore  unequally  and  only  slightly  stretched, 
and  has  no  fundamental  tone.  It  is  also  loaded  with  the  audi- 
tory bones,  which  deprive  it  of  every  trace  of  such  a  tone  and  act  as 
dampers  to  prevent  long-continued  vibrating.  Moreover,  since  the 
apex  of  its  funnel  bulges  inward,  the  force  of  the  vibrations  from  all 
sides  is  concentrated  in  vibrations  of  greater  intensity  in  the  centre, 
where  it  is  spent  in  setting  the  chain  of  ear-bones  in  motion. 

The  acoustic  vibrations  of  the  auditory  bones,  which  are  occa- 
sioned by  the  movements  of  the  ear-drum,  are  not  longitudinal, 
but  transverse;  they  do  not,  however,  resemble  the  vibrations  of  a 
stretched  cord  or  a  fixed  pin.  They  do  not  vibrate  by  reason  of  their 
elasticity,  but  like  very  light  small  levers — vibrating  as  a  system, 
with  a  simultaneous  motion  around  a  common  axis.  Direct  obser- 
vation of  these  bones  in  motion  shows  that  their  sympathetic  vibra- 
tions vary  greatly  for  tones  of  different  pitch  and  similar  intensity, 
from  a  scarcely  observable  motion  to  a  surprisingly  great  elongation. 


200  END-ORGANS  OF  THE  NERVOUS  SYSTEM 

The  effect  of  the  muscles  of  the  tympanum  upon  the  transmis- 
sion of  tones  of  different  pitch  is  not  quite  clearly 'demonstrated. 
In  general,  the  stretching  of  the  tensor  muscle,  within  the  limits 
which  have  thus  far  been  investigated,  seems  to  weaken  the  higher 
much  less  than  the  lower  tones.  But  the  tension  of  the  drum  un- 
der the  influence  of  this  muscle  does  not  indicate  the  slightest 
change  on  passing  from  low  to  high  tones.  The  tensor  tympani 
can,  therefore,  scarcely  be  regarded  as  the  mechanism  which  has 
complete  control  of  accommodation  to  pitch.  On  the  other  hand, 
its  reflex  contractions  are  most  easily  excited  by  tones  of  high  pitch. 
The  resulting  favoring  of  high  tones,  and  corresponding  dampening  of 
simultaneously  sounding  low  tones,  would  seem  thus  to  be  of  assist- 
ance in  picking  out  a  high  tone  from  a  mixture  of  sounds  of  differing 
pitch.  The  stretching  of  the  tendon  of  the  stapedius  muscle  has 
no  observable  influence  on  the  acoustic  vibrations  of  the  tympanum. 

§  28.  The  Eustachian  Tube,  when  in  its  normal  position,  is 
neither  closely  shut  nor  wide  open.  Its  office  is  to  effect  a  renewal  of 
the  air  in  the  tympanum,  to  maintain  the  equilibrium  of  atmos- 
pheric pressure  on  both  sides  of  the  tympanic  membrane,  and  to 
convey  away  the  fluids  which  collect  in  the  tympanic  cavity.  If  it 
remained  open,  so  as  to  permit  the  acoustic  waves  of  the  air  from 
the  mouth  to  enter,  our  own  voices  would  be  heard  as  a  roaring 
sound,  and  the  passage  of  air  inward  and  outward  during  respira- 
tion would  affect  the  position  and  tension  of  the  tympanic  mem- 
brane. That  it  is  opened,  however,  on  swallowing,  Valsalva  proved 
two  centuries  ago.  For  if  we  keep  the  nose  and  mouth  closed  and 
then  swallow,  with  the  cheeks  blown  violently  out,  a  feeling  of 
pressure  is  felt  in  the  ears  and  the  hearing  is  weakened.  These  ef- 
fects are  due  to  the  forcing  of  the  air  through  the  Eustachian  tube 
into  the  tympanic  cavity.  The  tube  is  thus  of  indirect  service  in 
respect  to  the  physiological  functions  of  the  middle  ear. 

§  29.  III.  The  Internal  Ear,  or  Labyrinth,  is  the  complex  organ 
in  which  the  terminal  fibrils  of  the  auditory  nerve  are  distributed 
and  the  end-organs  of  hearing  situated.  The  so-called  "bony 
labyrinth"  is  a  series  of  cavities  channelled  out  of  the  petrous  bone. 
It  consists  of  three  parts — the  Vestibule,  the  Semicircular  Canals, 
and  the  Cochlea.  In  each  osseous  part  a  membranous  part  is  sus- 
pended, corresponding  to  it  in  shape,  but  filling  only  a  small  portion 
of  the  bony  cavity  which  contains  it.  It  is  in  the  labyrinth  that  the 
acoustic  waves  transmitted  by  the  tympanum  are  analyzed  and 
changed  from  a  physical  molecular  process  to  a  nerve-commotion, 
by  the  special  end-apparatus  of  hearing  (see  Fig.  87). 

(A)  The  Vestibule  is  the  central  cavity  of  the  internal  ear;  it  is 
the  part  of  the  labyrinth  which  appears  first  in  animals  and  is  most 


STRUCTURE  OF  THE  INTERNAL  EAR     201 

constant.  The  membranous  vestibule  is  composed  of  two  sac-like 
dilatations — the  upper  and  larger  of  which  is  named  utriculus,  the 
lower  sacculus.  In  the  outer  wall  of  the  vestibule  is  the  fenestra 
ovalis;  its  anterior  wall  communicates  with  the  scala  vestibuli  of 
the  cochlea,  and  at  its  posterior  wall  the  fine  orifices  of  (B)  the  Semi- 
circular Canals  open  into  the  utriculus.  These  canals  are  three  in 
number,  are  bent  so  as  to  form  nearly  two-thirds  of  a  circle,  and  are 
about  an  inch  in  length  and  -fa  of  an  inch  in  diameter.  They  are 
called  the  superior,  the  posterior  or  vertical,  and  the  external  or 
horizontal  canals.  The  contiguous  ends  of  the  superior  and  pos- 


FIG.  87. — No.  1,  Cast  of  the  Osseous  Labyrinth  of  the  Left  Ear,  from  below;  No.  2,  of 
the  Right  Ear,  from  the  inside;  No.  3,  of  the  Left  Ear,  from  above.  (Henle.)  Av, 
aqueduct  of  vestibule;  Fc,  fossa  of  the  cochlea;  Fee,  its  fenestra  (rotunda);  Fv,  fe- 
nestra of  the  vestibule  (ovalis);  ha,  external  ampulla;  h,  external  semicircular  canal; 
Tsf,  tractus  spiralis  foram^nos^ls;  vaa,  ampulla  of  the  superior  semicircular  canal;  vc, 
posterior  semicircular  canal;  and  vpa,  its  ampulla. 

terior  canals  blend  together  and  have  a  common  orifice  into  the 
vestibule.  They  all  have  a  regular  relative  position,  their  planes 
being  nearly  at  right  angles  to  each  other.  Near  the  vestibule  they 
dilate  to  about  twice  their  average  diameter  and  form  the  so-called 
ampulla.  Both  the  osseous  vestibule  and  the  osseous  canals  con- 
tain a  fluid  (the  perilymph)  in  which  the  membranous  vestibule 
and  canals  are  suspended;  the  membranous  labyrinth  is  also  dis- 
tended with  a  similar  fluid  (the  endolymph). 

(C)  The  Cochlea  is  by  far  the  most  complex  part  of  the  laby- 
rinth, and,  according  to  present  evidence,  the  only  part  directly  con- 
cerned with  auditory  sensations  and  perceptions  (compare  Figs. 
88,  89,  and  90).  It  is  about  •£•  of  an  inch  long,  and  is  shaped  like 
the  shell  of  a  common  snail.  It,  too,  consists  of  a  membranous  sac 
embedded  in  an  osseous  cavity.  The  whole  passage  of  the  cochlea  is 
imperfectly  divided  into  two  canals  by  a  partition-wall  of  bone,  which 
is  wound  2-J-  times  around  an  axis  (the  modiolus),  from  the  base  to  the 
apex,  somewhat  like  a  spiral  stair-case.  It  is  called  the  osseous  lamina 
spiralis.  Of  the  two  canals  or  passages  thus  formed,  the  one  which 
faces  the  base  of  the  cochlea  is  called  the  scala  tympani;  since  it  has 


202 


END-ORGANS  OF  THE  NERVOUS  SYSTEM 


its  origin  in  the  circular  aperture  (fenestra  rotunda)  which  leads  to 
the  tympanic  cavity.  The  other,  which  faces  toward  the  apex, 
opens  into  the  vestibule,  and  is  called  the  scala  vestibuli.  At  the 
apex  of  the  cochlea  these  two  scalse  communicate  with  each  other 
through  a  small  hole  (helicotrema).  The  division  of  the  membranous 
cochlea  is  completed  by  a  membrane  (the  basilar  membrane,  or 
membranous  spiral  lamina),  which  bridges  the  interval  between  the 
free  edge  of  the  osseous  spiral  lamina  and  the  outer  wall  of  the  pas- 


FIG. 88. — Osseous  Cochlea  of  the  Right 
Ear,  exposed  from  in  front,  -f. 
(Henle.)  t,  section  of  the  division- 
wall  of  the  cochlea;  ft,  upper  end  of 
the  same.  Fee,  fenestra;  H,  ham- 
ulus;  Md,  modiolus;  Ls,  lamina 
spiral  is. 


FIG.  89.— Cross  Section  through  the 
Acoustic  Nerve  and  the  Cochlea,  f . 
(Henle.)  Nc,  nerve  of  the  cochlea; 
Nv,  nerve  of  the  vestibule;  St,  scala 
tympani;  Sv,  scala  vestibuli;  and 
between  them  the  ductus  cochlearis, 
DC.  Ls  and  Md,  as  in  preceding 
figure. 


sage;  it  is  attached  to  this  wall  by  the  spiral  ligament.  Another  mem- 
brane (the  membrane  of  Reissner)  arises  from  a  spiral  crest  (lim- 
bus,  or  crista  spiralis)  attached  to  the  free  edge  of  the  osseous 
lamina,  and  extends  to  the  spiral  ligament,  so  as  to  form  a  small 
aqueduct  between  it  and  the  basilar  membrane  (the  scala  inter- 
media, or  ductus  cochlearis,  or  canal  of  the  cochlea).  It  is  in  the 
vestibule,  in  the  ampullae  of  the  canals,  and  in  the  scala  intermedia 
that  the  nervous  end-organs  of  hearing  are  to  be  found. 

§  30.  The  auditory  nerve,  on  approaching  the  labyrinth,  divides 
into  a  vestibular  and  a  cochlear  branch  (compare  Fig.  89).  As 
stated  in  another  chapter  (see  p.  82),  these  two  parts  of  the  eighth 
nerve  separate  also  at  their  entrance  to  the  medulla  and  pass  to 
different  portions  of  the  central  organs.  They  serve  two  senses, 


DISTRIBUTION  OF  AUDITORY  NERVE 


203 


Liyamentum 
spirale 


the  end-organs  of  both  of  which  are  located  in  the  inner  ear.  The 
vestibular  branch  or  nerve  enters  the  vestibule,  and  divides  into  five 
branches,  one  for  the  utriculus,  one  for  the  sacculus,  and  one  for 
each  of  the  three  ampullae.  In  each  of  these  dilatations  the  mem- 
branous wall  forms  a  thickened  projection,  which  is  called  the  crista 
in  the  ampulla  and  the  macula  in  the  utricle  and  saccule.  The 
characteristic  feature  of  each  of  these  is  the  presence  of  epithelial 
cells  provided  with  tufts  of  fine  hairs.  The  hairs,  instead  of  pro- 
jecting directly  into  the  endolymph,  are  embedded  (as  discovered  by 
Retzius)  in  a  soft  gelatin- 
ous or  mucous-like  mass. 
In  the  ampullae  this  mass 
is  dome-shaped,  and  the 
hairs  are  of  considerable 
(microscopic)  length.  In 
the  utricle  and  saccule,  the 
hairs  are  shorter,  and  the 
soft  mass  in  which  they 
are  embedded  is  flatter, 
but  is  principally  remark- 
able for  containing  little 
particles  of  carbonate  of 
lime  (limestone),  which  are 
called  the  otoliths,  or  "ear- 
stones."  The  base  of  the 
hair-cells,  in  the  ampullae,  utricle,  and  saccule,  are  embraced  by  the 
terminal  ramifications  of  the  fibres  of  the  vestibular  nerve;  there  is 
no  doubt,  accordingly,  that  the  hair-cells  are  sensory  cells  (compare 
Fig.  91). 

§  31.  The  terminal  nerve-apparatus  which  constitutes  the  spe- 
cial end-organ  of  hearing  is  noteworthy  for  its  exceedingly  compli- 
cated structure  and  striking  appearance.  The  cochlear  branch  of 
the  auditory  nerve  pierces  the  axis  of  the  cochlea  and  gives  off 
laterals  which  enter  the  canals  of  the  osseous  spiral  membrane. 
Here  they  radiate  to  the  membranous  spiral  lamina,  and  are  con- 
nected with  a  ganglion  which  contains  the  cell-bodies  of  the  fibres 
of  the  cochlear  nerve.  Beyond  this  ganglion,  they  form  a  plexiform 
expansion,  from  which  the  delicate  fibrils — losing  their  medullary 
sheath  and  becoming  extremely  fine  axis-cylinders — pass  through  a 
gap  in  the  edge  of  the  lamina  into  the  organ  of  Corti,  where  they 
terminate  around  the  base  of  certain  hair-cells,  soon  to  be  described 
(compare  Fig.  92). 

It  is  the  so-called  "organ  of  Corti,"  however,  in  which  the  in- 
genuity of  the  natural  processes  engaged  in  constructing  the  mech- 


Lamina 

basilaris 


FIG.  90. — Section  through  one  of  the  Coils  of  the 
Cochlea.       °.     (Schematic,  from  Gegenbaur.) 


204 


END-ORGANS  OF  THE  NERVOUS  SYSTEM 


anisms  of  the  human  body  would  seem  to  have  reached  the 
ultimatum  of  endeavor  to  create  an  elaborate  and  effective,  but 
mystifying,  structure  for  the  conversion  of  physical  stimuli  into 
nervous  impulses.  A  study  of  the  accompanying  figures,  together 
with  the  following  description  of  some  of  its  more  obvious  features, 
will  suffice  the  purposes  of  our  treatise. 

It  will  be  noticed  that  the  organ  of  Corti  is  a  wonderful  arrange- 
ment of -cells,  some  of  which  are  elongated  and  curved,  and  are 
gathered  into  two  groups  that  may  be  designated,  respectively, 
as  an  inner  and  an  outer.  Since  the  cells  of  the  inner  group  pro- 


Fig.  91. — Nerve  Endings  in  an  Ampulla.     (Retzius.)    n,  sensory 
axons;  h,  hair-cells. 

ject  forward  and  outward,  while  those  of  the  outer  group  incline 
forward  and  inward,  the  two  form  a  sort  of  bow  which  arches  over 
an  exceedingly  minute  channel  (the  canal  of  Corti)  between  them 
and  the  membrane  on  which  they  both  rest.  This  membrane 
itself  is  composed  of  fibres  arranged  in  a  transverse  direction,  and 
in  such  manner  that  a  single  rod-like  cell  rests  upon  one  or  two  of 
these  fibres. 

Internal,  and  almost  parallel  to  the  inner  one  of  the  groups  just 
described,  is  a  row  of  columnar  cells  with  short  and  stiff  hair-like 
processes  (inner  hair-cells).  External  and  almost  parallel  to  the 
outer  group  are  four  or  five  rows  of  hair-cells  (outer  hair-cells) 
which  are  attached  to  the  basilar  membrane,  while  their  other  ex- 
tremity projects  as  a  brush  of  hairs  through  the  reticular  membrane 
(membrane  of  Kolliker).  This  latter  membrane  is  a  very  delicate 


THE  ORGAN  OF  CORTI 


205 


framework,  perforated  with  holes,  through  which  the  hairs  of  the 
outer  hair-cells  project,  and  which  extends  from  the  inner  rods  to 
the  external  row  of  hair-cells.  It  acts  as  a  support  for  the  ends  of 
these  cells.  The  interval  between  the  outer  hair-cells  and  the  spiral 
ligament  is  occupied  by  cells  of  a  columnar  form  (the  supporting 
cells  of  Hensen).  The  organ  of  Cord  is  covered  over  and  separated 
from  the  endolymph  of  the  ductus  cochlearis  by  the  so-caljed  mem- 
brana  tectoria. 

§  32.  The  problem  before  the  labyrinth  of  the  ear  is  in  part  the 
same  as  that  solved  by  the  tympanum,  namely,  the  problem  of  con- 
veying the  acoustic  waves  to  the  true  end-apparatus  of  hearing. 

Tectorial          Inner  Outer 

membrane      hair-cells       hair-cells 


Nerve-fibres  Inner  rod    Outer  rod        Basilar  membrane 

Fig.  92. — The  Organ  of  Corti.     (Retzius.) 

The  repeated  shocks  of  the  stirrup  at  the  fenestra  ovalis — -and  per- 
haps, in  far  less  degree,  the  pulsations  of  air  at  the  fenestra  ro- 
tunda— produce  waves  in  the  fluid  of  the  labyrinth.  Any  molec- 
ular oscillations  of  this  fluid,  thus  occasioned,  cannot,  however, 
act  directly  as  the  appropriate  stimulus  of  the  sensations  of  sound. 
Since  the  dimensions  of  the  whole  mass  thrown  into  vibration  are 
so  small  in  comparison  with  the  length  of  the  acoustic  waves  that 
the  extension  of  the  shock  from  the  stirrup  would  be  practically 
instantaneous  throughout,  and  since  the  surrounding  walls  may  be 
regarded  as  absolutely  immovable  by  any  such  impact,  the  laby- 
rinth-water would  act  as  an  incompressible  fluid.  It  would,  there- 
fore, be  unsuitable  for  the  transmission  of  various  kinds  of  acoustic 
waves.  But  different  parts  of  the  labyrinth  are  capable  of  yielding 
to  the  waves  in  the  fluid  caused  by  the  repeated  shocks  of  the 
stirrup.  This  is  especially  true  of  the  membrane  of  the  fenestra 
rotunda  which  is  left  free  to  bulge  out  into  the  tympanic  cavity. 
Waves  started  by  impact  at  the  fenestra  ovalis  would  pass  into  the 
scala  vestibuli  of  the  cochlea;  and  from  there — it  is  probable — 
through  the  membrane  of  Reissner,  the  fluid  in  the  cochlear  due- 


206  END-ORGANS  OF  THE  NERVOUS  SYSTEM 

tus,  and  the  basilar  membrane,  into  the  air  of  the  middle  ear.  In 
their  passage  through  the  basilar  membrane,  they  would,  of  course, 
cause  it  to  vibrate  and  so  excite  the  sensory  hair-cells  which  rest 
on  this  membrane. 

§  33.  We  may  be  fairly  certain  that  the  process  by  which  vi- 
brations excite  the  end-organ  of  hearing  is  as  just  described — 
namely,  in  brief,  that  vibrations  of  the  fluid  in  the  cochlea  excite 
the  hairs  of  the  hair-cells.  But  a  more  difficult  problem  is  that  of 
explaining  the  great  variety  of  responses  which  the  ear  makes  to 
different  stimuli,  and  which  we  know  by  means  of  the  many  dis- 
tinguishable noises,  tones,  and  tone-combinations.  Since  we  dis- 
tinguish a  great  number  of  pitches,  there  must  be  some  special  re- 
action of  the  ear  corresponding  to  each  of  these  tones.  One  of 
the  most  important  facts  to  bear  in  mind  in  framing  a  theory  of  the 
action  of  the  cochlea  is  the  analytic  power  of  the  ear.  The  vi- 
brations which  reach  the  ear  from  the  air,  when  a  chord  of  several 
tones,  or  even  when  a  single  tone,  is  sounded  with  its  accompanying 
overtones,  are  highly  complex;  but  a  "trained  ear"  can  analyze  the 
complex  into  the  separate  tones  of  which  it  is  composed.  The 
"training"  occurs,  no  doubt,  in  the  brain  and  not  in  the  ear;  but 
the  brain  could  not  distinguish  the  components  unless  the  ear  had 
first  broken  up  the  complex  vibration  into  its  elements,  and  sent 
to  the  brain  an  impulse  corresponding  to  each. 

A  theory  which  accounts  in  an  elegant  manner  for  this  analytic 
power  of  the  ear,  and  for  its  power  to  respond  differently  to  tones 
of  differing  pitch,  is  the  resonance  or  sympathetic  vibration  theory 
of  Helmholtz.1  What  is  meant  by  "sympathetic"  vibration  is 
illustrated  when  a  piano  string,  for  instance,  which  is  tuned  to 
vibrate  at  a  certain  rate,  takes  up  vibrations  of  this  rate  from  the 
air,  and  is  itself  set  in  vibration  by  them.  If  the  dampers  are  lifted 
from  the  strings  of  a  piano,  and  a  particular  tone  be  sung  into  it, 
it  answers  with  the  same  tone;  if  two  or  more  tones  are  simulta- 
neously sung  into  it,  it  answers  with  the  same  combination  of  tones, 
the  strings  tuned  to  these  tones  having  been  set  into  sympathetic 
vibration.  Helmholtz  conceived  that  the  transverse  fibres  of  the 
basilar  membrane  might  be  likened  to  the  strings  of  a  piano,  and 
since  they  are  of  different  length,  might  be  tuned  to  tones  of  dif- 
ferent pitch,  and  vibrate  sympathetically,  each  to  its  own  vibration 
rate.  The  vibration  of  any  fibre  of  the  basilar  membrane  would 
naturally  excite  the  hair-cells  in  its  immediate  neighborhood; 
different  hair-cells  would  thus  be  set  in  vibration  for  different  pitches, 
and  -different  combinations  of  them  for  different  combinations  of 

1  See  his  Sensations  of  Tone,  1895,  pp.  145  ff. 


THEORY  OF  SYMPATHETIC  VIBRATION  207 

tones;  and  thus  the  sense  for  pitch  and  the  analytic  power  of  the 
organ  would  find  their  explanation. 

The  theory  of  sympathetic  vibration  has  been  worked  out  with 
great  thoroughness  to  cover  the  various  peculiarities  of  auditory 
sensation,  and  in  general  has  been  found  to  be  adequate.  It  is  not, 
however,  altogether  free  from  internal  difficulties,  the  chief  of  which 
lies  in  the  extreme  minuteness  and  small  range  in  length  of  the 
fibres  which  are  supposed  to  be  tuned  to  the  various  audible  pitches. 
The  longest  of  them  is  but  half  a  millimetre  in  length,  and  it  is 
difficult  to  conceive  that  a  fibre  of  such  length,  even  though  loaded 
with  its  segment  of  the  organ  of  Corti,  and  even  though  suspended, 
not  in  air,  but  in  a  liquid,  could  be  tuned  so  low  as  the  lowest  string 
of  the  piano.  Moreover,  though  the  number  of  fibres  in  the  basilar 
membrane — about  24,000 — is  adequate  for  the  range  of  audible 
pitch  and  the  number  of  discriminable  pitches  which  a  trained  ear 
can  detect,  the  difference  in  length  of  these  fibres  is  only  about  as 
one  to  twelve,  much  less  than  would  be  required  for  tuning  to  the 
range  of  audible  pitch — unless,  indeed,  we  can  conceive  of  these 
fibres  as  being  under  different  tension  or  differently  loaded.  M. 
Meyer 1  has  called  attention  to  a  physiological  difficulty  in  the  way 
of  accepting  the  suggestion  that  the  fibres  are  under  different  ten- 
sion: when  a  living  tissue  is  subjected  to  a  continued  tension,  it 
yields  or  accommodates  itself,  so  that  the  tension  is  relieved;  and 
there  is  no  evidence  that  this  rule  is  broken  in  the  case  of  the 
basilar  membrane.  Hardesty,2  on  minute  anatomical  examination, 
finds  that  the  basilar  membrane  contains,  not  only  the  radial  fibres 
which  the  Helmholtz  theory  likens  to  the  strings  of  a  harp,  but  also 
numerous  other  fibres  running  athwart  these  and  binding  them 
so  tightly  together  that  it  seems  a  physical  impossibility  that  they 
could  vibrate  singly  or  in  small  groups,  as  required  by  the  theory. 

Such  difficulties  have  caused  dissatisfaction,  in  the  minds  of 
many  students  of  acoustics,  with  the  Helmholtz  theory,  and  several 
other  theories  have  been  put  forward,  but  the  working  of  them  out 
in  detail,  to  explain,  as  well  as  the  Helmholtz  theory  does,  the  vari- 
ous facts  of  hearing,  has  not  yet  been  accomplished.  The  most 
prominent  of  the  opposing  theories  is  that  of  Ewald.3  By  experi- 
menting with  a  little  model  of  the  basilar  membrane,  he  finds  that 
even  so  minute  a  membrane  as  this  is  set  into  vibration  of  a  fixed 
form,  giving  a  fixed  vibration  figure,  by  the  action  of  vibrations  of 
any  given  rate;  and  that  the  figure  varies  with  the  rate;  and,  further, 

1  University  of  Missouri  Studies,  1907,  II,  1. 

2  American  Journal  of  Anatomy.  1908,  VIII,  109. 

3  Pftiiger's  Archiv  /.  d.  gesammte  Physiologic,  1899,  LXXVI,  147,  and  1903, 
XCIII,  485. 


208  END-ORGANS  OF  THE  NERVOUS  SYSTEM 

that  the  combined  action  of  two  vibration  rates  is  such  as  to  in- 
duce in  the  membrane  a  compound  vibration-figure,  in  which,  how- 
ever, the  component  figures  are  not  obliterated.  As  applied  to  the 
cochlea,  these%  facts  would  indicate  that  vibrations  of  any  given 
rate,  acting  on  the  basilar  membrane,  would  cause  it  to  vibrate  in 
certain  places — not  simply  in  one  place  as  according  to  the  Helm- 
holtz  theory — and  that  therefore  corresponding  parts  of  the  sensory 
apparatus  would  be  excited  by  each  vibration  rate.  Apparently  a 
combination  of  nerve-fibres,  from  different  parts  of  the  cochlea, 
would  be  excited  by  even  a  single  tone;  and  thus  the  nervous  result 
would  be  less  simple  and  easy  to  conceive  than  according  to  the 
sympathetic  vibration  theory.  But  Ewald  has  made  it  probable 
that  the  basilar  membrane  does  vibrate  in  the  manner  supposed  by 
his  theory. 

With  regard  to  these  and  all  similar  physiological  theories,  how- 
ever, it  must  be  remembered  that  we  are  not  in  search  of  a  nervous 
apparatus  which  can  be  listened  to  by  the  brain,  or  by  the  soul  in 
the  brain,  as  we  listen,  so  to  say,  to  the  tones  produced  by  the 
vibrations  of  the  strings  of  a  piano.  What  we  are  seeking,  the 
rather,  is  some  sufficient  account  for  a  series  of  more  or  less 
complex  nervous  changes  which  can  be  adequately  correlated 
with  the  varieties  of  elements  into  which  we  can  analyze  our  sense 
experience. 

§  34.  From  the  fact  that  the  utricle,  saccule,  and  semicircular 
canals  are  parts  of  the  inner  ear,  it  was  natural  to  suppose  for  them 
an  auditory  function;  and  they  have  often  been  regarded  as  connected 
with  the  reception  of  noises  and  with  the  perception  of  the  direction 
from  which  sound  comes.  There  is,  however,  no  positive  evidence 
of  an  auditory  function  for  these  structures,  while  there  is  positive 
evidence  of  a  function  of  another  sort.  Experimental  knowledge 
of  this,  their  true  function,  began  with  Flourens,  who  applied,  about 
1825,  the  method  of  extirpation  to  the  canals  of  pigeons,  and  found 
the  result  to  consist  in  certain  disturbances  of  movement.  Since 
then  numerous  results  of  the  same  tenor  have  accumulated,  and  the 
technique  of  stimulating  and  extirpating  these  minute  structures 
has  been  perfected,  till,  in  the  hands  of  Ewald,1  very  delicate  opera- 
tions with  precise  results  have  been  achieved. 

The  position  of  the  canals  should  first  be  noted.  As  has  already 
been  said,  the  three  canals  of  one  labyrinth  lie  in  three  planes 
nearly  at  right  angles  to  each  other;  and  the  planes  of  the  canals  of 
the  two  labyrinths  are  related  to  each  other,  so  that  the  two  hori- 

1  Physiologische  Untersuchungen  uber  das  Endorgan  des  Nervus  Octavus 
(Wiesbaden,  1892).  See  also  Kreidl,  in  Asher  and  Spiro's  Ergebnisse  der  Physi- 
ologic, 1905,  V,  572,  and  Nagel,  Handbuch  der  Physiologic,  1905,  III,  778. 


OFFICE  OF  SEMICIRCULAR  CANALS 


209 


zontal  canals  are  parallel,  and  the  superior  of  either  side  is  parallel 
with  the  posterior  of  the  other  side.  The  planes  of  the  canals  are 
not,  however,  the  three  primary  planes  of  the  head,  but  lie  at  angles 
of  about  45°  to  these  (compare  Fig.  93). 

The  three  planes  at  right  angles  remind  one  of  the  planes  of 
reference  in  co-ordinate  geometry;  and  this  peculiarity  of  arrange- 
ment has  suggested  to  some 
authors  the  theory  that  the 
canals  are  concerned  with 
the  perception  of  the  direc- 
tion of  sound;  and  to  one 
author  the  still  vaguer 
theory  that  theyy  furnish 
sensations  of  three-dimen- 
sional space.1  To  under- 
stand the  real  function  of 
the  canals,  it  is  necessary 
first  to  have  in  mind  the 
class  of  reflexes  which  are 
called  compensatory  move- 
ments. If  a  frog  is  placed 
on  a  board,  and  the  board 
is  so  tilted  as  to  lower  the 
frog's  head,  he  responds  by 
raising  his  head;  and  simi- 
larly, he  lowers  his  head  if 
the  board  is  so  tilted  as  to 
raise  it,  and  turns  his  head 
to  the  right  if  the  board  is 
so  rotated  as  to  turn  it  to 
the  left,  etc.  These  reac- 
tions "compensate  for,"  or 

correct,  the  movement  impressed  on  him.  Similar  movements 
are  found  through  a  wide  range  of  animals,  including  fishes, 
birds,  and  mammals.  Along  with  compensatory  movements  of 
the  head  go  compensatory  rotations  of  the  eyes,  and  movements 
of  the  body  as  a  whole.  These  reflexes  are  of  great  importance  in 
maintaining  the  position  and  equilibrium  of  the  body,  and  in  keeping 
it  to  a  straight  line  in  locomotion.  Now  one  marked  result  of  the 
extirpation  of  the  semicircular  canals  is  the  loss  of  compensatory 
movements;  and  another  closely  related  result  is  the  loss  of  the 
"tonus"  of  many  of  the  muscles,  and  the  consequent  inefficiency 

1  Von  Cyon,  Pfliiger's  Archiv  /.  d.  gesammte  Physiologie,  1900,  LXIX,  211. 


Fig.  93. — The  Position  of  the  Semicircular  Canals  in 
the  Head.     (Ewald.) 


210  END-ORGANS  OF  THE  NERVOUS  SYSTEM 

of  many  movements.  Locomotion  becomes  uncertain,  and,  if  the 
destruction  of  the  canals  is  unilateral  or  otherwise  partial,  "forced 
movements,"  such  as  running  in  a  circle,  or  rolling  to  one  side, 
appear.  These  forced  movements  can  be  explained  as  the  result 
of  the  loss  of  some  of  the  compensatory  movements,  along  with 
retention  of  the  rest.  Thus,  if  every  incipient  movement  to  the  right 
calls  up  a  compensatory  movement,  while  movements  to  the  left 
call  up  none,  the  result  must  be  a  constant  turning  to  the  left.  On 
stimulation  of  a  canal,  as  with  a  current  of  electricity,  movements 
occur  which  are  apparently  identical  with  the  compensatory  move- 
ments. Stimulation  of  the  ampulla  of  any  single  canal  arouses  a 
movement  in  the  plane  of  that  canal.  Stimulation,  at  once,  of 
two  canals  which  lie  at  right  angles  to  each  other  arouses  a  move- 
ment in  a  plane  between  those  of  the  stimulated  canals. 

Such  facts  as  these  indicate  the  use  of  the  canals,  but  do  not  also 
show  how  they  are  normally  excited.  The  now  generally  accepted 
theory  of  these  organs  dates  from  about  1873-75,  when  it  was  put 
forth  independently  by  Mach,  Breuer,  and  Crum  Brown.  It  is 
based  partly  on  physical  considerations.  Each  canal  opens  at 
both  ends  into  the  utricle,  and  may  be  considered  as  a  circular  pipe, 
at  one  point  of  which  (the  ampulla)  a  sensory  end-organ  projects 
into  the  pipe.  Rotation  of  the  head  in  the  plane  of  any  canal  must 
therefore  cause,  by  inertia  of  the  contained  endolymph,  a  back- 
flow  in  the  canal.  On  account  of  the  very  small  calibre  of  the  pipe, 
and  the  consequent  friction  against  the  wall,  the  back-flow  of  en- 
dolymph would  certainly  be  very  slight,  but  inertia  must  have  some 
such  effect.  A  back-flow  through  the  ampulla  would  act  on  the 
dome-like  mass  in  which  the  hairs  of  the  sensory  cells  are  embedded, 
and  thus  bend  the  hairs  and  no  doubt  excite  the  cells.  Even  a  ro- 
tation not  in  the  exact  plane  of  a  canal  would  cause  some  back- 
flow,  unless  the  rotation  were  in  a  plane  at  right  angles  to  that  of 
the  canal;  and  the  more  nearly  the  plane  of  rotation  approached 
that  of  the  canal  the  greater  would  be  the  effect  within  that  canal. 
Given,  therefore,  three  canals  at  right  angles  to  one  another,  and 
no  rotation  of  the  head  can  occur  without  exciting  currents  in  at 
least  one  canal;  and  no  two  rotations,  in  different  directions,  can 
excite  exactly  the  same  direction  and  proportion  of  back-flow  in 
the  three  canals.  Thus  this  physical  theory  explains  the  facts  made 
evident  by  compensatory  movements:  first,  that  the  canals  are  some- 
how stimulated  by  head  rotations,  and  second,  that  they  are  differ- 
ently stimulated  by  rotations  in  different  directions. 

More  direct  evidence  is  at  hand  in  support  of  the  theory;  for 
Ewald  has  been  able  to  produce  artificial  currents  of  the  endolymph 
in  a  canal,  and  the  canal  is  thereby  excited,  as  is  shown  by  the  oc- 


OFFICE  OF  SEMICIRCULAR  CANALS  211 

currence  of  a  reflex  movement.  Moreover,  this  movement  is  in 
the  same  direction  as  the  current  in  the  canal ;  and  this  is  as  it  should 
be  to  agree  with  the  theory;  for  both  the  back-flow  and  the  com- 
pensatory movement  are  opposed  to  the  impressed  rotation,  and 
therefore  act  in  the  same  direction,  one  with  the  other. 

§  35.  Regarding  the  utricle  and  saccule,  physical  considerations 
again  suggest  a  similar  theory  (Breuer).  Since  the  gelatinous  mass 
in  which  the  hairs  of  the  sensory  cells  are  here  embedded  is  weighted 
with  the  otoliths,  it  would  tend  to  sag  downwards,  and  would  pull 
differently  on  the  hairs  according  to  the  position  of  the  head  with 
reference  to  gravity.  Probably  we  have  here  a  sense-organ  for 
indicating  the  position  of  the  head;  this  would  explain  certain 
compensatory  positions,  which  disappear  on  destruction  of  the  laby- 
rinth. Not  only  gravity,  but  any  rectilinear  acceleration  would  be 
expected  to  act  on  the  otoliths;  and  there  is  some  evidence  that  this 
is  the  case. 

§  36.  It  should  be  noted  that  it  is  acceleration  and  retardation, 
or  change  in  movement  rather  than  movement  as  such,  which  would 
be  expected  to  excite  the  canals.  The  inertia  current  would  gradu- 
ally cease,  as  the  fluid  became  carried  along  by  friction  with  the 
walls  of  the  canal,  till  finally  the  fluid  would  move  with  the  head. 
If  then  the  rotation  of  the  head  should  cease,  inertia  would  tend  to 
cause  a  continued  movement  of  the  fluid,  and  the  effect  would  be 
the  same  as  if  rotation  were  begun,  from  rest,  in  the  opposite 
direction.  This  is  what  actually  happens  in  dizziness,  and  the  facts 
of  dizziness  lend  further  support  to  the  theory.  Many  deaf  per- 
sons— those,  probably,  whose  disease  extends  to  the  semicircular 
canals — are  not  made  dizzy  by  rotation.1  The  swimming  sensa- 
tion of  the  head  which  accompanies  dizziness  is  probably  to  be 
ascribed  to  the  semicircular  canals,  and  the  somewhat  similar  sen- 
sations which  occur  on  starting  up  or  down  in  an  elevator  are  prob- 
ably connected  with  the  utricle  and  saccule.  With  milder  stimu- 
lation of  these  organs,  the  sensations  are  not  obtrusive,  but  the 
power  of  perceiving  changes  in  the  speed  or  direction  of  rotation 
is  very  keen. 

We  apparently  possess,  then,  in  the  labyrinth,  the  end-organ  of 
a  sense  for  the  positions  and  movements  of  the  head,  which  provides 
for  perceptions  of  rotation,  etc.,  and  which  reflexly  excites  compen- 
satory movements  and  muscular  tonus.  The  nerve  of  this  sense 
is  the  vestibular  branch  of  the  eighth.  As  the  central  connections 
of  this  nerve  with  the  cerebellum  are  close,  and  as  the  results  of 
extirpating  the  labyrinth  resemble  those  of  injuring  the  cerebellum 
(compare  p.  156),  it  is  probable  that  the  reflex  functions  connected 
1  James,  American  Journal  of  Otology,  1882,  IV,  239. 


212  END-ORGANS  OF  THE  NERVOUS  SYSTEM 

with  this  sense-organ  have  their  centre,  in  large  measure,  in  the 
cerebellum. 

§  37.  A  brief  description  of  the  End-Organs  of  Motion,  or  motor 
end-plates,  will  suffice  for  our  purposes.  In  general,  the  termina- 
tions of  the  efferent  nerves  are  connected  either  with  electrical 
organs  (as,  for  example,  in  the  torpedo),  or  with  secretory  glands, 
or  with  the  muscular  fibre.  We  consider  only  the  last  of  these 
three  cases. 

After  an  efferent  nerve  has  entered  the  substance  of  the  so-called 
voluntary  or  striated  muscle,  it  subdivides  among  the  individual 
muscular  fibres,  separating  these  fibres  from  each  other.  Such 
nerve-twigs  usually  lose  their  medullary  sheath,  and  their  axis- 
cylinder  splits  up  into  fibrils,  whose  exact  mode  of  termination  has 
been  much  debated.  It  appears  now  to  be  demonstrated  (by 
Kiihne,  Margo,  Rouget,  and  others)  that  the  axis-cylinder  itself 
pierces  the  sarcolemma  or  sheath  of  the  muscular  fibre;  that  the 
neurilemma  becomes  continuous  with  the  sarcolemma;  and  that 
the  fibrils,  into  which  the  axis-cylinder  divides,  form  a  flat,  branch- 
ing mass  within  certain  peculiar,  disk-shaped  bodies  situated  inside 
the  sarcolemma,  and  called  "motor  end-plates."  In  the  non-striated 
(or  non-voluntary)  muscles,  the  nerves  divide  and  subdivide  to  form 
more  and  more  minute  plexuses  of  nerve-fibres,  which  are  distrib- 
uted in  the  connective  tissue  that  separates  the  muscular  fibres  from 
each  other,  and  finally  applied  to  the  surfaces  of  the  muscular 
fibres.  The  exact  manner  of  this  application  is  of  no  particular 
interest  to  psychology,  even  when  approached  from  the  physi- 
ological point  of  view. 


CHAPTER  IX 

THE  CEREBRAL  HEMISPHERES  AND  THEIR  FUNCTIONS 

§  1 .  Ordinary  observation  recognizes  the  fact  that  the  phenomena 
of  consciousness  are  more  or  less  definitely  correlated  with  the  con- 
dition of  the  bodily  organs.  Certain  alterations  in  our  mental 
states,  on  account  of  the  injury  of  any  of  its  masses,  as  well  as  a  con- 
stant dependence  of  those  states  upon  the  way  some  of  the  masses 
stand  related  to  each  other  and  to  the  outside  world,  impress  the 
fact  upon  our  daily  experience.  It  is  by  no  means  so  obvious  that 
the  nervous  substance  has  any  particular  relation  to  the  thoughts 
and  feelings  of  the  mind.  For  the  functions  of  the  nervous  system 
are  not  exercised  in  giving  information  as  to  itself,  its  own  condi- 
tion and  changes.  By  aid  of  these  functions,  however,  we  have  pre- 
sented in  consciousness  a  more  or  less  clear  picture  of  the  condition 
and  changes  of  the  superficial  parts  of  the  body.  In  the  same  way  a 
knowledge  is  gained  of  the  successive  states  of  tension  belonging 
to  the  muscles  in  movement,  and  even — though  rather  obscurely — 
of  the  place  and  condition  of  the  internal  organs.  But  as  long  as 
they  are  healthy  and  excited  with  only  a  moderate  intensity  of 
their  stimuli,  the  nerves  do  not  even  reveal  their  own  existence; 
and  when  they  are  injured  or  unduly  excited,  the  notice  they  fur- 
nish of  the  fact  comes  in  the  form  of  painful  feeling  which  we  have 
learned  to  localize,  not  in  the  nervous  substance  itself,  but  in  the 
adjacent  parts  of  muscle  and  skin.  Attention  may  be  called,  how- 
ever, to  the  peripheral  nerves  by  the  accident  or  the  dissecting- 
knife  which  exposes  them  to  sight.  But  in  the  case  of  the  central 
nervous  organs,  and  especially  in  the  case  of  the  brain,  there  is  little 
in  ordinary  experience  which  leads  to  a  suspicion  of  their  signifi- 
cance or  even  of  their  existence. 

It  is  not  very  strange,  then,  that  no  general  recognition  of  the 
supreme  importance  of  the  brain,  in  relation  to  the  phenomena  of 
consciousness,  is  to  be  found  in  early  history.  It  is  true  that 
Plutarch1  and  Theophrastus2  inform  us  of  the  opinion  of  the 
physician  Alcmseon,  who  is  said  to  have  been  a  younger  contem- 
porary of  Pythagoras,  and  who  regarded  the  brain  as  the  common 
meeting-place  of  the  senses.  The  same  view  is  also  ascribed  to  the 

1  De  Placitis  Philosophorum,  IV,  17,  1.  *De  Sensu,  §  25  f. 

213 


214  THE  CEREBRAL  HEMISPHERES 

celebrated  Hippocrates.  Later  on  Plato  accepted  it.  But  Aris- 
totle,1 the  greatest  of  all  thinkers  in  antiquity,  the  son  of  a  phy- 
sician, especially  educated  in  physical  science,  and  well  acquainted 
for  the  time  in  the  dissection  of  animals,  regarded  the  brain  as  a 
lump  of  cold  substance,  quite  unfit  to  be  the  seat  and  organ  of  the 
sensus  communis.  This  important  office  he  ascribed  rather  to  the 
heart.  The  brain  he  considered  to  be  chiefly  useful  as  the  source 
of  fluid  for  lubricating  the  eyes,  etc. 

§  2.  The  opinion  of  Exner,2  however,  who  supposes  that  feeling 
in  no  way  immediately  informs  us  that  we  think  with  the  head, 
still  less  with  the  brain  or  the  cortex  of  the  cerebrum,  seems  some- 
what extreme.  For  we  certainly  localize  in  the  head  certain  phe- 
nomena of  consciousness  that  are  inextricably  interwoven  with  the 
processes  of  thought.  The  act  of  attention,  for  example,  results 
in  feelings  which  indicate  that  the  muscles  of  the  eye  are  being  in- 
nervated; or  in  the  more  indefinite  and  diffused  sense  of  strain 
produced  by  contracting  the  skin  of  the  forehead  and  adjacent 
parts  of  the  face.  The  special  sensations  of  hearing,  smelling,  and 
tasting,  which  impress  so  strongly  our  conscious  life,  are  frequently 
referred  to  the  head.  The  same  thing  is  true  of  many  of  the  sen- 
sations of  sight — particularly  of  such  as  appear  when  the  eyes  are 
closed,  in  the  form  of  after-images,  or  spectra,  or  indefinite  and 
changing  color-spots,  seated  in  the  upper  front  part  of  the  face. 
Moreover,  that  inchoate  and  sometimes  half-articulated  language, 
with  which  we  support  our  trains  of  thought,  even  when  we  are  not 
conscious  of  resorting  to  the  expedient  of  "talking  to  ourselves,"  is 
felt  to  be  going  on  within  the  head.  When  one  has  been  engaged 
for  some  time  in  intense  thought,  or  in  eager  and  concentrated 
observation,  one  is  suddenly  made  aware  of  more  or  less  painful 
feelings  which  are  somewhat  indefinitely  ascribed  to  the  same 
cerebral  region.  Men  commonly  lean  the  head  upon  the  hand 
in  supporting  meditation;  or  rub  it  vigorously  to  awaken  the  pow- 
ers of  memory  and  reasoning;  or  stroke  it  to  relieve  the  disagreea- 
ble sensations  which  follow  severe  mental  excitement.  Headache, 
of  more  or  less  intensity,  thus  becomes  associated  with  active  ex- 
ercise of  the  intellect.  The  head  is  wearied  with  thought;  and 
not  only  so,  but  also  with  intense  physical  exercise.  The  dis- 
comfort which  bodily  strain  produces  in  the  hinder  regions  of  the 
head  are  an  indication,  although  of  only  a  very  general  kind,  that 
processes  have  gone  on  in  that  locality  which  are  of  great  impor- 
tance to  the  succeeding  states  of  consciousness.  All  this  apparent 

1  See  De  Partibus  Animalium,  652,  b.  5;    (II,  7);    656,  b.  22  (II,  10);    De 
Juvent.,  467,  b.  28;   and  De  Anima,  III,  1  and  2. 

2  See  Hermann's  Handb.  d.  PhysioL,  II,  ii,  p.  192. 


THE  PROBLEM  OF  CEREBRAL  FUNCTIONS   215 

testimony  of  immediate  feeling  is,  doubtless,  somewhat  exaggerated 
in  an  age  so  distinctively  "nervous"  as  our  own;  but  it  cannot  well 
be  doubted  that  a  certain  amount  of  testimony  from  immediate 
feeling  as  to  the  important  relation  which  exists  between  the  state 
of  mind  and  the  contents  of  the  cranial  cavity,  belongs  to  all  human 
experience. 

However  uncertain  the  witness  of  immediate  feeling  upon  the 
point  in  question  may  be,  very  little  observation  of  others  is  needed 
to  amplify  and  confirm  its  witness.  We  are  not  infrequently  led  to 
notice  how  quickly  and  profoundly  the  states  of  consciousness  are 
changed  by  injuries  to  the  brain.  The  effect  of  a  blow  upon  the 
head  in  suspending  consciousness  is  decisive  of  this  question.  It 
is  but  a  step  from  this  conclusion  to  a  recognition  of  the  truth  that 
the  physiological  significance  of  the  contents  of  the  cranial  cavity 
consists  in  their  affording  a  field  upon  which  all  the  impressions  of 
sense  can  meet  together,  and  so  furnish  the  basis  and  material  of 
comparative  thought.  Indeed,  it  was  this  line  of  inquiry  which 
probably  led  certain  ancient  anatomists,  like  Herophilus  and  Galen, 
to  locate  the  soul,  or  psychical  principle,  in  the  brain.1 

§  3.  A  great  multitude  of  physical  considerations,  advanced  by 
modern  science,  place  beyond  doubt  the  supreme  importance  of 
the  brain  in  its  influence  upon  the  phenomena  of  consciousness. 
The  free  circulation  of  arterial  blood,  with  its  supply  of  oxygen, 
is  a  necessary  condition  for  the  fulfilment  of  the  functions  of  all 
the  central  organs ;  but  this  necessity  is  especially  marked  in  the  case 
of  the  brain.  The  stoppage  of  one  of  the  great  arteries  leading  to 
this  organ,  either  by  compression  in  the  neck,  or  by  embolism  at 
some  point  along  its  course,  at  once  produces  profound  distur- 
bances and  even  complete  cessation  of  consciousness. 

Certain  other  arguments  of  a  similar  nature,  which  have  been  ad- 
vanced from  time  to  time,  have  either  been  rendered  doubtful  or 
quite  discredited  by  recent  investigations.  Such  are  the  claims 
made  by  Lombard  and  Schiff  that  a  rise  of  temperature  either  in 
the  entire  cerebral  area  or  in  particular  circumscribed  regions  of 
the  cortex,  results  from  all  kinds  of  psychical  activities.  Indeed, 
the  amount  of  such  variation  (less  than  yi^0  C.)  would  seem  to  fall 
below  the  limits  of  accurate  observation.  So,  too,  the  claim  of 
Byasson  and  others  to  measure  with  any  reasonable  approach  to 
accuracy  the  amount  of  thought  accomplished,  by  the  increase  of 
waste  in  the  cerebral  tissues,  and  by  the  resulting  quantity  of  sul- 
phates and  phosphates  excreted,  may  be  said  to  have  involved  orig- 

1  In  the  subsequent  discussions,  such  terms  as  "seat,"  "localize,"  "local- 
ization," "  resides  in,"  and  similar  terms,  must  be  understood  only  as  involv- 
ing a  convenient  figure  of  speech. 


216  THE  CEREBRAL  HEMISPHERES 

inally  far  too  many  uncertain  factors;  and  besides,  it  is  not  confirmed 
by  more  recent  work.  For  the  present,  then,  all  such  arguments 
for  the  special  connection  of  the  cerebral  hemispheres  with  the  phe- 
nomena of  consciousness  must  be  left  in  abeyance.1 

§  4.  In  the  case  of  man,  the  cerebral  hemispheres  are,  appar- 
ently, the  only  portions  of  the  nervous  system,  between  the  size, 
condition,  and  molecular  activity  of  which  and  the  phenomena  of 
consciousness  there  is  a  direct  correlation.  If,  then,  we  are  to  speak 
of  mental  activities  as  "localized"  at  all,  the  locality  must  be  in 
the  cortex  of  the  cerebrum.  The  position  that,  in  the  case  of  man, 
the  spinal  cord  and  all  the  intercranial  organs  below  the  cerebral 
hemispheres,  are  incapable  of  acting  as  the  immediate  physical 
basis  of  mental  states,  is  confirmed  even  by  those  experiments  upon 
other  animals,  which  seem  at  first  sight  to  discredit  it.  The  hypoth- 
esis that  consciousness  has  a  seat  in  the  spinal  cord  of  the  frog; 
that,  in  fact,  we  may  properly  speak  of  the  decapitated  animal  as 
having  a  soul — has  been  urged  by  eminent  physiologists  (Pfliiger, 
for  example).  That  the  cord  alone  is  capable  of  various  purpose- 
ful activities,  such  as  serve,  lender  certain  circumstances,  as  signs 
of  a  psychical  experience,  may  be  demonstrated  by  experiment. 
But  unless  one  is  prepared  to  maintain  that  all  purposeful  activity, 
as  resulting  from  excited  nervous  substance,  must  be  correlated 
with  phenomena  of  conscious  sensation  and  volition,  one  can  scarce- 
ly assume  with  confidence  that  such  phenomena  accompany  the 
movements  of  the  decapitated  frog. 

§  5.  The  evidence  from  comparative  anatomy  in  favor  of  re- 
garding the  cerebrum  as,  in  some  peculiar  manner,  the  "seat"  of 
mental  life  has  been  so  fully  set  forth  in  preceding  chapters  (pp. 
33,  61),  that  it  needs  here  but  a  brief  note.  In  general,  the  size  of 
the  cerebrum  bears  a  fairly  close  relation,  as  between  different  orders 
of  vertebrates,  with  the  apparent  intelligence  of  the  animal.  The 
size  of  the  cerebrum  is  indeed  correlated  also  with  the  size  of  the 
animal;  and  there  are  some  apparent  exceptions,  in  which  animals, 
such  as  the  ruminants,  though  possessing  large  hemispheres  do 
not  give  the  impression  of  special  intelligence.  It  must  be  admitted, 
however,  that  comparative  psychology  has  not  yet  progressed  to 
such  a  stage  that  we  can  definitely  assign  the  grade  of  intelligence 
of  each  of  these  animals ;  and  our  casual  observations  may  have  de- 
ceived us  in  regard  to  them.  In  a  general  way,  it  certainly  seems 
that  the  intelligence  of  animals  is  correlated  with  the  size  of  their 
cerebra,  and  not,  on  the  other  hand,  to  the  same  degree  with  the 
size  of  any  other  part  of  the  nervous  system.  The  power  of  learn- 

1  This  paragraph  is  in  correction  of  the  views  expressed  in  the  earlier  edi- 
tion of  this  work.  See  Elements  of  Physiological  Psychology,  p.  242. 


EVIDENCE  FROM  REMOVAL  OR  INJURY          217 

ing  also  seems  to  depend  on  the  development  of  the  cerebrum. 
It  cannot,  indeed,  be  said  that  nothing  can  be  learned  without  de- 
veloping this  particular  part  of  the  brain;  for  even  a  fish  can  learn 
to  some  extent;  and  the  behavior  of  invertebrate  animals  has  been 
found  to  be  modifiable.1  It  would  be  rash  to  assert  that  absolutely 
no  plasticity,  no  modifiability  by  experience,  is  retained  by  the  spinal 
cord;  and  it  would  be  impossible  to  demonstrate  in  logical  form  that 
no  dim  and  rudimentary  consciousness  resides  in  the  cord;  but  com- 
parative anatomy  leaves  no  serious  doubt  that  the  highly  developed 
consciousness  of  man,  and  his  great  power  of  learning  by  experi- 
ence, are  associated  with  the  functions  of  his  cerebral  hemispheres. 

§  6.  Reference  should  also  be  made  to  our  previous  discussion 
(compare  p.  158)  of  the  functions  destroyed  and  spared,  in  ani- 
mals, by  removal  of  the  cerebrum.  We  found  that  the  functions 
remaining  after  such  an  operation  are  the  vegetative,  locomotor, 
and  protective  reflexes — along  with  some  of  the  simpler  expressive 
movements — comprising,  all  in  all,  the  elementary  movements  of 
the  members,  but  involving  a  few  of  the  most  essential  combina- 
tions of  such  movements.  What  was  destroyed  by  the  operation 
was,  most  clearly,  the  learned  reactions,  together  with  the  power 
of  new  learning,  and  the  "anticipatory  reactions,"  or  reactions  of 
only  indirect  utility.  From  these  facts,  the  conclusion  seemed 
justified  that  the  cerebrum  is  the  organ  for  learned  reactions  and  for 
reactions  to  the  wider  environment,  that  is,  for  the  reactions  made 
possible  by  the  existence  of  the  "distance  receptors."  Now  it  is 
especially  with  reference  to  this  "wider  environment,"  as  revealed 
by  these  receptors,  that  learned  reactions  have  their  chief  importance 
in  securing  physical  well-being  and  mental  development. 

§  7.  Perhaps  the  most  conclusive  evidence  that  the  cerebral 
hemispheres  are  the  seat  of  consciousness  and  of  the  intellectual 
operations,  as  well  as,  chiefly,  of  learned  movements  and  reactions, 
is  afforded  by  the  results  of  partial  injuries  to  the  brain.  When  the 
injury  is  total  or  general,  as  in  the  case  of  a  blow  on  the  head,  an 
objector  might  possibly  reply  that  breaking  of  the  neck,  or  piercing 
the  heart,  also  promptly  abolishes  consciousness  and  all  mental 
function.  The  argument,  in  this  bald  form,  cannot  be  taken  seri- 
ously at  the  present  day;  yet  it  has  this  much  of  force,  that  the  loss 
of  consciousness  following  brain  shock  is,  in  strict  logic,  evidence 
only  that  the  brain  is  necessary  to  consciousness — even  as  the 
heart  is.  That  is  to  say,  as  a  vital  organ,  it  is  part  of  the  bodily 
mechanism  with  which  consciousness  is  associated.  It  might 
still  be  true  that  the  whole  nervous  system — cerebrum,  cerebellum, 
brain-stem,  cord  and  nerves — are  the  seat  and  organ  of  the  mental 
1  See  Jennings,  Behavior  of  the  Lower  Organisms  (New  York,  1906). 


218  THE  CEREBRAL  HEMISPHERES 

life.  But  partial  injuries,  which  do  not  abolish  all  consciousness, 
but  simply  make  impossible  some  of  its  phenomena,  cannot  be 
considered  in  the  same  light.  Life  persists;  the  organ  destroyed 
is  not  vital;  and  yet  some  functions  of  the  mental  life  are  rendered 
impossible. 

To  introspection,  it  certainly  seems  as  if  the  consciousness  of 
the  hand,  for  example,  had  its  seat  in  the  hand  itself;  and  as  if  the 
skilled  movements  which  the  hand  learns  are  learned  by  the  hand 
itself;  and  common  forms  of  speech  agree  with  this  naive  view. 
This  seeming  consciousness  may  even  be  projected  into  the  tool 
which  the  hand  is  using.  But  if  the  nerves  going  to  the  hand  are 
severed,  at  any  point  between  the  hand  and  the  spinal  cord,  stimuli 
affecting  the  hand  are  no  longer  perceived,  and  no  power  of  skilled 
or  even  of  reflex  movement  is  retained.  The  hand  has  become  as 
though  non-existent  for  consciousness.  Yet  the  power  to  think 
of  the  hand  has  not  been  lost;  nor  the  power  to  will  movements  of 
the  hand,  which  are,  however,  no  longer  carried  out.  In  cases  of 
amputation  of  the  hand,  stimulation  of  the  stump — and,  thus,  of 
the  remnants  of  the  nerves  which  formerly  ran  to  the  hand — often 
arouses  sensations  located  in  the  hand  as  if  it  were  still  there.  Such 
observations  show  that  the  consciousness  of  the  hand  resides  not 
in  the  hand,  but  somewhere  in  the  central  nervous  system. 

The  same  argument  can  be  carried  further  by  noting  the  results 
of  injuries  to  the  cord.  If  the  cord  has  suffered  such  an  injury 
that  the  lower  part  of  it  is  severed  from  the  upper,  the  whole  lower 
part  of  the  body  is  cut  off  from  consciousness  and  all  mental  in- 
fluences, just  as  the  hand  was  in  the  preceding  case.  The  con- 
sciousness of  the  legs  does  not  then  reside  in  their  reflex  centres  in 
the  cord,  but  somewhere  higher  up,  in  the  brain.  In  the  same  way, 
injuries  may  occur  to  the  brain-stem,  which,  without  severing  it 
entirely — for  this  would  be  a  mortal  injury,  and  would  therefore 
destroy  the  value  of  the  observation  from  our  present  point  of  view 
— may  yet  interrupt  the  sensory  or  motor  tracts  connecting  the  cere- 
bral cortex  with  the  cord;  and  in  such  cases,  the  same  general  re- 
sult, as  regards  consciousness  and  learned  movements,  is  to  be 
noted.  The  power  to  think  of  the  parts  of  the  body  concerned  is 
retained,  and  the  power  to  will  movements;  but  the  execution  of 
the  movements  is  impossible,  and  sensations  of  the  members  no 
longer  arise  in  consciousness.  It  seems  possible,  in  this  way,  to 
push  back  the  seat  of  consciousness  from  the  periphery  to  the  cere- 
bral cortex,  and  to  conclude  that  the  latter  is  the  essential  seat  of 
consciousness. 

The  argument  is  further  strengthened  by  observing  the  effect  of 
partial  injuries  to  the  cortex  itself.  Injuries  to  certain  portions  of 


THE  PROBLEM  STATED  219 

it — as  will  be  set  forth  in  more  detail  in  the  next  chapter — cause  a 
loss  of  certain  of  the  more  elementary  functions  of  normal  life. 
A  person  may  lose,  as  the  result  of  such  injury,  the  sense  of  sight, 
or  that  of  hearing,  or  the  sensations  from  any  part  of  the  periphery. 
The  hand  and  its  nerves,  it  may  be,  as  well  as  the  cord,  brain- 
stem,  and  cerebellum,  are  uninjured,  but  the  destruction  of  a  small 
part  of  the  cortex  has  destroyed  all  sensation  of  the  hand.  Indeed, 
the  person  thus  afflicted  seems  unable  even  to  imagine  the  sensation 
as  localized  in  his  hand,  in  any  very  realistic  way.  As  far  as  con- 
sciousness is  concerned,  therefore,  he  has  suffered  a  more  severe 
loss  than  would  have  been  suffered  if  the  injury  had  affected 
simply  the  nerves  of  the  hand.  So  again,  destruction  of  certain 
portions  of  the  cortex  deprives  a  person  of  visual  sensations,  and 
sometimes  of  visual  images  as  well.  Much  the  same  sort  of  evi- 
dence is  available  in  regard  to  other  than  strictly  sensory  functions. 
Learned  movements,  memories  of  any  particular  sort,  mental  oper- 
ations in  general,  may  be  thrown  out  of  power  to  function,  even 
though  the  injury  is  not  wide-spread  or  severe  enough  to  abolish 
all  consciousness. 

§  8.  Having  been  convinced  by  this  evidence  that  the  cerebral 
hemispheres  are,  in  some  special  sense,  the  seat  and  organ  of  mental 
functions,  the  psychologist  next  appeals  to  anatomy  and  to  physi- 
ology for  an  answer  to  these  questions :  (1)  What  plan  is  followed  in 
the  apportionment  of  the  various  mental  functions  over  the  whole 
extent  of  the  cortex  ?  and  (2)  What  is  the  precise  character  of  the 
functions  of  this  organ  in  its  relations  to  the  phenomena  of  con- 
sciousness ?  Both  problems — that  of  the  "localization  of  cerebral 
functions/'  and  that  of  the  intimate  physiology  of  the  cortex — are 
fraught  with  such  difficulty  that  the  answers  given  are  still  very 
fragmentary  and  often  tentative.  The  results  of  investigation, 
however,  are  well  worthy  the  serious  attention  of  the  psychologist. 

§  9.  The  cerebral  hemispheres  consist,  as  has  been  stated  in 
an  earlier  chapter  (see  pp.  50  f.),  of  the  olfactory  lobes,  the  corpora 
striata,  and  the  pallium.  It  is  the  latter  with  which  we  are  now  con- 
cerned. The  pallium  itself  can  be  divided,  on  the  basis  of  compara- 
tive anatomy  (compare  pp.  31  f.)  into  the  archipallium  and  the  neo- 
pallium;  of  these  the  former  is  older  in  the  animal  series,  and  is  con- 
cerned especially  with  the  sense  of  smell  and  with  the  "snout  sense." 
In  mammals  the  archipallium  is  eclipsed  in  size  by  the  neopallium, 
which,  by  expanding  laterally,  forward  and  backward,  leaves  the 
archipallium  in  a  central  location  and  for  the  most  part  conceals  it. 
It  cannot  be  seen  on  the  upper,  lower,  or  lateral  surfaces  of  the  brain, 
and  only  a  little  of  it  is  visible  on  the  mesial  surface  of  a  hemisphere. 
It  includes  the  fornix  and  the  dentate  gyre,  but  it  is  best  seen,  as 


220  THE  CEREBRAL  HEMISPHERES 

the   "horn  of  Ammon,"   in  a  section  through  the  brain  (see  Figs. 
9  and  97). 

§  10.  The  cortex  is  divided,  for  convenience  of  reference,  into 
lobes.  The  great  Sylvian  fissure  makes  a  clear  division  between  the 
frontal  lobe  above  it,  and  the  temporal  lobe  beneath.  The  fissure 
of  Rolando,  or  central  fissure  (compare  Fig.  94),  constitutes  the 


Frontal  lobe 


Precentral  gyre 

Central  fissure 

Postcentral  gyre 

Parietal  lobe 


Occipital  lobe 


FIG.  94. — The   Upper   Surface   of   the   Cerebral    Hemispheres. 
(Sobotta-McMurrich.) 

boundary  between  the  frontal  and  the  parietal  lobes.  The  pari- 
etal lobe  is  separated  from  the  temporal  by  the  posterior  portion 
of  the  fissure  of  Sylvius.  The  rearmost  portion  of  the  cortex  is 
designated  as  the  occipital  lobe.  A  partial  boundary  between  the 
occipital  and  parietal  lobes  is  afforded  by  the  parieto-occipital 
fissure.  There  is  no  clear  boundary  between  the  occipital  and  the 
temporal  lobes.  In  fact,  the  whole  division  into  lobes  is  somewhat 
artificial.  There  is  no  break  in  the  cortex  between  one  lobe  and 
the  next;  for  the  cortex  extends  down  the  sides  of  the  boundary 
fissures,  and  continuously  around  the  bottom  of  the  fissures  into 
the  adjoining  lobes.  Within  the  Sylvian  fissure,  the  cortex  ex- 
pands into  a  considerable  area,  cut  up  by  smaller  fissures;  and  this 


PRINCIPAL  DIVISIONS  OF  THE  CORTEX        221 


concealed  part  of  the  cortex  is  known  as  the  "island  of  Reil,"  or, 
more  briefly,  as  the  Island. 

Besides  the  fissures  which  have  been  chosen  as  the  grand  bound- 
aries between  the  lobes,  many  other  fissures — in  the  superficial  man- 
ner above  indicated — divide  the  lobes  into  smaller  parts  known  as 
convolutions  or  gyres.  Some  of  these  smaller  fissures  are  incon- 
stant, when  one  individual  brain  is  compared  with  another;  but 
certain  of  the  more  important  are  fairly  constant,  and  so  form  ap- 


Central  fissure 


Supramarginal  gyre 

Angular  gyre 


Superior  frontal 
gyre 

Middle  frontal 
Precentral  gyre 


Inferior 
temporal 
gyre 


Cerebellum 


Fia.  95. — Lateral  Surface  of  the  Left  Cerebral  Hemisphere.     (Edinger.) 

propriate  landmarks  on  the  surfaces  of  the  cortex.  Those  of  most 
interest  are  the  following:  In  the  "central  region"  (see  Fig.  95), 
or  that  immediately  adjacent  to  the  central  fissure,  we  may  dis- 
tinguish two  gyres,  one  on  each  side  of  the  central  fissure,  and  ex- 
tending along  it;  they  are  separated  from  the  rest  of  the  frontal 
and  parietal  lobes,  respectively,  by  the  precentral  and  postcentral 
fissures.  These  gyres  are  named  the  precentral  and  the  postcentral. 
To  the  front  of  the  precentral  fissure  lies  the  great  extent  of  the  frontal 
lobe,  which  is  partially  divided  into  superior,  middle,  and  inferior 
gyres  by  two  horizontal  fissures,  called  the  superior  and  the  infe- 
rior frontal.  In  the  temporal  lobe,  also,  there  is  a  series  of  hori- 
zontal fissures,  which  divide  the  lobe  into  the  superior,  middle, 
and  inferior  temporal  gyres.  The  subdivision  of  the  parietal  and 
occipital  lobes  is  not  quite  so  simply  made.  The  intraparietal 
fissure  separates  the  superior  parietal  gyre  from  the  rest  of  the  lobe ; 
below  this,  two  parts  are  easily  distinguished — namely,  the  supra- 
marginal  gyre,  which  curves  around  the  end  of  the  fissure  of  Syl- 


222 


THE  CEREBRAL  HEMISPHERES 


vius,  and  the  angular  gyre,  which  curves  around  the  end  of  the  supe- 
rior temporal  fissure.  On  the  lateral  surface  of  the  occipital  lobe 
the  divisions  are  not  specially  clear  or  constant;  but  superior,  mid- 
dle, and  inferior  gyres  are  customarily  recognized. 

On  the  mesial  surface  (Fig.  96),  a  prominent  fissure  is  the 
cingulate,  extending  parallel  to  the  callosum,  and  separating  the 
limbic  lobe  from  the  frontal  and  parietal.  The  central  fissure 
often  shows  itself  on  the  mesial  surface;  and  the  gyre  which  curves 


Cingulate  fissure. 


Central  fissure 


Limbic  lobe 


Paracentral  lobule 


Precuneus 


Parieto-occipital 
Assure 


Optic  chiasm 


FIG.  96.— Mesial  Surface  of  the  Right  Cerebral  Hemisphere.     (Edinger.) 

around  its  end  is  called  the  paracentral  lobule.  The  parie to- 
occipital  fissure  is  strongly  marked  on  this  surface,  and  forms  a 
clear  division  between  the  parietal  and  occipital  lobes.  Within 
the  occipital  lobe,  there  appears  on  the  mesial  surface  a  prominent 
fissure,  called  the  calcarine,  which  is  of  great  importance  in  dis- 
cussions of  localization.  The  calcarine  fissure  joins  the  parieto- 
occipital,  and  the  triangular  gyre  which  lies  between  them  is  called 
the  cuneus,  while  the  gyre  which  lies  immediately  beneath  the  cal- 
carine fissure  is  the  lingual;  beneath  this,  again,  is  the  fusiform. 

These,  then,  are  the  chief  landmarks  on  the  surface  of  the  brain, 
which  are  of  use  in  our  studies  for  the  localization  of  cerebral  func- 
tions. 

§  11.  A  section  through  the  cerebral  hemispheres  (Fig.  97) 
shows  the  gray  matter,  or  cortex,  on  the  surface,  and  extending 
to  and  around  the  bottom  of  the  various  fissures;  but  beneath  the 


PROJECTION  AND  ASSOCIATION  FIBRES 


223 


cortex  appears  a  large  mass  of  white  matter.  This  white  matter 
is  composed  of  medullated  (or  myelinated)  nerve-fibres,  which  are 
of  varied  origin.  Some,  as  was  stated  in  considering  the  develop- 
ment of  the  brain  (see  p.  53),  grow  into  the  cerebrum  from  the 
interbrain,  and  pass  to  various  regions  of  the  cortex.  Others  orig- 
inate in  the  cortex,  and  grow  down,  converging  into  the  internal  cap- 
sule, whence  they  pass  to  the  peduncle  and  so  on  down  to  the  pons. 
These  two  classes  of  fibres  form  paths  of  communication  between  the 
cortex  and  the  lower  parts  of  the  nervous  system;  they  are  known 
as  "projection  fibres/'  in  distinc- 
tion from  the  "association  fibres," 
which  pass  from  one  part  of  the  cor- 
tex to  another.  Much  the  great- 
est part  of  the  cerebral  mass  of 
white  matter,  however,  is  composed 
of  association  fibres,  some  of  which 
are  short,  and  connect  neighboring 
gyres,  while  others  are  long  and 
connect  distant  parts  of  the  cor- 
tex with  each  other.  Of  these  long 
association  fibres,  several  distinct 
bundles  can  be  distinguished:  a 
massive  bundle  of  fibres  running 
between  the  temporal  and  occipital 
lobes;  a  bundle  running  between 
the  temporal  and  frontal  lobes; 
another  between  the  temporal  and 
parietal  lobes;  one  between  the  frontal  and  occipital  lobes;  and  one 
extending  from  one  end  to  the  other  of  the  curved  limbic  lobe.  Not 
all  the  fibres  in  a  bundle,  however,  extend  the  whole  length  of  its 
course,  but  some  enter  or  leave  at  various  points  (compare  Fig.  98). 

Though  projection  fibres  have  been  traced  to  or  from  nearly 
all  parts  of  the  cortex,  they  are  by  far  most  numerous  in  certain 
definite  portions  of  the  cortex;  such  are,  especially,  the  central 
region,  on  both  sides  of  the  central  fissure;  the  calcarine  region; 
and  the  superior  temporal  gyre.  This  fact  in  itself  suggests,  at 
least,  a  certain  amount  of  localization  of  function ;  since  the  regions 
directly  connected  by  fibres  with  lower  parts  of  the  system  would 
probably  be  the  regions  most  directly  connected  with  the  senses 
and  with  movement.  This  suggestion  is  borne  out,  as  we  shall  see, 
by  the  results  of  other  methods  of  localization. 

As  was  stated  in  considering  the  development  of  the  brain  (see 
pp.  58  f.),  different  bundles  of  fibres  become  myelinated  at  different 
times.  At  birth  the  only  myelinated  fibres  passing  to  the  cortex 


Subtt.  nigra 


FIG.  97. — Frontal  Section  through  the 
Cerebral  Hemispheres.  (Gegenbaur.) 
A  portion  of  the  archipallium  is  seen 
at  the  part  marked  "Hippocampus." 


224  THE  CEREBRAL  HEMISPHERES 

of  the  human  infant  go  to  the  central  zone;  but  soon  other  regions 
receive  myelinated  fibres.  Flechsig,  on  the  basis  of  extended 
studies  of  the  date  of  myelinization  of  different  regions,  is  able  to 
divide  the  cortex  into  areas  (Fig.  99),  some  of  which  show  mye- 
linated fibres  early,  others  late,  and  others  at  intermediate  dates. 
He  believes  that  the  map  of  the  cortex  so  obtained  is  also  to  be 
regarded  as  a  map  of  the  distribution  of  functions;  and  in  particular 
he  supposes  that  the  regions  whose  fibres  receive  their  myelin 
sheath  early  are  the  centres  of  the  lower  functions  of  sensation 


Fia.  98.— Association  Fibres  of  the  Cerebrum.     (Starr's  Atlas  of  Nerve 
Cells,  by  permission  of  the  Columbia  University  Press.) 

and  movement,  while  the  late-myelinating  regions  are  the  seat  of 
the  highest  intellectual  functions.  As  far  as  concerns  the  early- 
myelinating  areas,  this  theory  is  confirmed  by  the  results  of  other 
methods  of  study;  as  far  as  concerns  the  localization  of  the  intel- 
lectual functions,  the  theory  should  be  entertained  with  considerable 
reserve.  There  is  no  improbability,  however,  in  the  view  that  areas 
distinguished  on  the  basis  of  their  differing  dates  of  myelinization 
should  also  have  different  functions. 

§  12.  Microscopic  examination  of  the  cortex  reveals  a  wealth 
of  nerve-cells,  embedded  in  an  intricate  net-work  of  fine  fibres. 
The  study  of  these  cells  and  fibres,  and  of  the  differences  which  they 
present  in  different  parts  of  the  cortex,  has  been  prosecuted  with 
great  vigor  and  success  during  recent  years.  The  most  logical  order 
for  presentation  of  this  subject  would  demand  at  once  some  account 
of  the  results  of  these  histological  studies.  But  they  will  mean  more 
to  the  reader  if  he  is  first  made  acquainted  with  the  results  of  other 


NERVOUS  ELEMENTS  OF  THE  CORTEX 


225 


work  in  the  localization  of  functions  in  the  cortex.  It  will,  there- 
fore, be  better  to  abandon  the  strict  logical  order  of  presentation  for 
a  historical  order.  The  physiological  and  pathological  results  have, 
in  fact,  preceded  the  histological,  and  have  served  as  the  chief  im- 
petus to  the  minute  examination  of  all  parts  of  the  cortex — with  a 
view  to  determine  whether  regions  which  have  special  functions 
are  also  characterized  by  a  specialized  structure. 

To  superficial  examination,  indeed,  the  cerebrum  appears  to  be 
a  fairly  homogeneous  mass,  of  soft  consistency — an  appearance  which 


FIG.  99.— Division  of  the  Cortex  on  the  Basis  of  Date  of  Myelinization.  (Flechsig.) 
The  frontal  lobe  lies  to  the  left.  The  numbers  indicate  the  approximate  order  of 
myelinization.  The  early-myelinating  areas  are  heavily  shaded,  the  intermediate  areas 
lightly  shaded,  and  the  late-myelinating  areas  left  clear. 

easily  gave  rise  to  the  notion  that  it  functions  as  a  whole,  much  as 
a  gland  functions.  Such  an  epigram  as  "The  brain  secretes 
thought,  as  the  liver  secretes  bile,"  has  even  at  times  gained  currency. 
But  when  the  microscope,  aided  by  differential  stains,  revealed 
the  inner  structure  of  this  mass,  and  when  it  was  seen  to  consist, 
like  the  nerves,  of  multitudes  of  fibres  running  in  various  direc- 
tions in  the  white  matter,  and  entering  and  leaving  the  gray,  the 
glandular  simile  lost  its  force,  and  the  opinion  that  each  fibre  or 
set  of  fibres — like  each  nerve — had  something  specific  to  do,  became 
almost  a  matter  of  compulsion. 

§  13.  We  begin,  then,  our  search  for  more  definite  knowledge  as  to 
the  localization  of  cerebral  functions  with  strong  presumptions  in 
its  favor.  The  cerebral  cortex  is  itself  a  very  complex  organ,  or 


226 


THE  CEREBRAL  HEMISPHERES 


system  of  organs.  Its  different  regions  are  marked  by  compara- 
tively slight,  and  yet  not  insignificant,  differences  of  structure;  they 
stand  in  different  local  relations  and  nervous  connections  with  one 
another  and  with  the  ganglia  lying  below.  This  outlying  rind  of 
gray  nervous  matter  is,  of  course,  not  a  homogeneous  mass.  It  is 
made  up  of  innumerable  nervous  elements  combined  in  various 
ways  and  multiform  connections.  It  may  be  regarded,  then,  as  a 
complex  of  organs. 

Most  of  our  definite  knowledge  concerning  the  functions  of  the 
other  parts  of  the  nervous  mechanism  also  creates  a  presumption  in 


Fig.  99a.— The  Same  as  Fig.  99,  Mesial  Surface. 

favor  of  some  localization  of  cerebral  functions.  All  the  different 
parts  of  this  mechanism  are,  indeed,  constructed  by  combining  vari- 
ously a  few  elements  of  essentially  the  same  structure;  all  of  them 
likewise  are  capable  of  exercising  essentially  the  same  neural  func- 
tions. But  each  part  of  this  mechanism  has  also  its  special  func- 
tions. Thus  we  found  that  the  different  nerves  become  classified 
functionally;  some  are  motor,  voluntary  or  involuntary,  some  in- 
hibitory, some  secretory,  some  sensory,  etc.  Hints  of  a  certain 
kind  of  classification  may  be  discovered  for  the  smaller  ganglia  or 
collections  of  nerve-cells.  In  making  transverse  sections  of  the 
cord,  different  regions  with  different  functions  appear.  Consid- 
ered longitudinally,  the  cord  is  capable  of  being  more  or  less  defi- 
nitely divided  into  several  so-called  centres,  with  specifically  different 
functions.  Localized  centres,  where  specific  kinds  of  reflex-motor 


HISTORY  OF  THE  INVESTIGATION  227 

activity  have  their  particular  seats,  are  fairly  crowded  together  in 
the  medulla  oblongata.  All  the  lower  parts  of  the  encephalon 
appear  subject,  in  a  measure,  to  the  principle  of  localization.  Shall 
we,  then,  stop  short  in  our  attempts  at  differencing  the  functions  of 
the  locally  separate  parts  of  the  nervous  system  just  at  the  point 
where  we  reach  the  most  complex  and  extended  organ,  or  rather 
collection  of  organs,  which  this  system  contains? 

§  14.  Notwithstanding  the  strong  presumption  in  favor  of  the 
localization  of  cerebral  function,  the  beginnings  of  a  successful 
attempt  to  establish  this  theory  are  comparatively  recent.  The 
doctrines  of  Gall,  Spurzheim,  and  others  in  the  older  school  of 
phrenologists,  proved  so  inconclusive  as  to  bring  contempt  upon 
subsequent  attempts  to  divide  the  hemispheres  of  the  brain  into 
different  functional  areas.  Moreover,  certain  indisputable  facts 
seemed  to  render  impossible  the  assured  beginnings  of  a  theory  of 
cerebral  localization.  Considerable  portions  of  the  human  brain, 
it  was  found,  might  be  lost  without  destroying  any  one  sensory  or 
motor  function.  Moreover,  the  gray  matter  of  the  cerebral  hemi- 
spheres, it  was  then  thought,  could  not  be  directly  excited  by  elec- 
tricity or  by  other  forms  of  stimuli.  The  greatest  experimenters 
in  physiology,  such  as  Longet,  Magendie,  Flourens,  Matteucci,  Van 
Deen,  Budge,  and  Schiff,  declared  against  the  localizing  of  cerebral 
function.  In  1842  Longet1  affirmed  that  he  had  experimented  upon 
the  cortical  substance  of  dogs,  rabbits,  and  kids,  had  irritated  it  me- 
chanically, cauterized  it  with  potash,  nitric  acid,  etc.,  and  had  passed 
galvanic  currents  through  it  in  different  directions,  without  obtain- 
ing any  sign  whatever  of  resulting  muscular  contraction.  In  the 
same  year  Flourens2  asserted,  on  the  basis  of  numerous  experiments 
in  extirpation,  that  the  lobes  of  the  cerebrum  perform  their  func- 
tions with  their  whole  mass;  that  there  is  no  special  seat  for  any  of 
the  cerebral  activities;  and  that  even  a  small  remnant  of  the  hemi- 
spheres can  serve  all  the  uses  of  their  collective  functions. 

So  great  was  the  authority  of  the  distinguished  names  just  men- 
tioned, that  their  confident  opinions  gained  general  credence.  The 
evidence  brought  forward  by  Broca  and  others  seemed,  however, 
to  show  some  special  connection  between  a  single  convolution  of 
the  frontal  lobe  and  the  complex  activities  of  articulate  speech; 
and  the  anatomist,  Meynert,  held  the  opinion  that  the  structure 
and  connections  of  the  cerebrum  show  its  anterior  portion  to  be 
in  general  used  for  motor,  its  posterior  for  sensory,  functions.  In 
1867  Eckhard  repeated  the  significant  observation  which  had  been 

1  Anatomic  et  physiologic  du  systeme  nerveux,  etc.,  Paris,  1842,  I.,  pp.  644  f. 

2  Recherches  experimentales  sur  Us   proprieties  et  les  fonctions  du  systeme 
nerveux,  etc.,  pp.  99  f. 


228  THE  CEREBRAL  HEMISPHERES 

made  by  Haller  and  Zinn  more  than  a  century  before:  namely, 
that,  on  removing  parts  of  the  cortical  substance  of  an  animal's 
brain,  convulsive  movements  occur  in  its  extremities. 

It  was  not  until  1870  that  the  "epoch-making"  experiments 
of  Fritsch  and  Hitzig1  began  the  modern  era  of  investigation  into 
this  subject.  These  observers  announced  the  fact  that  the  cere- 
bral cortex  of  dogs  is,  at  least  in  certain  minute  areas  of  it,  excita- 
ble by  electricity.  They  pointed  out  the  further  fact  that,  while 
some  parts  of  the  convexity  of  the  cerebrum  are  capable  of  motor 
excitation  and  others  not,  the  motor  parts  lie  in  general  to  the 
front,  the  non-motor  to  the  rear  of  this  convexity.  By  stimulating 
with  an  electrical  current  the  so-called  motor  parts,  co-ordinated 
contractions  of  the  muscles  in  the  opposite  half  of  the  body  were 
obtained.  Of  such  so-called  "motor-centres"  they  indicated,  in 
their  first  announcement,  the  following  five:  One  for  the  muscles 
of  the  neck,  another  for  the  extension  and  abduction  of  the  fore 
limb,  another  for  the  bending  and  rotation  of  the  same  limb,  an- 
other for  the  hind  limb,  and  lastly  one  for  the  face.  From  such 
facts  they  drew  the  conclusion  that  the  principle  announced  by 
Flourens  is  demonstrably  false.  We  must  rather  admit,  say  they, 
that  "certainly  several  psychical  functions,  and  probably  all,  are 
shown  to  have  their  point  of  entrance  into  matter  or  of  origin  from 
it  at  circumscribed  centres  of  the  cerebral  cortex."  The  same 
principle  was  subsequently  defended  at  length  by  Hitzig,  and  the 
number  of  so-called  cerebral  centres  increased.  The  most  note- 
worthy facts  which  these  experimenters  first  made  clear  and  de- 
monstrable have  since  been  verified  by  many  investigators.  Among 
the  physiologists  who  have  amplified  the  results  of  Fritsch  and  Hit- 
zig, the  following  deserve  special  mention:  Ferrier,  for  his  work 
on  the  monkey's  brain;  and,  more  recently,  Sherrington  and  Grtin- 
baum,  for  their  work  on  the  anthropoid  apes,  the  brains  of  which 
are  anatomically  much  closer  to  the  human  brain  than  are  those  of 
the  lower  monkeys.  The  testimony  of  human  pathology,  and  the 
evidence  of  comparative  anatomy  and  of  histology,  have  also  been 
largely  drawn  upon  either  to  confirm  or  to  confute  the  conclusions 
originally  based  on  experiments  with  animals.  Before  considering 
the  conclusions  themselves,  it  is  necessary  to  understand  the  true 
nature  and  extent  of  the  various  kinds  of  evidence. 

§  15.  Three  great  lines  of  evidence,  leading  from  three  great 
groups  of  facts,  must  be  considered.  These  are  the  evidence  from 

1  See  the  article  by  G.  Fritsch  and  E.  Hitzig  in  the  Archiv  f.  Anat.,  Phys- 
iol.,  etc.,  1870,  pp.  300-332;  and  subsequent  articles  by  Hitzig  in  the  same 
Archiv,  1871,  1873,  1874,  1875,  1876;  also  his  collected  works  Physiologische 
und  klinische  Untersuchungen  uber  das  Gehirn  (Berlin,  1904). 


THE  THREE  LINES  OF  EVIDENCE  229 

experimentation,  the  evidence  from  pathology,  and  the  evidence 
from  histology  and  comparative  anatomy.  Each  of  the  three  has 
its  peculiar  advantages  and  value;  each  also  its  peculiar  difficul- 
ties and  dangers.  It  is  only  by  regarding  the  combined  testimony 
of  the  three  that  the  highest  probability  at  present  possible  can  be 
attained.  . 

Experimentation  with  a  view  to  discover  the  localized  functions 
of  the  cerebral  cortex  is  of  two  kinds,  stimulation  and  extirpation. 
In  stimulation  experiments,  the  procedure  is  as  follows:  having 
first,  under  anaesthesia,  removed  the  bony  and  membranous  cover- 
ings of  a  portion  of  the  brain,  a  weak  current  of  electricity  is  applied 
to  a  minute  portion  of  the  exposed  cortex,  and  a  watch  is  kept  for 
resulting  movements  in  any  part  of  the  body.  It  was  this  experi- 
ment which,  in  the  hands  of  Flourens  and  other  early  observers, 
gave  negative  results,  and  which  first  succeeded  in  the  hands  of 
Fritsch  and  Hitzig.  The  failure  of  the  older  experimenters  is  not 
difficult  to  explain;  for  only  a  small  proportion  of  the  entire  cortex, 
on  stimulation  by  currents  of  low  or  moderate  strength,  responds 
with  any  bodily  movements  whatever.  The  rest  of  the  cortex  is 
said  to  be  "silent"  under  stimulation.  In  this  fact  lies  a  limitation 
of  the  method;  it  supplies  information  only  regarding  the  fraction  of 
the  cortex  which  gives  motor  responses. 

A  further  difficulty  with  the  method  of  stimulation  is  its  depend- 
ence on  the  use  of  the  electric  current.  This  is  an  admirable  stimu- 
lus in  most  respects,  but  it  is  subject  to  one  or  two  limitations,  which 
need  to  be  guarded  against  by  the  experimenter;  and  sometimes  by 
those  who  would  accept  his  results.  One  difficulty  is  the  "spread- 
ing" of  the  current,  which  is  thus  likely  to  excite  parts  not  imme- 
diately in  contact  with  the  electrodes,  and  so  deceive  the  observer. 
In  the  most  recent  work,  the  method  of  "unipolar  stimulation" 
— in  which  one  pole  of  the  battery  is  broad,  and  applied  to  a  distant 
part  of  the  body,  while  the  other  is  a  needle  point  applied  to  the  sur- 
face of  the  brain — has  been  employed  with  success  in  finer  localiza- 
tions. At  one  time,  it  was  feared  that  the  spreading  of  the  electrical 
current  seriously  jeopardized  all  the  results  of  cortical  stimulation; 
for — it  was  argued — the  movements  might  be  due  not  to  arousal  of 
the  cortex,  after  all,  but  to  the  arousal  of  some  deeper-lying  struct- 
ure. A  variety  of  checks  have  shown,  however,  that  the  cortex 
is  actually  the  part  aroused.  Perhaps  the  best  worth  citing  of 
these  checks  is  the  fact  that,  in  the  large  brain  of  the  chimpanzee, 
with  its  thick  cortex,  and  relatively  long  distances  between  parts,  weak 
unipolar  stimulation  easily  arouses  movements,  but  only  when  the 
stimulus  is  applied  to  certain  limited  regions;  and  the  same  move- 
ments are  obtained  with  great  constancy  from  the  same  spots. 


230  THE  CEREBRAL  HEMISPHERES 

Such  regular  results  could  hardly  be  got  by  diffusion  of  current  to 
subcortical  parts;  and,  in  fact,  the  same  responses  cannot  be  got 
by  stimulation  of  any  remote  subcortical  parts.  On  the  contrary,  the 
same  responses,  or  nearly  the  same,  can  be  got  by  stimulating  the 
white  matter  immediately  below  a  given  region  of  the  cortex  as  are 
got  from  the  region  itself;  but  this  is  to  be  expected,  since  any  area 
must,  of  course,  exert  its  influence  through  the  fibres  issuing  from  it. 

Some  use  has  been  made  of  a  similar,  but  reverse  use  of  the 
method  of  stimulation; — namely,  that  of  observing  the  electrical 
changes  in  the  cortex  on  stimulation  of  certain  peripheral  nerves. 
Just  as  a  nerve,  excited  at  one  end,  is  traversed  by  an  electric  wave, 
so  it  has  been  found  by  several  physiologists  (Caton,  Danielewski, 
and  others)  that  on  flashing  a  light  into  the  eye  of  an  animal,  elec- 
tric currents  were  produced  in  a  certain  definite  region  of  the  cor- 
tex, which — it  is  then  concluded — must  be  closely  connected  with 
the  retina.  The  value  of  this  method  is  diminished  by  the  fact 
that  the  currents  seem  excessively  weak  except  in  certain  regions; 
and  that  these  same  regions  happen  to  have  their  functions  fairly 
well  located  by  other  methods. 

§  16.  In  the  method  of  extirpation,  as  practised  by  physiologists, 
a  well-defined  area  of  the  cortex  is  cut  out;  the  animal  is  allowed  to 
recover  from  the  general  effects  of  the  operation,  and  is  watched  and 
tested  with  a  view  to  determine  precisely  what  loss  of  function  has 
attended  the  loss  of  brain  substance.  The  assumption  that  the 
two  correspond  is  antecedently  probable;  but  he  would  be  an  un- 
wary experimenter  who  should,  at  the  present  day,  assert  unquali- 
fiedly that  the  injured  function  had  its  peculiar,  not  to  say  its  only, 
seat  in  the  injured  part.  Indeed,  later  on  the  injured  animal  often 
shows  a  partial  or  nearly  complete  recovery  of  the  lost  functions. 
This  species  of  restitution  is  a  puzzling  side  of  the  results  of  the 
method  of  extirpation;  it  will  be  referred  to  again.  Certain  other 
difficulties  of  this  method  can  be  largely  avoided  by  good  procedure. 
For  example,  the  inflammatory  after-effects  of  an  operation,  which 
often  impaired  the  value  of  the  early  work,  can  now  be  avoided  by 
operating  with  aseptic  precautions.  The  danger  of  cutting  too 
deep,  and  so  of  not  simply  extirpating  the  desired  area  of  the  cortex, 
but  also  of  interfering  with  other  areas  by  incidentally  severing 
their  projection  and  association  fibres,  cannot  be  wholly  avoided; 
but  examination  of  the  brain  after  the  death  of  the  animal  may  reveal 
the  truth  in  this  respect. 

One  difficulty  with  all  the  methods  of  the  physiologist  is  that  he 
deals  with  animals,  and  can  learn  of  their  mental  processes  only  in- 
directly and  imperfectly.  To  a  limited  extent,  there  exist  observa- 
tions on  man  which  are  of  the  same  nature  as  those  of  the  physiolo- 


EVIDENCE  OF  HUMAN  PATHOLOGY  231 

gists  on  animals.  In  some  surgical  operations,  it  is  necessary  to  re- 
move definite  parts  of  the  brain;  and  in  order  to  locate  these  parts 
precisely,  the  electric  current  is  applied,  when  the  part  is  in  the 
"motor  region."  Thus  some  opportunity  is  afforded  for  examining 
the  effects  of  stimulation  and  of  extirpation  in  the  human  subject. 

§  17.  The  chief  source  of  direct  evidence  regarding  localization 
in  the  human  brain  is  pathology.  Injuries  occur  to  parts  of  the 
brain,  by  gunshot  wounds,  or  by  fractures  of  the  skull  necessitating 
the  removal  of  splinters  of  bone  and  with  them  some  of  the  brain 
substance.  More  common  are  destructive  lesions  due  to  tumors, 
hemorrhages,  or  local  softening  from  impaired  circulation  in  one 
or  more  branches  of  the  cerebral  arteries.  It  will  be  observed  that 
accident  and  disease  do  much  the  same  thing  here  to  the  human  brain 
that  the  physiologist's  knife  does  to  the  brains  of  the  animals  which 
he  is  studying.  The  pathological  method  is  essentially  the  same  "as 
the  method  of  extirpation.  It  has  the  advantage  of  giving  results 
on  the  human  subject,  regarding  whose  mental  operations  we  have 
much  better  sources  of  information  than  are  available  in  the  case 
of  animals.  The  pathological  method  has,  on  the  other  hand,  the 
disadvantage  that  the  seat  and  limits  of  the  lesion  are  not  prede- 
termined by  the  observer.  Too  much  of  the  brain  substance  is 
usually  affected  to  afford  a  good  subject  for  precise  localization 
of  function.  Indeed,  in  many  cases  a  large  portion  of  the  entire 
cortex  is  more  or  less  affected,  either  by  pressure  exerted  by  a  grow- 
ing tumor,  or  by  general  disturbances  of  the  blood  supply,  or  by 
spreading  of  the  chemical  influences  of  a  diseased  spot.  The  result 
is  that  few  pathological  cases  afford  perfectly  clean  experimental 
evidence.  Another  difficulty  is  that  the  individual  whose  brain  is 
to  be  injured  by  these  natural  causes  is  not  known  beforehand;  he, 
therefore,  cannot  be  examined  beforehand  as  to  his  mental  character- 
istics, as  the  animal  subject  can  be.  For  these  reasons,  and  also  be- 
cause of  the  intricate  interweaving  of  mental  functions,  the  progress 
of  localization  in  the  human  brain  has  been  slow;  a  large  accumula- 
tion of  material,  and  good  judgment  in  interpreting  the  material, 
are  still  needed  in  order  to  reach  sound  conclusions.  In  spite  of 
these  difficulties,  however,  it  may  be  said  that  by  using  as  guides  the 
best-established  localizations  in  animals,  the  study  of  human  path- 
ology has  established  the  truth,  that  the  fundamental  facts  of  localiza- 
tion are  the  same  in  the  brains  of  both  the  lower  animals  and  of  man. 

§  18.  In  addition  to  the  physiological  and  the  allied  pathological 
methods,  there  is  a  group  of  methods  which  belong  under  the  science 
of  anatomy  in  a  broad  sense  of  the  latter  word. 

The  comparative  anatomy  of  the  brain  affords  some  scientific 
information  regarding  the  functions  of  its  different  parts.  The  best 


232  THE  CEREBRAL  HEMISPHERES 

instance  of  the  use  of  this  method  for  purposes  of  localization  is 
the  case  of  the  archipallium,  and  its  probable  connection  with  the 
sense  of  smell  and  other  related  functions.  This  matter  has  al- 
ready been  sufficiently  discussed.  The  logic  of  the  method,  in  all 
such  cases,  is  as  follows:  When  the  behavior  of  a  species  of  animals 
shows  that  a  certain  function  is  highly  develope'd  (or  the  opposite), 
peculiar  developments  of  the  brain  in  these  species  may,  with  some 
probability,  be  regarded  as  related  to  this  function.  This  argu- 
ment is,  indeed,  similar  to  that  of  Gall  and  the  phrenologists;  but 
they  endeavored  to  apply  the  method  in  the  comparison  of  human 
individuals,  and  in  the  first  instance,  they  sought  to  infer  the  de- 
velopment of  the  brain  from  the  external  appearance  of  the  skull. 
The  latter  inference  is  now  known  to  be  very  insecure.  There  seems 
no  antecedent  reason  to  conclude,  however,  that  a  comparison  of 
the  brains  of  different  human  individuals,  whose  mental  peculiari- 
ties were  well  known,  might  not  assist  in  the  localization  of  mental 
functions.  The  method  is  still  in  use;  but  so  far  its  results  are  not 
very  trustworthy — partly  because  of  insufficient  psychological  analy- 
sis of  the  individuals  concerned,  and  partly  because  of  insufficient 
microscopic  study  of  the  brains. 

Comparative  anatomy  is  useful  in  another  way,  which  has  al- 
ready been  noticed  in  our  study  of  the  nerve-tracts  (compare  pp. 
31,  88).  Brains  of  simpler  construction  afford  a  better  opportunity 
for  the  tracing  of  nervous  connections  than  is  afforded  by  the  ex- 
tremely intricate  mass  of  fibres  in  the  white  matter  of  the  human 
brain.  It  is  a  valid  assumption,  that  the  connections  of  any  part 
of  the  cortex  are  of  great,  and  even  decisive  importance,  in  assign- 
ing the  function  of  that  part.  If,  for  example,  it  is  possible — as  it 
is — to  trace  the  fibres  of  the  optic  nerve  back  to  certain  parts  of 
the  interbrain,  and  other  fibres  thence  to  a  certain  region  of  the  cor- 
tex, this  fact  is  the  best  possible  indication  that  the  region  of  the 
cortex  to  which  these  fibres  run  is,  somehow,  specially  concerned 
with  the  sense  of  sight.  Indeed,  the  most  decisive  localization  of 
the  sensory  functions  of  the  cortex  has  been,  to  a  large  extent,  ob- 
tained by  this  method.  To  take  another  example:  If  the  fibres  of 
the  cortico-spinal  or  pyramidal  tract,  the  principal  motor  tract  pass- 
ing from  the  cerebrum  to  the  cord,  can  be  traced  back  to  a  certain 
area  of  the  cortex,  there  is  good  reason  for  calling  this  the  "motor 
area."  Such  tracing  of  fibres,  as  previously  explained  (compare 
pp.  87  f.),  has  been  accomplished  by  several  methods. 

In  the  case,  too,  of  parts  of  the  cortex  which  are  directly  connected 
with  lower  ganglia  (either  sensory  or  motor),  the  tracing  of  fibre-con- 
nections has  decisive  weight.  In  general,  increased  knowledge  of  the 
connections  of  other  parts  of  the  cortex  with  its  sensory  and  motor 


EVIDENCE  FROM  HISTOLOGY  .       233 

areas,  and  with  one  another,  would  conduce  greatly  to  an  under- 
standing of  the  functions  of  all  these  areas.  If,  for  example,  a  cer- 
tain limited  region  can  be  shown  to  be  the  origin  of  the  fibres  con- 
necting the  cortex  with  the  muscles  of  the  tongue,  and  if  some  other 
area  can  be  shown  to  be  connected  with  this  motor  tongue  area  by 
especially  numerous  fibres,  then  there  is  evidence  that  this  second 
area,  too,  is  concerned  in  movements  of  the  tongue.  The  probability 
arises  in  this  way,  that  all  these  areas  are  concerned  with  the  com- 
plex functions  of  speech.  In  other  words,  a  truly  neurological  con- 
ception of  the  working  of  the  brain,  as  distinguished  from  a  rough 
assignment  of  this  or  that  gross  function  or  "faculty"  to  this  or  that 
part  of  the  cortex,  would  be  advanced  by  nothing  so  much  as  by 
an  unravelling  of  the  paths  of  the  association  fibres.  Unfortunately, 
such  a  task  is  extraordinarily  difficult.  Since  it  is  in  this  respect  that 
man  differs  most  from  the  animals,  the  work  must  be  largely  done 
on  the  human  brain;  it  must,  therefore,  be  subject  to  the  difficulties 
that  attend  other  work  with  pathological  material.  Still,  progress 
is  being  made  even  here,  and  at  an  accelerated  rate. 

Another  method  of  an  anatomical  character  which  has  recently 
begun  to  show  great  promise  is  the  histological  mapping  of  the 
cortex.  Some  of  the  results  of  this  method  will  be  referred  to,  later 
on,  in  their  relation  to  the  more  successful  of  the  efforts  to  solve 
the  problems  of  the  localization  of  cerebral  functions,  and  of  the 
more  precise  nature,  in  themselves  considered,  of  these  functions. 

§  19.  We  close  this  critical  survey  of  the  evidence  for  the  localiza- 
tion of  cerebral  functions,  with  these  observations  as  to  the  nature 
of  the  evidence  itself  and  as  to  the  trustworthiness  of  its  results. 
The  evidence  is  always  extremely  complex  and  often  very  conflict- 
ing; but  it  is  in  general  cumulative,  and  in  certain  cases  it  is  entitled 
to  be  pronounced  quite  convincing.  It  must  always  be  remembered, 
however,  that  the  possibility  of  differentiations  and  even  of  idiosyn- 
crasies is  as  great  in  the  human  brain  as  it  is  in  any  of  nature's 
most  complicated  products.  On  the  psychological  side,  too,  we 
find  ourselves  always  faced  with  great,  and  often  insuperable, 
difficulties  in  our  attempts  to  perfect  the  necessary  analyses.  The 
most  conclusive  results  are  obtained  in  those  cases  where,  on  the 
side  of  the  nervous  mechanism,  we  find  all  the  lines  followed  by 
the  different  methods  converging  on  the  same  result;  and  where, 
on  the  side  of  the  mental  life  and  development,  we  are  dealing  with 
those  forms  of  conscious  activity — such  as  the  co-ordination  and 
control  of  bodily  movements  and  the  experiences  of  the  more  simple 
and  fundamental  of  the  activities  of  sensation  and  association — 
which  are  shared  by  man  with  the  lower  animals,  and  which  are, 
whether  considered  from  the  biological  or  the  psychological  point 


234  THE  CEREBRAL  HEMISPHERES 

of  view,  most  essential  to  his  existence  and  to  the  capacity  for  learn- 
ing to  adapt  himself  to  a  varied  environment. 

More  precisely:  A  hundred  years  of  the  use  of  these  different 
methods  of  investigation,  advanced  in  the  most  recent  times  to  a 
high  degree  of  refinement  in  the  hands  of  many  diligent  and  skil- 
ful workmen,  have  brought  about  a  general  agreement  and  a  rea- 
sonable certainty  as  respect  the  following  of  the  simpler  functions: 
The  "motor  area"  is  definitely  located;  the  "visual  area"  likewise; 
and  the  location  of  the  areas  for  hearing  and  smell  is  only  a  little 
less  definite. 


CHAPTER  X 


THE  CEREBRAL  HEMISPHERES  AND  THEIR  FUNCTIONS 
(CONTINUED) 

§  1.  The  first  functional  area  to  be  localized  was  the  motor  area. 
The  pioneer  work  of  Fritsch  and  Hitzig1  located  it  in  the  case  of 
the  dog  (Fig.  100).  Its  position  is  far  forward  on  the  surface  of  the 
dog's  cortex,  in  the  "sigmoid  gyre."  Here  a  small  area  was  de- 
tected, on  stimulating  which  with  a  weak  current  of  electricity,  the 
muscles  of  the  neck  were  thrown  into 
contraction;  another  similar  area 
was  found  for  extension  of  the  fore 
limb;  another  for  flexion  of  the  same 
limb;  and  another  for  facial  move- 
ments. Movements  of  the  back, 
abdomen,  and  tail  were  also  ob- 
tained, though  their  precise  area  was 
not,  at  that  time,  sharply  localized. 
Many  later  observers  have  confirmed 
and  refined  these  results.  As  it  is 
not  our  purpose  here  to  give  a  com- 
parative description  of  cerebral  local- 
ization in  different  animals,  we  may 
simply  note  that  motor  areas  have 
been  located  in  other  carnivora,  in 
rodents,  and  indeed  in  various  orders 
of  animals.  We  turn  at  once  to  the 
primates.  Ferrier rendered  service2 
to  the  growing  science  of  localiza- 
tion by  extending  the  study  to  the 
monkeys — an  important  extension, 
because  of  the  similarity  of  shape  and  fissuration  between  the  brains 
of  these  animals  and  the  human  brain;  and  also  because  a  much 
more  detailed  localization  was  found  possible  in  the  primate  brain. 
Ferrier  located  the  motor  area  in  the  central  region  of  the  monkey's 
brain,  and,  indeed,  in  both  of  the  central  gyres,  the  precentral  and 
the  postcentral.  In  other  words,  his  motor  area  extended  along  the 

1  Archiv  f.  Anat.  und  PhysioL,  1870,  pp.  312  f. 

2  Functions  of  the  Brain  (London,  1876);    2d  edition,  1886. 

235 


Fia.  100. — Motor  Area  of  the  Dog. 
(Fritsch  and  Hitzig.)  The  two  hem- 
ispheres are  drawn  from  different 
animals,  a,  the  sulcus,  around 
which  the  sigmoid  gyre  bends;  A, 
area  for  muscles  of  the  neck;  +,  fore 
limb;  *,  hind  limb,  000°,  facial  mus- 
cles. 


236  THE  CEREBRAL  HEMISPHERES 

central  fissure,  or  fissure  of  Rolando,  and  on  both  sides  of  it.  With- 
in this  area,  he  distinguished  smaller  areas  for  movements  of  dif- 
ferent groups  of  muscles;  movements  of  the  hind  limbs  were  ob- 
tained from  the  upper  part  of  the  region,  near  the  middle  line, 
while  movements  of  the  fore  limbs  were  obtained  about  half-way 
down,  and  movements  of  the  face  at  the  bottom  of  the  region,  near 
the  fissure  of  Sylvius. 

§  2.  Ferrier's  results  have  been  abundantly  confirmed,  with 
one  principal  exception,  which  will  be  stated  directly  .  We  can 
pass  quickly  over  this  work  on  the  lower  monkeys  and  consider  the 
case  of  the  anthropoid  apes,  whose  brain  resembles  the  human 
brain  still  more  closely.  Owing  to  the  difficulty  of  obtaining  ani- 
mals for  experimentation,  these  highest  and  most  interesting  forms 
were  not  early  examined  by  physiologists.  Beevor  and  Horsley 
published  in  18901  a  physiological  study  of  the  brain  of  a  single 
orang;  and,  more  recently,  Grlinbaum  and  Sherrington2  have  ex- 
perimented on  all  three  species  of  anthropoid  apes,  including  a  con- 
siderable number  of  individuals.  Their  work,  done  in  the  light  of 
all  the  experience  of  previous  investigators,  and  with  improved 
methods,  assigns  much  the  same  position  to  the  motor  area  in  the 
anthropoid  brain  as  that  found  by  Ferrier  for  the  smaller  monkeys. 
Griinbaum  and  Sherrington,  however,  employed  unipolar  stimula- 
tion (compare  p.  229),  by  which  means  the  electrical  stimulus  can 
be  more  sharply  limited  in  its  application  than  by  older  methods; 
for  this  reason,  among  others,  they  were  able  to  show  that  the  ex- 
fcntable  region  did  not  extend  to  the  rear  of  the  central  fissure,  as 
Terrier  had  found  in  the  monkeys,  but  was  limited  to  the  precentral 
gyre.  This  led  to  renewed  examination3  of  the  brain  of  the  smaller 
monkeys,  with  the  result  that  here,  too,  the  motor  area  was  con- 
fined to  the  anterior  side  of  the  central  fissure.  In  this  respect, 
then,  the  results  of  Ferrier  have  received  an  important  correction. 
The  excitable  region  extends  down  into  the  central  fissure,  and  even 
to  its  very  bottom,  but  does  not  reach  the  free  surface  of  the  post- 
central  gyre.  The  limits  of  the  motor  region,  in  the  forward  di- 
rection, are  less  sharp  than  on  the  rear.  The  area  is  broad  at  the 
top,  where  it  extends  also  over  upon  the  mesial  surface  of  the  hemis- 
phere; lower  down  the  precentral  gyre,  it  becomes  narrower. 

1  Philosophical  Transactions  of  the  Roy.  Soc.  of  London,  1890,  B.  p.  129. 

2  Proceedings  of  the  Royal  Society  of  London,  1901,  LX1X,  206;    and  1903, 
LXXII,   152;    Transactions  of  the  Pathological  Society  of  London,  1902,   53, 
part  I,  pp.  127-136;    Sherrington,  Integrative  Action  of  the  Nervous  System, 
New  York,  1906. 

3  See  the  very  extensive  investigations  of  C.  and  O.  Vogt,  Journal  /.  Psy- 
chol  und  NeuroL,  1907,  VIII,  277. 


BRAINS  OF  MONKEYS  AND  ANTHROPOID  APES  237 

These  investigators  found  (compare  Figs.  101  and  102)  the  move- 
ments of  the  different  parts  of  the  body  represented  within  the  motor 
region  about  as  Ferrier  had  said:  the  hind  limbs  were  excitable 
from  the  upper  part,  near  the  middle  line;  the  area  for  the  trunk  lay 
below  these,  opposite  the  upper  of  two  well-marked  curves  in  the 
central  fissure,  which  serve  as  valuable  landmarks.  Below  this 
same  curve  is  the  area  for  the  arms;  further  below,  and  opposite 


Uits  cenfrdlis. 


cords. 


FIG.  101.  —  Lateral  Surface  of  the  Brain  of  a  Chimpanzee.  (Grunbaum  and  Sherrington.) 
The  left  hemisphere  is  shown,  with  the  frontal  lobe  to  the  left.  The  extent  of  the 
motor  area  is  indicated  by  the  darkened  portion,  though  it  should  be  understood  that 
a  large  part  of  the  area  lies  in  the  central  fissure. 

the  lower  of  the  two  bends  of  the  fissure,  is  a  region  for  the  neck; 
and  below  this,  again,  near  the  bottom  of  the  fissure,  lies  the  area 
for  the  head  and  face.  Still  more  minute  localizations  were  found 
to  be  possible:  thus,  within  the  arm  area,  the  sequence,  from  above 
downward,  is  shoulder,  elbow,  wrist,  hand;  and  in  the  region  cor- 
responding to  the  lower  extremity  the  sequence  is  pelvic  region, 
toes,  ankle,  knee,  and  hip. 

Movements  of  the  eyeballs  were  not  obtained  in  the  anthropoid 
ape  from  stimulation  of  the  precentral  gyre;  there  was,  however,  a 
considerable  area  further  forward,  in  the  middle  and  inferior  frontal 
gyres,  by  excitation  of  which  conjugate  movement  of  both  eyes  to 


238 


THE  CEREBRAL  HEMISPHERES 


the  other  side  was  obtained.  This  agrees  with  the  results  of  experi- 
ments  with  the  monkey. 

/$  3.  The  method  of  extirpation,  applied  to  the  motor  area  of  the 
anthropoid  brain,  results  in  paralysis  of  the  entire  opposite  side, 
or  of  parts  of  this  side.  For  example,  extirpation  of  the  hand  area 
of  the  right  hemisphere  causes  an  immediate  and  severe  paralysis 
of  the  left  hand,  but  without  any  sign  of  paralysis  in  either  the  face 


Sulc.Central.     A™** 

Sulccattoso  X^r-,/         Sulc.srecencrmar0 

mar$ 
Sulc.parieto 


C.S.S.4& 


FIG.  102.— Mesial  Surface  of  the  Brain  of  a  Chimpanzee.     (Grttnbaum  and  Sherring- 
ton.)     The  left  hemisphere  is  shown,  with  the  frontal  lobe  to  the  right. 

or  the  leg;  and  extirpation  in  the  leg  area  causes  paralysis  of  part 
of  the  opposite  leg.  These  symptoms  are,  however,  as  is  usual 
in  such  extirpations,  recovered  from,  in  large  measure,  in  the  course 
of  a  few  weeks.  All  these  extirpations  are  in  the  precentral  gyre; 
when,  on  the  contrary,  part  of  the  postcentral  is  cut  out,  no  paraly- 
sis results. 

§  4.  An  uncritical  transfer  from  the  ape's  brain  to  the  human 
brain,  of  the  localizations  obtained  in  these  ways,  would  not  be  en- 
tirely justified,  in  spite  of  the  great  similarity  of  the  two,  both  as  re- 
spects their  external  form,  and  also  the  course  of  fibre-tracts  with- 
in them.  But,  as  previously  stated,  it  sometimes  happens  that  sur- 
geons, in  operating  on  the  brain,  have  occasion  to  use  the  electric 
stimulus  for  the  purpose  of  orientation.  Previous  to  these  results 
of  Griinbaum  and  Sherrington,  observations  of  this  sort  had  not 


PARALYSES  OF  THE  MOTOR  AREA 


239 


definitely  contradicted  the  older  localization,  which  placed  the  motor 
area  on  both  sides  of  the  central  fissure.  But  since  the  problem 
has  been  more  sharply  defined,  and  their  attention  more  definitely 
directed,  by  the  recent  work  on  the 
brains  of  anthropoid  apes,  oper- 
ating surgeons  have  found  that  the 
excitable  region  in  the  human  cor- 
tex also  lies  in  the  precentral,  and 
not  the  postcentral,  gyre.  The  ar- 
rangement of  special  centres  within 
this  general  region  of  the  human 
brain  is  substantially  the  same  as 

^.^that  in  the  anthropoid  brain — name- 
ly, the  leg  at  the  top,  the  arm  half- 
way down,  and  the  face  at  the  bottom 

— - of  the  precentral  gyre. 

The  evidence  from  paralyses  re- 
sulting from  disease  or  injury  of 
this  region  in  man  is  also  in  general 
agreement  with  the  same  system  of 
localization.  The  anatomical  meth- 
od lends  further  support  to  the  same 
conclusion;  for  it  is  after  lesion  of 
the  precentral  gyre  that  the  pyram- 
idal or  cortico-spinal  tracts  de- 
generate. And  since  there  is  no 
doubt  that  these  tracts  are  the  prin- 
cipal motor  path  (Fig.  103)  of  con- 
nection between  the  cortex  and  the 
cord,  their  origin  from  the  precen- 
tral gyre  is  strong  evidence  that  this 
gyre  is  the  true  "motor  area."  A 
specially  favorable  opportunity  for 
examining  this  question  is  afforded 
by  cases  of  the  disease  known  as 
"amyotrophic  lateral  sclerosis." 

This  disease  produces  at  the  same    Flo.  103._Diagrara  of  the  Motor 
time  a  gradual  atrophy  or  the  mus-  way  from  the  cortex. 

cles,  and  also  progressive  degenera- 
tion of  those  portions  of  the  nervous  system  which  are  connected 
with  the  muscles ;  namely,  of  the  cells  of  the  ventral  horn  of  the  cord, 
and  of  the  cortico-spinal  tract  throughout  its  whole  extent.  Ac- 
cordingly the  origin  of  these  motor  tracts  in  the  cortex  ought  to  be 
similarly  affected;  and  recent  careful  examination  by  several  au- 


240  THE  CEREBRAL  HEMISPHERES 

thorities1  has  shown  that,  in  fact,  the  cortex  of  the  precentral  gyre 
shows  profound  pathological  changes  in  cases  of  this  disease.  The 
"giant  cells/7  which  are  characteristic  of  this  gyre,  are  very  much 
/  reduced  in  number,  and  the  other  cells  are  also  affected.  The  large 
fibres  which  issue  from  the  cortex  and  which  pass — many  of  them 
at  least — downward  to  form  the  pyramidal  tracts  have  largely 
disappeared;  but  no  corresponding  changes  are  found  in  the  post- 
central  gyre.  The  bottom  of  the  central  fissure  is,  by  this  method, 
indicated  as  being,  in  the  human  brain,  the  hinder  boundary  of 
the  motor  region;  and  although  the  forward  boundary  is  less  sharply 
defined,  the  changes  are  for  the  most  part  confined  to  the  precentral 
gyre.  Still  more  convincing  is  the  experimental  study  by  Holmes 
and  May,2  who,  after  severing  the  pyramidal  tract,  in  different  ani- 
mals, located  the  exact  origin  of  this  tract  by  aid  of  the  chroma- 
tolysis  which  occurs  in  cells  after  their  axons  have  been  cut.  This 
symptom  was  limited  to  the  precentral  gyre  (sometimes  extending 
slightly  into  the  superior  and  middle  frontal  gyres),  and  it  seemed 
also  to  be  limited  to  the  giant  pyramidal  cells  in  the  inner  pyram- 
idal layer.  The  authors  conclude  that  "  The  cortico-spinal  fibres 
arise  only  from  the  giant  pyramidal  cells  .  .  .  and  these  cells 
probably  give  origin  only  to  cortico-spinal  fibres." 

§  5.  To  these  lines  of  evidence  may  be  added  the  fact  that  the 
area  thus  marked  out  as  motor  is  characterized  by  a  peculiar  struct- 
ure; and  that  the  limits  of  this  structure  are  nearly  the  same  as 
those  indicated  by  the  other  methods  of  localization.  All  in  all, 
then,  there  can  be  little  further  hesitation  in  accepting  the  locali- 
zation of  the  motor  area  in  the  anthropoid  brain,  as  valid  also  for 
the  human  brain. 

§  6.  What,  exactly,  is  the  function  of  this  so-called  "motor" 
area  ?  This  question  is  not  easy  to  answer.  From  the  fact  that 
it  is  the  origin  of  the  principal  path  of  conduction  from  the  cortex 
to  the  motor  nuclei  in  the  cord  and  brain-stem,  the  inference  is 
clearly  valid,  that  the  control  of  the  cortex  over  the  bodily  move- 
ments is  largely  exerted  through  this  area.  It  may,  therefore,  be 
regarded  as  a  collecting  centre  for  impulses  from  various  parts  of 
the  cortex — the  impulses  thus  collected  giving  rise  to  discharges 
down  the  cortico-spinal  tracts  and  so  to  muscular  contractions 
and  relaxations.  Since,  however,  there  are  no  direct  fibres  from 
the  motor  area,  or  from  any  part  of  the  cortex,  to  the  muscles,  but 
only  fibres  running  to  the  lower  motor  nuclei,  it  would  be  more  proper 

1  Among  others,  see  Campbell,  Histological  Studies  on  the  Localization  of  Cere- 
bral Function,  pp.  38  ff.  (Cambridge,  1905);    Schroder,  Journal  fur  Psycholo- 
gie  und  Neurologic,  1910,  XVI,  60-78;  Janssens,  ibid.,  1910,  XV,  245-256. 

2  Brain,  1909,  XXXII,  1. 


DEFINITION  OF  THE  MOTOR  AREA  241 

to  speak  of  the  motor  area  as  controlling  the  lower  nuclei,  than  to 
speak  of  it  as  controlling  the  muscles.  The  movements  obtained 
by  stimulating  the  motor  area  are  co-ordinated  movements,  in 
much  the  same  way  as  reflexes  are  co-ordinated.  That  is  to  say: 

;  Neither  isolated  contractions  of  single  muscles,  nor  general  contrac- 
tions of  all  the  muscles  in  a  limb,  are  usually  obtained  on  exciting 
the  motor  area.  The  movements  are  such  as,  for  example,  flex- 
ions and  extensions  of  the  limbs,  clenching  or  opening  the  fist, 
pricking  up  the  ear,  mastication,  turning  both  eyes  to  the  side, 
etc.  In  the  higher  apes,  movements  of  separate  fingers  can  be  ob- 
tained ;  but  even  these  are  to  be  regarded  as  co-ordinated  movements. 
A  clear  indication  of  the  co-ordinated  nature  of  these  movements 
is  the  fact,  first  discovered  by  H.  E.  Hering  and  Sherrington,1  that 
the  contraction  of  a  group  of  muscles,  when  aroused  by  excitation 
of  its  appropriate  cortical  area,  is  attended  by  relaxation  of  the  an- 
tagonistic muscles.  This  is  the  same  result  as  appears  in  reflex 
action  (see  p.  162).  It  is  quite  likely,  then,  that  the  same  "central 
cells  "  of  the  cord,  which  were  conceived  of  as  the  mechanisms  con- 
trolling the  co-ordination  of  reflexes,  are  excited  by  the  fibres  of  the 
cortico-spinal  tract.  In  this  case,  the  function  of  the  motor  area 
may  be  said  to  be,  the  control  of  spinal  co-ordinating  mechanisms, 
in  accordance  with  impulses  reaching  the  motor  area  from  various 
parts  of  the  cortex.  Accordingly,  we  can  neither  regard  the  motor 
area  as  standing  in  direct  relation  with  the  muscles,  nor  as  the  orig- 
inal starting-point  of  cortical  -  influence  on  the  muscles.  But  it  is 
more  properly  thought  of  as  an  intermediary  between  the  cortex 
in  general  and  the  co-ordinating  mechanisms  of  the  cord  and  brain- 
stem  (compare  p.  94). 

§  7.  The  foregoing  may  be  taken  as  a  fairly  safe  and  even  ob- 
vious induction.  But  there  is  still  considerable  difficulty  in  reach- 
ing a  satisfactory  conception  of  the  precise  function  of  the  motor 
area.  The  difficulty  arises  chiefly  from  the  facts  of  that  restitu- 
tion of  function,  which  so  often  follows  removal  or  disease  of  this 
region,  or  of  parts  of  it.  In  connection  with  the  results  of  extir- 
pating limited  portions  of  the  motor  area  of  the  anthropoid  brain, 
it  was  mentioned  above  that  the  paralysis  of  the  arm  or  leg,  which  is 
the  immediate  result,  usually  disappears  in  the  course  of  a  few  weeks. 
The  same  result  has  often  been  noted  in  man ;  for,  in  certain  cases 
of  irritation  of  small  portions  of  the  motor  area,  resulting  in  epilep- 
tiform  convulsions,  it  has  long  been  found  practicable  to  cure  the 
convulsions  by  cutting  out  the  irritated  portion;  and,  though  this 
gives  rise  to  temporary  paralysis  of  some  group  of  muscles,  the 

lPfluger's  Archiv  f.  d.  gesammte  PhysioL,  1897,  LXVIII,  222;  Journ.  of  PhysioL, 
1899,  23  Suppl. 


242  THE  CEREBRAL  HEMISPHERES 

paralysis  is  recovered  from,  at  least  by  young  persons.  When  the 
part  destroyed  by  experiment  on  the  animals,  or  by  disease  or  acci- 
dent in  man,  is  more  extensive,  the  recovery  of  function  is  less  com- 
plete. Locomotion  is  restored,  at  least  to  a  considerable  degree; 
but  in  man's  case,  after  extensive  destruction  of  the  motor  area  in 
both  hemispheres,  the  gait  remains  insecure  and  subject  to  spasm. 
The  more  specialized  movements  are  less  completely  restored.  A 
dog,  deprived  of  the  motor  region  on  both  sides,  is  at  first  pretty 
completely  paralyzed,  except  for  reflexes  and  such  vital  movements 
as  breathing.  Soon,  however,  he  recovers  locomotion;  but  he  is 
stated  (Munk,  Monakow)  never  to  recover  the  use  of  the  forepaw 
as  a  hand  for  holding  a  bone,  etc. 

Now  since  restitution  of  function  is  not  the  result  of  restitution 
of  the  injured  parts  of  the  brain  by  new  growth,  it  is  a  puzzling 
phenomenon.  It  has  sometimes  been  explained  as  due  to  the  taking 
up  of  the  function  of  the  destroyed  part  by  other  parts  ("vicarious 
function").  Such  an  explanation  seems  improbable,  since  it  would" 
call  for  the  growth  of  new  projection  fibres,  a  growth  which  prob- 
ably does  not  occur;  and  since,  moreover,  it  calls  for  too  much  fresh 
learning  of  complicated  functions.  Another  explanation  is  to  the 
effect  that  other  parts  of  the  cortex,  besides  the  part  which  gives 
rise  on  excitation  to  a  given  movement,  are  also  connected  with  that 
movement,  though  in  a  minor  degree.  They  may,  therefore,  be 
considered  as  auxiliary  centres,  unused  to  taking  full  charge  of  a 
movement,  yet  always  employed  in  conjunction  with  the  principal 
motor  centres.  After  destruction  of  the  principal  centre,  these 
auxiliary  centres,  continuing  to  act,  might  in  time  come  to  exercise 
an  efficient  control.  Such  auxiliary  centres  should  be  looked  for 
in  the  immediate  neighborhood  of  the  principal  focus;  or  in  the  cor- 
responding area  of  the  other  hemisphere.  Thus  the  representation 
of  a  given  movement  in  the  cortex  might  be  centred  at  a  given  point 
or  small  area,  but  still  spread  somewhat  over  neighboring  areas. 
There  is  indeed  some  evidence,  anatomical  as  well  as  physiological, 
that  the  motor  area  of  the  left  hemisphere,  though  principally  con- 
nected with  the  right  side  of  the  cord  and  of  the  body,  is  connected 
to  a  much  slighter  degree  with  its  own  side.  It  may  even  be,  as 
Von  Monakow  is  inclined  to  believe,1  that  there  are  "motor"  spots,, 
scattered  generally  over  the  cortex,  and  connected  by  projection  j 
fibres,  perhaps  with  the  thalamus  and  mid-brain,  and  so,  through  \ 
the  thalamo-spinal,  tecto-spinal,  and  rubro-spinal  tracts,  with  the  v 
spinal  cord.  There  do  appear  to  be  projection  fibres  issuing  from 
various  parts  of  the  cortex;  but  hitherto  the  existence  of  scattered 
motor  spots  has  not  been  demonstrated. 

1  Ergebnisse  der  Physiologie,  1902,  I,  part  2,  p.  611. 


FUNCTION  OF  THE  MOTOR  AREA  243 

Another  conception  of  Von  Monakow1  seems  more  worthy  of  at- 
tention. It  is  a  mistake,  he  urges,  to  consider  the  functions  which 
are  lost  immediately  after  the  destruction  of  a  given  area  of  the  cor- 
tex as  exclusively  appertaining  to  that  area.  The  area  in  question 
was  connected  by  nerve-fibres  with  other  parts  of  the  cortex  and  with 
subcortical  centres;  and  the  destruction  of  the  area  cuts  all  these 
fibres,  abolishes  their  functions,  and  induces  pathological  changes 
in  them.  It  thus  interferes  with  the  function  of  the  various  parts 
to  which,  or  from  which,  they  lead.  These  connected  centres  are 
thus  left  in  a  subnormal  condition;  but  although  they  might  not  dis- 
charge their  functions  at  once,  they  might  come  in  time,  since  they 
are  not  destroyed,  to  function  again  in  a  normal  manner,  or  at  least 
in  a  manner  approaching  the  normal. 

Still  another  possible  explanation  of  restitution  of  function  fol- 
lowing destruction  of  the  motor  area  holds  that  the  lower  centres, 
in  the  brain-stem  and  cord,  are  responsible  for  the  movements  after- 
ward executed.  This  is  likely  to  be  correct  to  a  certain  extent — 
just  as  the  subcortical  centres  are  responsible,  in  normal  condi- 
tions, for  much  of  the  co-ordination  and  efficiency  of  voluntary 
movements.  But  it  can  hardly  be  held  that  the  cortex  has  noth- 
ing to  do  with  movements  after  restitution;  because  the  movements 
occur  with  some  regard  to  volition  and  to  other  mental  influences. 

Some  of  the  most  suggestive  and  puzzling  cases  of  the  restitu- 
tion of  function  in  a  manner  to  suggest  a  certain  amount  of  substi- 
tution of  one  cortical  area  for  another,  have  recently  occurred  in 
connection  with  the  surgical  practice  of  nerve-anastomosis.  It  has 
been  found,  for  example,  that  where  the  facial  nerve  had  been  com- 
pletely severed  and  all  its  motor  and  sensory  functions  quite  de- 
stroyed, a  partial,  and  in  extremely  favorable  instances,  an  almost 
entire  recovery  of  functions  could  be  obtained  by  uniting  the  pe- 
ripheral end  of  the  injured  facial  nerve  with  that  portion  of  the  ac- 
cessory nerve  which  supplies  the  trapezius  muscle.  Now  the  cere- 
bral areas  which  control  these  two  nerves  are  not  far  distant  in 
space,  but  in  their  normal  functions  are  largely  different.  It  would 
seem,  therefore,  that  such  cases  must  depend  for  their  physiological 
explanation  on  the  spread  of  the  impulse  originating  in  the  cortical 
areas  over  their  customary  limits,  when  the  demand  for  this  is  made 
by  abnormal  conditions.  And  on  the  other  hand,  there  can  be  no 
doubt  about  their  emphasizing  the  cortical  factors  in  all  cases  of 
recovered  functions,  since  in  these  cases,  imagination  and  will  were 
shown  to  be  about  the  most  influential  forms  of  excitement,  in  the 
interests  of  a  restoration  of  function.2 

1  Op.  cit.,  p.  569,  and  passim. 

2  See  a  discussion  of  some  of  these  cases:  Ladd,  "A  Suggestive  Case  of  Nerve- 
Anastomosis,"  The  Popular  Science  Monthly,  August,  1905. 


244  THE  CEREBRAL  HEMISPHERES 


§  8.  Restitution  of  function  has  to  be  reckoned  with  in  consid- 
ering the  localization  of  other  functions  besides  the  motor,  and  even 
to  a  greater  degree;  for  the  localization  of  the  sensory  functions  is 
dependent  more  largely  on  the  method  of  extirpation  and  on  the 
corresponding  pathological  method.  The  method  of  excitation 
is  not  applicable  for  purposes  of  localization  in  most  parts  of  the 
cortex,  since,  with  the  exception  of  the  motor  area  and  of  a  few 
other  regions  which  will  be  mentioned  in  due  course,  the  rest  of  the 
cortex  is  " silent"  under  excitation;  i.  e.,  does  not  give  rise  to  move- 

Central  fissure          Su-pramargindl  gyre 

Angular  gyre 


Fissure  of  Sylvius 

FIG.  104.— The  Somesthetic  Area. 

ments.  It  has  only  recently  been  found  that  the  weak  electric 
current  can  be  safely  applied  to  an  exposed  surface  of  the  human 
cortex,  in  surgical  cases,  without  the  use  of  an  anaesthetic;  pain  does 
not  result.  It  will  therefore  be  possible,  in  the  future,  as  it  has  not 
been  in  the  past,  to  explore,  little  by  little,  as  occasion  offers,  the 
surface  of  the  brain  with  the  exciting  current,  in  conscious  human 
beings.  In  this  way,  direct  and  convincing  testimony  can  be  ob- 
tained as  to  the  sensations  or  ideas,  if  any,  aroused  by  the  excitation 
of  definite  cortical  areas. 

§  9.  Besides  the  motor  area,  there  have  been  found  areas  which 
are  closely  related  to  several  of  the  senses.  We  pass,  then,  to  the 
consideration  of  the  "sensory  areas "  (Fig.  104). 

The  history  of  the  localization  of  the  so-called  "somesthetic 
area,"  or  area  for  the  sense  of  "touch" — which,  in  the  older  mean- 
ing of  the  term,  included  the  cutaneous  senses  and  the  muscular 
sense — is  of  interest.  It  was  often  observed  that  injuries  to  the 
motor  area,  in  both  animals  and  man,  were  accompanied  by  loss  of 


THE  SOMESTHETIC  AREA  245 

sensation  as  well  as  by  motor  paralysis.  If,  for  example,  the  injury 
affected  the  upper  part  of  the  central  gyres,  on  the  left  side,  paraly- 
sis, and  also,  in  many  cases,  loss  of  sensation  in  the  right  leg,  would 
result.  This  led  to  the  doctrine  that  the  sensory  and  motor  centres 
are  the  same;  and  many  authorities  preferred  to  call  the  cortical 
area  involved  sensory  rather  than  motor.  But  there  were,  also, 
many  negative  cases — i.  e.,  cases  in  which  paralysis  occurred  without 
loss  of  sensation,  or  in  which  loss  of  sensation  occurred  without  paral- 
ysis. Such  facts  as  these  seemed  to  disprove  the  doctrine  that  the 
sensory  and  motor  centres  are  coincident.  The  discovery  that  the 
motor  area,  instead  of  occupying  both  sides  of  the  central  fissure,  is  * 
confined  to  the  frontal  side,  made  necessary  a  complete  revision  of 
all  the  older  observations.  It  was  no  longer  possible  simply  to 
describe  an  injury  as  lying  in  the  central  gyres,  without  more  minute 
specification.  Critical  review  of  the  older  cases,  and  especially  the 
more  attentive  observation  of  recent  cases,  have  seemed  to  show  that 
injury,  when  strictly  confined  to  the  precentral  gyre,  is  not  attended 
by  loss  of  sensation;  the  precentral  gyre,  or  motor  area,  can  there- 
fore be  excluded  from  the  somesthetic  area.  But  destruction  of 
the  region  immediately  behind  the  central  fissure  is  attended  by 
loss  of  sensation.  Accordingly,  the  central  fissure  seems  to  be  the 
anterior  boundary  of  the  somesthetic  area;  although  its  posterior 
limits  cannot  yet  be  distinctly  assigned.  Some  authorities1  are  in- 
clined to  include  within  it,  in  addition  to  the  postcentral  gyre,  the 
front  part  of  the  superior  and  inferior  parietal;  while  others2  would 
limit  it  to  the  postcentral,  or  even  to  the  anterior  part  of  that  gyre. 
Within  this  somesthetic  area,  the  representation  of  the  different 
parts  of  the  body  seems  to  have  about  the  same  arrangement  as  in 
the  adjoining  motor  area:  namely,  the  leg  area  at  the  top;  the  arm 
area  half-way  down;  and  the  head  area  at  the  bottom.  In  this 
manner  the  sensory  and  the  motor  areas  for  a  given  member  are 
brought  into  close  proximity  to  each  other,  but  on  opposite  sides 
of  the  central  fissure. 

§  10.  The  losses  of  sensation  which  attend  destruction  of  the 
somesthetic  area  do  not  affect  all  varieties  of  sensation  equally.  The 
pain  sense  is  little  or  not  at  all  affected,  except  temporarily;  the  sense 
of  pressure  and  contact  is  considerably  more  diminished;  the  tem- 
perature sense  is  so  much  reduced  that  only  extremes  of  heat  and 
cold  are  perceived;  the  muscular  sense  is  almost  entirely  destroyed; 
and  the  perception  of  form,  size,  location,  etc.,  by  use  of  the  hand 
is  usually  abolished.  These  are  the  permanent  results  of  extensive 

1  Von  Monakow,  Ergebnisse  der  Physiologic,  1902,  I,  part  2,  pp.  621-641. 

2  Campbell,  Histological  Studies  in  the  Localization  of  Cerebral  Function,  1905, 
pp.  97  ff. 


246  THE  CEREBRAL  HEMISPHERES 

destruction  involving  the  postcentral  and  front  part  of  the  parietal 
regions.1  This  would  seem  to  indicate  that  the  ability  to  make 
fine  distinctions  in  sensations  and  to  recognize  their  "intellectual" 
.qualities  is  most  easily  lost. 

§  11.  To  the  pathological  and  experimental  evidence  just  men- 
tioned may  be  added  the  evidence  of  an  anatomical  nature  in  favor 
of  locating  the  somesthetic  area  just  behind  the  central  fissure. 
/"The  primary  receiving  station   for   the  body  senses  must  be  that 
I   region  to  which  the  sensory  tracts  from  the  cord  and  brain-stem 
>NeaiL   In  a  previous  chapter  (compare  pp.  81, 89  f.),  these  tracts  were 
^traced  through  the  "fillet"  to  the  thalamus;  they  run  to  a  particular 
part  of  the  thalamus,  namely,  the  ventral  nucleus,  where  the  fibres 
from  the  cord  and  bulb  terminate.     Prom  this  part  of  the  thalamus 
issue  fresh  fibres,  which  pass  to  the  cortex;  and  these  have  been 
traced2  by  the  degeneration  method  to  the  region  behind  the  cen- 
tral fissure.     The  myelinization  method  has  led  to  much  the  same 
result.3     This  part,  therefore,  may  with  some  probability  be  ac- 
cepted as  the  cortical  termination  of  the  sensory  path  from  the  skin 
and  muscles.     On  the  whole,  it  seems  a  highly  reasonable  conject- 
ure that  the  function  of  this  somestheJJQ.  area  is  that  of  receiving 
sensory  impulses,  and  of  distributing  them,  by  association  fibres, 
to  neighboring  and  distant  parts  of  the  cortex.     Accordingly,  if 
the  postcentral  gyre,  close  to  the  fissure,  is  the  receiving  station,  the 
posterior  part  of  this  gyre,  and  the  adjoining  margin  of  the  parietal 
convolutions,  may  be  held  to  be  concerned  with  the  combination, 
"elaboration,"  perception,  interpretation,  etc.,  of  somesthetic  data. 
§  12.  The  visual  area  was  first  located  with  some  exactness  by 
I  Munk.4     He  found  that  removal  of  portions  of  the  occipital  lobe] 
I  was  followed,  in  dogs  and  monkeys,  by  disturbances  of  vision;  and 
insisted  that  destructions  of  other  portions  of  the  cortex  were  not 
necessarily  followed  by  visual  loss.     Though  this  view  was  for  a 
time  subject  to  much  doubt  and  contradiction,  the  accumulation 
of  experimental  and  pathological  data,  and  especially  of  anatomi- 
cal facts,  has  shown  that  Munk's  localization  is  correct.     Ferrier, 
indeed,  located  the  visual  area  further  forward,  in  the  angular  gyre; 
but  this  is  now  agreed  to  have  been  an  error,  due  to  the  fact  that  the 
"optic  radiation" — i.  e.,  the  fibres  leading  from  the  external  genicu- 

1  Von  Monakow,  op.  cit.,  pp.  629-630. 

2  Von  Monakow,  op.  cit.,  p.  636. 

3  The  myelinization  method  has  not  yet  excluded  the  precentral  gyre  from  be- 
ing part  of  the  receiving  station;  and  the  inner  structure  of  the  precentral  is  in 
some  important  respects  like  that  of  known  sensory  centres.     There  is  still  a 
chance  that  some  sensory  fibres  lead  directly  to  the  motor  area. 

4  Ueber  d.  Functionen  d.  Grosshirnrinde  (Berlin,  1881;  2d  ed.,  1890). 


THE  VISUAL  AREA  247 

latum  to  the  occipital  lobe,  and  constituting  the  continuation  of 
the  optic  nerves — lies  close  beneath  the  cortex  of  the  angular  gyre, 
and  thus  is  likely  to  be  unintentionally  affected  by  either  stimulation 
or  extirpation  of  this  region. 

The  particular  sort  of  visual  disturbance  resulting  from  injury 
or  disease  within  the  occipital  lobe  varies  greatly  in  different  cases. 
In  some  cases  it  is  described  as  "psychic  blindness,"  under  which 
head  may  be  included  several  varieties,  such  as  inability  to  recog- 
nize objects,  or  to  read,  or  to  perceive  colors,  or  to  utilize  vision  for 


Parielo-occipital 
fissure 


Cuneus 

Calcarine 
fissure 

Lingual 
gyre 


FIG.  105.— The  Visual  Area  in  the  Occipital  Lobe. 

purposes  of  orientation.  All  of  these  symptoms  may  occur  with- 
out blindness  in  the  strict  sense;  in  such  cases,  the  patient  can  still 
see,  but  has  lost  the  uses  of  sight,  or  some  of  them.  In  another  class 
of  cases,  there  is  complete  blindness  for  a  certain  part  of  the  field 
of  view.  If  the  occipital  lobe  is  destroyed  in  the  right  hemisphere, 
for  instance,  the  patient  cannot  see  what  lies  to  his  left,  or,  more 
precisely,  what  lies  to  the  left  of  the  vertical  meridian  of  his  eyes. 
It  is  as  if  the  right  half  of  each  retina  were  blinded.  This  form  of 
blindness  is  called  "hemianopsia."  The  peculiar  distribution  of 
such  blindness  is  well  explained  by  the  semi-decussation  of  the  optic 
nerves  at  the  chiasm  (compare  pp.  91  f.).  In  hemianopsia,  however, 
central  or  foveal  vision  is  not  destroyed  in  either  eye,  which  leads  to 
the  supposition  that  the  fibres  from  the  fovea  of  each  eye  are  dis- 
tributed to  the  occipital  lobes  of  both  hemispheres.  In  hemian- 


248  THE  CEREBRAL  HEMISPHERES 

opsia,  blindness  for  the  affected  half  of  the  field  of  vision  is  com- 
plete; all  that  is  left,  in  man's  case,  is  the  pupillary  reflex. 

§  13.  Clearly,  hemianopsia  is  a  more  radical  form  of  blindness 
than  is  "psychical  blindness."  It  must,  therefore,  be  considered 
as  the  sign  of  destruction  of  the  primary  receiving  station  for  nerve 
impulses  from  the  eye,  or  of  the  fibres  leading  from  the  eye  to  that 
receiving  station.  The  localization  of  this  primary  visual  area  is 
therefore  the  first  step  required  toward  a  more  precise  localization  of 
visual  functions  within  the  wider  limits  of  the  occipital  lobe.  To 
Henschen1  is  due  the  credit  for  a  sharper  localization  of  this  pri- 
mary visual  area.  On  the  basis  of  clinical  cases,  he  located  it  in  a 
relatively  small  portion  of  the  occipital  lobe,  a  portion  which  lies, 
in  man,  almost  entirely  on  the  mesial  surface,  and  in  the  immediate 
neighborhood  of  the  calcarine  fissure.  Later  observers  have  on 
the  whole  confirmed  Henschen's  conclusion  that  hemianopsia, 
when  it  is  due  strictly  to  injury  of  the  cortex,  is  dependent  on  lesion 
of  the  calcarine  region  (Fig.  105). 

Anatomical  evidence  of  convincing  character  is  available  in  con- 
firmation of  this  localization  It  will  be  recalled  (see  p.  92)  that 
the  optic  nerves,  after  their  semi-decussation,  terminate  in  thein- 
terbrain,  and  for  the  most  part  in  the  external  geniculate  bodies. 
The  fibres  which  arise  from  the  geniculatum  have  been  traced  by 
Flechsig,  Vogt,  and  others,  by  the  method  of  myelinization;  they 
are  found  to  pass  back,  underneath  the  angular  gyre  and  the  lateral 
portion  of  the  occipital  lobe,  and  to  terminate  in  the  lips  and  im- 
mediate neighborhood  of  the  calcarine  fissure.  Other  methods  of 
tracing  fibres  have  led  to  concordant  results.  The  facts,  however, 
that  some  few  fibres  pass  from  the  lower  optic  centres  to  other  parts 
of  the  occipital  lobe,  and  that  central  vision  is  often  preserved  after 
the  destruction  of  the  calcarine  region,  would  seem  to  favor  the  view 
of  Von  Monakow.2  He  holds  that  central  or  foveal  vision  has  no 
sharply  defined  cortical  centre. 

§  14.  Within  the  calcarine  region,  some  degree  of  more  minute 
localization  is  indicated  by  the  observations  of  Henschen  and  others. 
Apparently  the  posterior  part  of  the  primary  visual  area  is  connected 
with  the  lower  part  of  the  retinas,  and  the  anterior  part  of  the  vis- 
ual area  with  the  upper  part  of  the  retinas.  It  would  appear  likely 
that  the  retinas  are  projected,  point  for  point,  though  perhaps  not 
quite  so  minutely  as  this,  upon  the  visual  cortex.  The  projection 
of  corresponding  points  of  the  two  retinas  must  be  superimposed 
in  the  visual  areas,  the  right  half  of  each  retina  being  projected  on 

1  Klinische  und  anatomische  Beitrdge  zur  Pathologic  des  Gehirns  (Upsala,  1890- 
1892);  "On  the  Visual  Path  and  Centre,"  Brain,  1893,  p.  170. 
3  Ergebnisse  der  Physiologic,  1902,  I,  part  2,  pp.  653,  661. 


PHENOMENA  OF  PSYCHICAL  BLINDNESS          249 

the  right  visual  cortex,  and  the  left  half  of  each  retina  on  the  left 
visual  area. 

^^plxcitation  of  the  occipital  lobe  gives  rise  to  movements  of  the 
»eyes,  and  Schafer1  found  that  different  movements  were  obtained 
by  exciting  different  portions  of  this  lobe;  for  example,  excitation 
of  the  posterior  part  gave  an  upward  movement  of  both  eyes.  This 
result  is  harmonious  with  that  of  Henschen  just  stated,  since  an 
upward  movement  of  the  eyes  is  what  occurs  when  a  bright  light 
from  above  suddenly  shines  into  the  eye,  striking  the  lower  part 

Central  fissure 


Superior 
temporal  gyre 

Middle 
temporal  gyre 

Inferior 
temporal  gyre 


Fissure  of  Sylvius 


FIG.  106. — The  Auditory  Area. 

of  the  retina.  It  is  probable  that  our  ordinary  movements  of  the 
eyes  in  looking  at  an  object,  i.  e.,  in  directing  the  centre  of  clear 
vision  upon  it,  are  reactions  through  the  visual  area,  and  not  through 
the  motor  area.  And  motor  fibres  leading  downward  from  the 
visual  area  toward  the  mid-brain  have  been  demonstrated  by  von 
Monakow. 

If  the  calcarine  region  is  the  receiving  station  for  optic  impulses, 
it  is  probable  that  neighboring  portions  of  the  occipital  lobes  are 
concerned  with  those  more  complicated  visual  functions,  the  loss 
of  which  is  betrayed  by  the  different  sorts  of  "psychical  blindness." 

§  15.  The  evidence  in  favor  of  an  auditory  centre  is  of  the  same 
kind  as  that  in  favor  of  the  visual  ce'ntre;  and  the  history  of  its  local- 
ization is  much  the  same.  There  has  been  general  agreement,  from 
an  early  period  of  the  study,  that  the  auditory  functions  were  spe- 

1  Brain,  1888,  XI,  1;  Textbook  of  Physiology,  1900,  vol.  II,  p.  755. 


250-  THE  CEREBRAL  HEMISPHERES 

cially  connected  with  the  temporal  lobe.  The  symptoms  which  re- 
sult from  lesions  here  cover  much  the  same  range  as  the  visual 
symptoms  which  attend  lesions  of  the  occipital  lobe.  They  are  in 
some  cases  of  the  "psychic"  variety,  such  as  "word-deafness"  » 
(inability  to  understand  spoken  words)  or  as  "amusia"  (inability 
to  apprehend  melodies);  or  such  as  loss  of  memory  for  words  or 
for  melodies.  In  other  cases,  there  is  genuine  deafness  in  the  oppo- 
site ear;  but  the  findings  here  seem  not  to  be  perfectly  clear,  per- 
haps because  of  an  incomplete  decussation  of  the  incoming  fibres 
from  the  organ  of  hearing.  Complete  deafness  is  not  caused  ex- 
cept by  destruction  of  both  temporal  regions. 

Given  this  general  localization  of  auditory  (Fig.  106)  functions  in 
the  'temporal  lobe,  the  efforts  of  investigators  were  devoted  to  subdi- 
viding the  area,  and  especially  to  finding  the  primary  receiving  sta- 
tion for  impulses  from  the  ear.  Clinical  evidence  tends  to  limit  the 
primary  auditory  area  to  the  upper  temporal  gyre,  and  to  the  rear 
two-thirds  of  this  gyre.  Here  again,  as  with  the  visual  centre,  the 
tracing  of  the  incoming  path  is  of  decisive  importance.  We  have 
already  seen  (pp.  90  f .)  that  the  path  from  the  cochlea,  the  organ  of 
hearing,  is  traced  up  to  the  inter-brain,  and  to  that  part  of  it  which 
is  called  the  internal  geniculatum.  By  the  myelinization  method 
(Flechsig,  Vogt),  the  fibres  arising  thence  are  seen  to  pass  to  the 
first  temporal  gyre;  and,  indeed,  to  a  limited  portion  of  it,  which  lies, 
in  man,  mostly  within  the  fissure  of  Sylvius.  This  is,  then,  the 
primary  auditory  area,  the  receiving  and  distributing  station  for 
impulses  coming  in  from  the  ear.  Neighboring  parts  of  the  temporal 
lobe,  and  perhaps  also  of  the  parietal  and  of  the  island,  are  "elab- 
orative,"  combinational,  or  perceptive  with  respect  to  the  audi- 
tory impulses.  Excitation  of  the  temporal  lobe,  in  animals,  gives 
rise  to  movements  of  the  ears,  and  to  turning  of  the  eyes  and  head 
to  the  opposite  side.  These  are,  in  appearance,  "listening"  move- 
ments, and  their  occurrence  indicates  that  the  primary  motor  ad- 
justment to  sound  occurs  through  the  auditory  area  rather  than 
through  the  motor  area. 

§  16.  Regarding  the  cortical  representation  of  the  sense  of  smell, 
we  have  already  mentioned  the  evidence  afforded  by  comparative 
anatomy  that  the  archipallium  is  concerned  with  this  and  allied 
senses.  The  fibres  from  the  olfactory  part  of  the  nose  pass  to  the 
olfactory  lobe;  and  fibres  can  be  traced  back  from  this  lobe  to  the 
pyriform  lobule,  which,  accordingly,  is  regarded  as  the  receiving 
station  for  olfactory  impulses.  The  pyriform  lobule  usually  shows 
a  high  development  in  animals  which  make  great  use  of  the  sense 
of  smell.  In  man,  it  is  comparatively  small.  There  is  not  much 
pathological  evidence  regarding  the  localization  of  smell  in  the 


C 


THE  SO-CALLED  "SILENT  AREAS"  251 

human  cortex,  and  the  fibre  connections  just  mentioned,  on  which 
the  localization  of  the  olfactory  area  is  based,  have  been  traced  in 
animals  not  closely  related  to  man.  But  what  pathological  evi- 
dence exists  points,  for  the  most  part,  to  the  pyriform  lobule  or  to 
its  neighborhood.  Excitation  of  this  general  region,  by  Ferrier, 
gave  movements  of  the  nostrils — an  adjustment  of  the  sense-organ 
analogous  to  those  which  are  got  by  exciting  the  visual  and  auditory 
areas. 

§  17.  Little  of  a  definite  nature  can  be  stated  regarding  the  corti- 
cal representation  of  the  sense  of  taste;  but  it  is  believed  to  lie  some- 
where within  the  archipallium  or  its  immediate  neighborhood. 

§  18.  On  reviewing  the  localizations  which  have  been  established, 
we  find  that  the  retina  is  connected  most  directly  with  the  calcarine 
region;  the  organ  of  hearing  with  that  portion  of  the  first  temporal 
which  lies  in  the  side  of  the  Sylvian  fissure;  the  organ  of  smell  with 
the  pyriform  lobule;  and  the  cutaneous  and  muscular  senses  with 
the  postcentral  gyre;  while  the  principal  motor  pathway  arises  from 
the  precentral  gyre.  Other  special  motor  pathways,  controlling  the 
adjustment  of  sense-organs,  arise  from  the  visual,  auditory,  and 
olfactory  areas.  In  addition,  there  is  an  area  in  the  frontal  lobe, 
the  connections  of  which  are  not  as  yet  well  made  out,  but  which, 
on  excitation,  gives  movements  of  the  eyes.  With  the  exception  of 
this  last  area,  the  receiving  and  departing  stations  are  of  limited 
extent;  they  occupy  a  comparatively  small  proportion  of  the  entire 
surface  of  the  cortex,  especially  in  the  higher  animals.  They  are 
relatively  smallest  in  the  human  brain.  But  this  relative  diminu- 
tion, as  we  ascend  the  scale  of  animals,  is  owing  to  increase  in  the 
intervening  areas,  which  are  not  so  directly  connected  with  the  sense- 
organs  or  with  the  muscles. 

What,  now,  are  the  functions  of  these  "silent"  areas?  It  is  easy 
to  call  them  "intellectual  centres";  but  if  such  a  term  is  used,  the 
implication  of  the  word  "intellectual"  must  be  made  very  broad. 
It  must  be  made  broad  enough,  for  example,  to  cover  the  mental 
performances  of  the  dog,  since  even  in  the  dog  there  is  much  "silent" 
space  left  between  the  primary  sensory  and  the  motor  areas.  Flech- 
sig  proposed  to  call  such  areas  "association  centres";  but — here 
again — if  this  term  is  used,  it  should,  for  the  present,  be  under- 
stood in  an  anatomical  rather  than  in  a  psychological  sense.  It 
should  have  reference,  namely,  to  the  association  fibres;  and  the 
idea  conveyed  should  be  that  the  association  centres  are  those  areas 
whose  connections  are  provided  by  these  fibres;  that  is  to  say,  their 
connection  with  the  organs  of  sense  and  of  movement  is  indirect. 
Even  this  cannot  be  stated  as  an  absolutely  valid  distinction;  for, 
though  the  principal  bundles  of  projection  fibres  are  traceable  to 


252  THE  CEREBRAL  HEMISPHERES 

the  sensory  and  motor  areas,  there  seems  to  be  a  scattered  distri- 
bution of  similar  fibres  over  a  large  portion  of  the  whole  cortex. 

§  19.  With  regard  to  the  specific  functions  of  other  parts  of  the 
cortex,  not  thus  far  discussed,  we  have  little  definite  information; 
and  yet  we  are  not  wholly  without  grounds  of  conjecture.  It  js 
obvious,  for  example,  that  the  areas  surrounding  the  calcarine 
region  are  necessary  for  the  proper  utilization  of  visual  data;  that 
portions  of  the  temporal  lobe,  near  the  primary  auditory  station, 
are  necessary  for  the  utilization  of  auditory  data;  and  that  parts  of 
the  parietal  lobe,  adjoining  the  somesthetic  area,  are  necessary  for 
the  utilization  of  cutaneous  and  muscular  sensation.  These  out- 
lying regions  are  richly  connected  by  short  association  fibres  with 
their  respective  sensory  distributing  stations.  Most  of  the  evi- 
dence for  these  statements  has  been  obtained  from  observation  of 
human  patients  suffering  from  brain  diseases,  but  some  evidence 
,_js  also  to  be  got  from  the  results  of  extirpation  in  animals.  Munk 
/  found  that  excising  small  portions  of  the  temporal  lobe,  in  dogs, 
resulted  not  in  deafness,  for  the  animals  would  still  prick  up  the 
ears  at  a  sound,  but  in  what  he  called  "psychic  deafness,"  as  shown 
by  the  loss  of  habitual  reactions  to  words  of  command,  etc.  Re- 
movals of  small  parts  of  the  occipital  region  were  followed,  similarly, 
not  by  blindness,  but  by  failure  to  recognize  familiar  objects,  such 
as  food,  or  the  master's  hand  extended  as  if  to  "shake  hands." 

Losses  of  function  which  follow  destructions  of  limited  portions 
of  the  human  cortex  have  been  grouped  under  a  considerable  num- 
ber of  descriptive  terms.  These  defects  seldom  occur  singly;  usu- 
ally a  number  of  them  occur  together;  yet  cases  of  isolated  loss  are 
not  unknown,  and  are  properly  regarded  as  specially  important, 
because  they  afford  the  best  opportunity  for  purposes  of  localiza- 
tion, and  for  analysis  of  complicated  mental  functions  into  compo- 
nent functions  of  less  complexity. 

§  20.  A  number  of  defects  have  been  distinguished,  which  result 
from  injury  of  the  occipital  lobe  or  of  the  adjacent  part  of  the  pari- 
etal lobe.  The  principal  ones  are  given  below. 

Alexia  is  inability  to  read,  occurring,  of  course,  in  a  person  who 
previously  could  read,  and  in  one  who  has  not  become  blind.  In 
such  cases,  the  printed  characters  are  seen  clearly  enough,  but  have 
lost  their  significance.  In  "pure"  cases,  the  individual  can  under- 
stand what  is  spoken,  and  may  even  be  able  to  write,  though  unable 
to  read  what  he  has  just  written. 

Asymbolia.  The  use  of  this  term  varies,  at  present,  between  a 
strict  definition,  covering  cases  in  which  numerals,  or  other  con- 
ventional signs,  are  no  longer  grasped,  and  a  wider  definition,  ac- 
cording to  which  it  includes  cases  of  "object  blindness."  In  the 


RESULTS  OF  INJURIES  TO  THE  LOBES  253 

latter  cases,  the  patient  is  unable  to  recognize  familiar  objects; 
the  visual  impression  is  no  longer  for  him  a  sign  of  the  object, 
though  he  may  know  the  object  at  once  on  handling  it. 

Achromatopsia.  Occasionally  there  occurs,  without  blindness, 
a  loss  of  color  vision  in  half  of  the  field  of  view. 

In  some  cases,  there  is,  without  blindness,  a  loss  of  the  power  to 
perceive  depth,  or  of  the  power  to  orientate  oneself  in  familiar  sur- 
roundings. In  other  cases,  there  is  deficiency  in  the  clearness  of 
central  vision;  and  in  still  others,  a  loss  of  visual  imagination.  Two 
or  more  of  these  various  defects  are  likely  to  be  combined  in  the 
same  individual;  but  there  is  no  necessary  connection  between  any 
two  of  them,  nor  between  any  one  of  them  and  hemianopsia.  Nor 
is  it  possible  as  yet  to  connect  any  of  them  with  destruction  of  any 
particular  limited  area;  but,  when  taken  together,  they  give  some 
notion  of  the  function  of  the  cortex  in  the  neighborhood  of  the  visual 
area. 

§  21.  Analogous  defects  are  met  in  certain  cases  of  lesion  of  the 
temporal  lobe  and  neighboring  parts  of  the  island  and  parietal 
lobe: 

Amusia  is  a  loss  of  ability  to  apprehend  melodies  and  other 
musical  complexes;  it  has  been  observed  in  musicians  afflicted  with 
brain  disease. 

Word-deafness  is  a  similar  inability  to  recognize  the  meaning  of 
spoken  words. 

Verbal  amnesia  and  paraphasia  belong  to  the  aphasias.  In  the 
former,  there  is  abnormal  difficulty  in  finding  the  right  word  to 
express  one's  meaning.  Resort  must  be  had  to  round-about  modes 
of  expression,  or  absurdly  general  terms,  such  as  "the  what-d' 
you-call-it."  In  paraphasia,  there  is  less  hesitation,  but  much  sub- 
stitution of  wrong  words,  or  of  jargon. 

§  22.  Injuries  to  the  parietal  lobe,  in  the  neighborhood  of  the 
postcentral  gyre,  are  often  attended  by 

Aster eognosis.  This  is  a  loss  of  the  "  stereognostic  sense,"  or, 
better,  of  the  ability  to  recognize  the  size,  shape,  and  consistency  of 
objects  by  handling  them.  Such  judgments  must  depend  on  util- 
izing a  combination  of  impressions  from  both  the  cutaneous  and 
the  muscular  senses. 

From  a  survey  of  these  defects,  and  of  the  location  of  the  lesions 
which  cause  them,  we  may  properly  reach  the  general  conclusion 
that  the  immediate  neighborhood  of  the  receiving  station  for  each 
sense  is  concerned  with  functions  which  are  closely  related  to  that 
sense. 

§  23.  Similar  defects  occur  on  the  side  of  movement,  and  pass 
by  the  general  name  of  apraxia,  of  which  several  forms  are  recog- 


254  THE  CEREBRAL  HEMISPHERES 

nized.  In  general,  "apraxia"  may  be  defined  as  a  loss  of  ability 
to  perform  learned  or  skilled  acts,  in  the  absence  of  paralysis,  or 
ataxia,  or  pronounced  sensory  or  perceptional  defect.  The  pa- 
tient knows  what  he  wishes  to  do,  but  cannot  make  his  hands  do 
it;  yet  his  hands  are  not  paralyzed,  and  may  be  able  to  perform  very 
simple  acts,  and  to  take  part  in  instinctive  movements,  such  as 
locomotion,  or  as  pointing  toward  an  object  at  which  he  is  looking. 
Often,  indeed,  there  is  paralysis  of  the  right  hand,  with  apraxia 
of  the  left  hand,  which  is  not  paralyzed.  Sometimes  the  apraxia 
is  associated,  not  with  motor  paralysis  of  one  hand,  but  with  object- 
blindness;  but  in  these  cases,  the  difficulty  consists  rather  in  not 
knowing  what  is  to  be  done,  than  in  inability  to  carry  out  an  in- 
tention. Sometimes,  also,  the  difficulty  lies  in  the  combination  of 
the  single  movements  which,  in  proper  sequence,  make  up  a  skilled 
act.  In  this  connection,  it  is  worth  noting  that  even  very  familiar 
and  apparently  simple  acts,  such  as  lighting  a  candle  or  picking  a 
flower,  are  really  composed  of  an  orderly  sequence  of  movements, 
and  therefore  need  to  be  learned,  in  childhood,  by  a  long  course  of 
training.  The  results  of  this  training  are  lost,  in  some  forms  of 
apraxia,  in  consequence  of  localized  disease  of  the  brain.  There 
are  other  cases  in  which  apraxia  is  associated  with  a  "general"  de- 
fect of  intellect. 

The  close  study  of  apraxia  is  only  of  recent  date,  and  has  been 
prosecuted  with  special  vigor  by  Liepmann1  and  Pick.2 

One  case,  narrated  by  Liepmann,  is  of  special  interest;  because 
the  patient,  while  free  from  paralysis  or  ataxia  on  either  side,  was 
apractic  on  the  right  side  only.  He  was  also  free  from  "general 
intellectual"  defect,  as  was  shown  by  his  ability  to  describe  the  act 
which  he  wished  to  perform,  and  by  his  performing  it  himself  with 
his  left  hand.  With  his  right  hand,  however,  he  could  not  success- 
fully perform  even  simple  learned  acts.  The  post-mortem  examina- 
tion of  the  brain  in  this  case  revealed  destructions  of  the  white  mat- 
ter in  such  situations  as  to  separate  the  two  central  gyres  of  the  left 
hemisphere  from  the  rest  of  that  hemisphere,  and  also,  through  in- 
jury of  the  callosum,  from  the  right  hemisphere.  Being  thus  iso- 
lated, the  motor  area  for  the  right  hand  was  beyond  the  influence  of 
other  parts  of  the  cortex  concerned  with  the  intention  and  the  proper 
ordering  of  separate  movements  into  a  skilled  act. 

Agraphia,  or  inability  to  write,  may  be  regarded  as  a  special 
form  of  apraxia.  It  seldom  occurs  alone,  but  usually  in  associa- 

1  H.  Liepmann,  Das  Krankheitsbild  bei  Apraxie  (Berlin,  1900);  Der  weitere 
KrarikheitsverlauJ  bei  dem  einseitig  Apraktischen  (Berlin,  1906);  Ueber  Storungen 
des  Handelns  bei  Gehirnkranken  (Berlin,  1905). 

3  A.  Pick,  Studien  -fiber  motorische  Apraxie  (Leipzig  and  Vienna,  1905). 


DISTURBANCES  OF  SPEECH  FUNCTIONS  255 

tion  with  some  form  of  aphasia.  In  the  few  recorded  cases  where  it 
has  occurred  alone,  examination  of  the  brain  has  revealed  destruc- 
tion of  the  cortex  in  the  middle  frontal  gyre,  just  forward  of  the  motor 
centre  for  the  arm  and  hand.  This  localization  is  disputed  by 
many  authorities,  and  is  to  be  accepted  only  tentatively. 

§  24.  No  class  of  learned  and  skilled  acts  is  more  highly  special- 
ized than  the  various  linguistic  performances,  such  as  using  articu- 
late language  in  speaking,  understanding  what  is  spoken,  reading 
and  writing.  No  losses  of  learned  acts  are,  therefore,  easier  to  de- 
tect than  are  those  of  language.  For  this  reason,  the  study  of 
aphasia,  or  loss  of  speech,  has  a  much  longer  history  than  has  the 
study  of  apraxia  or  of  asymbolia.  And  since  aphasia  is  a  result  of 
localized  brain  lesions,  it  has  been,  and  is  still,  one  of  the  chief  con- 
cerns of  the  student  of  cerebral  localization. 

For  about  a  decade,  however,  previous  to  the  discoveries  of 
Fritsch  and  Hitzig,  in  1870,  the  facts  which  seemed  definitely  to 
connect  the  loss  of  speech  with  a  certain  region  of  the  left  cerebral 
hemisphere  were  nearly  all  to  which  any  advocate  of  the  localiza- 
tion of  the  cerebral  function  could  confidently  appeal  in  behalf  of 
his  theory.  As  long  ago  as  1825,  Bouillaud  located  the  articulation 
of  words  in  the  frontal  lobes.  Subsequently  (1836)  M.  Dax  main- 
tained the  proposition  that  "  lesions  of  the  left  half  of  the  enceph- 
alon  are  coincident  with  forgetfulness  of  the  symbols  of  thought." 

In  treatises  of  the  years  1861-1865,  Broca  first  announced  the 
substantially  true  discovery  that  the  gyrus  frontalis  inferior  on  the 
left  side  of  the  cerebrum  is  especially  concerned  in  using  the  pow- 
er of  speech.  This  circumstance  he  connected  with  the  fact  that 
men  generally  use  the  left  hemisphere  more  than  the  right  for  the 
expression  of  thought  with  the  right  hand  and  arm,  whether  in 
writing  or  in  the  mechanical  arts.  The  literal  meaning  of  the 
statements  made  by  Broca — such  as  that  this  part  of  the  brain  is 
"the  seat  of  the  faculty  of  articulate  language"1 — is,  however,  not 
simply  inappropriate  to  the  facts;  it  is  even  absurd.  There  is  no 
one  "faculty"  of  language  which  can,  in  any  possible  meaning  of 
the  word,  be  regarded  as  having  its  "seat"  or  locality  confined  ^o 
some  particular  region  of  the  brain.  Speech  involves,  in  a  very: 
complicated  and  large  way,  all  the  faculties;  strictly  speaking,  then,\ 
it  cannot  be  located,  with  all  its  attendant  operations  of  self-con-  \ 
scious,  rational  mind,  in  any  one  cerebral  area.  But  that  the  J 
phenomena  of  aphasia  show  some  special  connection  of  certain 
cerebral  centres  with  the  complex  process  of  apprehending  and  ex- 

1  "Sur  le  siege  de  la  faculte*  du  langage  articule*,"  etc.,  Bull  de  la  Soc.  anat., 
August,  1861;  "Du  siege  de  la  faculte"  du  langage  articule  dans  rhe"misphere 
gauche  du  cerveau,"  Bull,  de  la  Soc.  d'anthropol,  June,  1865. 


256  THE  CEREBRAL  HEMISPHERES 

pressing  articulate  language,  seems  entitled  to  credit  as  an  induc- 
tion based  upon  a  wide  range  of  facts.  Of  course,  in  this  particu- 
lar attempt  at  localization  of  function,  no  real  help  can  be  derived 
from  experiments  upon  the  lower  animals. 

§  25.  The  phenomena  of  various  classes,  among  which  the  truly 
aphasic  cases  must  be  discriminated,  vary  all  the  way  from  those 
resembling  the  results  of  momentary  inattention — such  as  that  of 
the  German  professor  who  certified  in  writing,  "A.  B.  has  attended 
my  remarkable  lectures  in  chemistry  with  inorganic  assiduity" — to 
the  impairment  and  utter  loss  of  speech  in  progressive  paralysis 
with  dementia.  A  few  of  the  more  curious  and  instructive  in- 
stances furnish  facts  like  the  following:  The  aphasic  patient  may 
be  entirely  speechless,  and  yet  understand  what  is  said  to  him,  and 
be  able  to  write  his  wishes  down  on  paper.  Some  thus  afflicted  re- 
tain the  power  to  pronounce  words  of  one  syllable,  but  are  obliged 
to  resort  to  writing  in  order  to  communicate  anything  further.  Oth- 
ers possess  a  small  stock  of  words,  which  they  make  more  servicea- 
ble with  expressive  gestures.  Others,  still,  are  simply  able  to  speak 
"a  few  senseless,  and  often  very  extraordinary,  syllables  and  words." 

Among  the  surprising  phenomena  of  the  disease  of  aphasia, 
none  are  perhaps  more  so  than  those  occasioned  by  the  ability 
to  utter  certain  syllables  or  words,  when  accompanied  by  an  utter 
inability  to  put  the  same  letters  into  slightly  different  combination. 
One  patient,  who  could  say  "Bonjour,  monsieur,"  tolerably  well, 
could  not  pronounce  the  word  "bonbon"  at  all.  Another,  whose 
vocabulary  was  almost  entirely  limited  to  the  meaningless  sylla- 
bles, "cousisi,"  was  quite  unable  to  utter  either  "coucon"  or  "sisi." 
The  celebrated  case  of  the  aphasic  Le  Long,  reported  by  Broca, 
was  that  of  a  man  confined  to  five  words  for  his  entire  vocabulary. 
These  words  were,  "oui,  non,  tois,  instead  of  trois,  toujours,  and 
Le  Lo  instead  of  Le  Long."  The  first  two  and  the  last  were  used 
with  their  appropriate  meaning;  "tois"  indicated  all  ideas  of  num- 
ber whatever;  and  "toujours"  was  the  word  used  when  the  patient 
could  not  express  his  meaning  by  gestures  and  the  other  four 
words.  It  appears,  then,  that  Le  Long  could  pronounce  the 
r  in  "toujours,"  but  not  in  "trois,"  and  the  nasal  sound  in  "non," 
but  not  in  his  own  name.  In  another  class  of  cases,  the  aphasic 
person  can  utter  only  a  few  or  no  words  spontaneously  and  cor- 
rectly, but  can  repeat  and  write  without  difficulty  words  that  are 
spoken  before  him.  Such  inability  is  somtimes  called  "simple 
aphasia  of  recollection."  Different  classes  of  words,  as  a  rule,  slip 
from  the  memory  in  succession,  as  it  were.  Proper  names  are 
"most  frequently  forgotten;  then  substantives  generally,  and  some- 
times verbs,  adjectives,  pronouns,  and  all  other  parts  of  speech. 


DISTURBANCES  OF  SPEECH  FUNCTIONS 


257 


"The  more  concrete  the  idea/'  says  Kussmaul,1  "the  more  readily 
the  word  to  designate  it  is  forgotten,  when  the  memory  fails." 
Many  cases  of  disease  occur  where  the  patient  has  lost  the  power 
mentally  to  find  the  appropriate  words,  although  his  power  of  ar- 
ticulation is  unimpaired.  Such  disturbances  of  speech  may,  or  may 
not,  be  accompanied  by  a  corresponding  impairment  of  general  in- 
telligence. This  complication  increases  the  difficulty  of  studying 
the  phases  of  this  disease. 

Aphasia  may  also  be  accompanied  by  so-called  "word-deafness" 
and  "word-blindness."     Persons  thus  afflicted  hear  words  as  con- 

Central  fissure 


Middle 

frontal  gyre 
Precentral  gyre 


Inferior 

frontal  gyre 


Fissure  of  Sylvius 


FIG.  107. — Areas  of  the  Cortex  Supposed  to  be  Specially  Connected  with  Speech. 

fused  murmurings,  or  see  them  as  blurred  images.  The  individ- 
ual letters  may  be  intelligently  heard  or  read,  but  their  combina- 
tion has  become  unintelligible.  The  same  thing  sometimes  happens 
with  figures;  as  in  the  case  of  the  accountant  who  could  read  the 
sum  766,  figure  for  figure,  but  did  not  know  what  the  figure  7 
meant  as  placed  before  the  two  6's.  At  other  times  the  disturb- 
ance of  speech  takes  the  form  of  grammatical  ataxy,  as  it  were,  or 
of  verbal  delirium — a  medley  of  words,  partly  in  themselves  signifi- 
cant and  partly  unmeaning. 

§  26.  In  the  analysis  of  these  varied  defects  of  speech,  an  im- 
portant step  was  taken  by  Wernicke,2  who,  in  1874,  made  a  dis- 
tinction between  motor  aphasia  and  sensory  aphasia  (compare 
Fig.  107).  Accepting  Broca's  localization  as  correct  for  the  motor 

1  Ziemssen's  Cyclopaedia,  XIV,  p.  759. 

3  Der  aphasische  Symptomencomplex  (Breslau,  1874). 


258  THE  CEREBRAL  HEMISPHERES 

form,  he  assigned  for  sensory  aphasia  quite  a  different  seat — namely, 
in  the  temporal  lobe  of  the  left  hemisphere.  The  distinguishing 
symptom  of  motor  aphasia  is  inability  to  combine  articulatory  move- 
ments into  spoken  words,  in  the  absence,  however,  of  paralysis  of 
the  muscles  of  articulation.  The  distinguishing  symptom  of  the 
sensory  form,  according  to  Wernicke,  is  a  loss  of  the  auditory  images 
of  words.  Now,  since  speech  is  before  all  an  auditory  affair,  loss 
of  the  auditory  images  of  words  is  equivalent  to  a  loss  of  memory  for 
words,  and  so  of  the  use  of  speech.  So  reasoned  Wernicke.  Wheth- 
er the  psychological  terms  in  which  he  expressed  his  theory  are  cor- 
rect, or  not,  the  distinction  between  motor  and  sensory  aphasia  has 
proved  to  be  valid;  and  also  the  separate  localization  of  the  two 
forms.  Even  in  milder  cases,  in  which  both  the  sensory  and  the 
motor  aphasic  patient  have  retained  some  use  of  speech,  there  is 
still  a  difference  between  the  two.  The  motor  aphasic  shows  much 
hesitation  and  effort  in  enunciation,  and  speaks  disjointedly,  having 
lost  his  hold  on  grammatical  form;  whereas  the  sensory  aphasic 
may  speak  fluently  enough,  and  with  preservation  of  grammatical 
form,  but  with  the  wrong  use  of  words,  and  in  some  cases  in  a  per- 
fect jargon.  Wernicke  also  introduced  the  conceptions  of  "corti- 
cal," "subcortical,"  and  "  transcortical "  aphasia — a  subdivision 
which  can  be  applied  to  both  main  types.  In  the  cortical  vari- 
eties, the  centre  itself  is  supposed  to  be  destroyed,  and  therefore 
the  loss  of  function  is  complete.  In  the  subcortical  variety,  the 
centre,  remaining  intact,  allows  of  an  internal  speech,  which,  how- 
ever is  disconnected,  by  subcortical  lesion,  from  the  ear,  in  the  one 
case,  and  from  the  muscles  of  articulation,  in  the  other;  so  that 
the  patient  becomes  either  word-deaf  or  else  incapable  of  actually 
enunciating.  In  the  transcortical  or  associative  variety,  the  centre, 
by  severance  of  the  association  fibres,  is  thought  to  be  isolated  from 
other  parts  of  the  cortex  concerned  in  the  use  of  language.  Though 
it  is  not  difficult  to  find  cases  presenting  these  varieties  of  symptoms, 
the  anatomical  findings  do  not,  as  yet,  clearly  substantiate  the  in- 
terpretations indicated. 

§  27.  In  the  recent  history  of  the  aphasia  problem,  great  impor- 
tance attaches  to  the  work  of  P.  Marie.1  This  authority  concludes, 
from  extensive  material  of  his  own,  and  from  a  review  of  cases  pre- 

1  Marie's  original  articles  appeared  in  the  Semaine  medicate,  in  1906.  His 
point  of  view,  and  the  evidence  for  it,  are  well  presented  in  the  extensive  mono- 
graph of  his  pupil  Moutier,  L'Aphasie  de  Broca,  Paris,  1908.  Marie's  conten- 
tions have  provoked  wide-spread  discussion  among  authorities.  Reference  may 
be  made  to  the  reviews  and  discussions  by  C.  von  Monakow,  Ergebnisse  der  Physi- 
ologic, 1907,  VI,  334-605;  and  by  A.  Meyer,  Psychological  Bulletin,  1907,  IV, 
181-193;  1908,  V,  275-282. 


DISCUSSION  OF  BROCAS  SPEECH  CENTRE         259 

viously  published,  comprising  the  original  cases  of  Broca,  that 
actual  injury  always  extends  beyond  the  limits  of  Broca's  "speech 
centre,"  and  that  the  assignment  of  special  speech  functions  to  this 
region  is  entirely  unjustified.  He  even  goes  so  far  as  to  deny  any 
speech  function  to  the  third  left  frontal  convolution.  He  holds 
that  the  motor  symptoms  in  aphasia  are  due,  not  to  any  cortical 
lesion,  but  to  destruction  of  parts  lying  beneath  the  Broca  region, 
especially  the  lenticular  nucleus  and  the  neighboring  white  matter. 
The  motor  symptoms,  taken  alone,  would  not,  in  his  view,  con- 
stitute a  true  aphasia,  but  simply  a  defect  in  the  co-ordination  of 
the  speech  muscles.  True  aphasia,  he  believes,  is  always  of  the 
Wernicke  or  "sensory"  type,  and  is  due  to  injury  of  "Wernicke's 
region,"  namely  the  rear  of  the  temporal  lobe  and  adjoining  parts 
of  the  parietal.  He  objects,  however,  to  calling  this  aphasia  "sen- 
sory," insisting  that  it  is,  rather,  to  be  called  intellectual.  In  sup- 
port of  this  last  contention,  he  points  out  that  practically  all  cases 
of  true  aphasia  show  other  intellectual  defects. 

Without  doubt  it  is  time  to  inaugurate  a  much  needed  reform 
in  the  study  of  aphasia,  especially  on  the  anatomical  side.  Above 
all,  do  the  negative  cases,  of  which  there  are  many  with  regard  to 
the  Broca  region,  need  to  be  taken  much  fuller  account  of  than  has 
previously  been  done.  The  fact  of  a  negative  case  is,  however, 
always  hard  to  establish;  since  defects  of  speech  may  be  recovered 
from  after  a  brain  injury,  and  may  never  form  part  of  the  record 
of  the  patient's  clinical  history.1  The  anatomical  examination  of 
the  brain  of  aphasic  patients  has  usually  been  quite  superficial, 
from  a  modern  stand-point,  which  demands  that  the  underlying 
portions,  as  well  as  the  cortex,  be  microscopically  examined  in 
"serial  sections."  This  laborious  method  is  only  beginning  to  be 
applied  to  cases  of  aphasia,  and  for  this  reason,  most  of  the  older 
attempts  to  localize  its  various  symptoms  are  subject  to  suspicion. 

Marie's  view  of  aphasia  as  essentially  a  loss  of  general  intellectual 
power  corresponds  indeed  to  the  fact  that  patients  who  suffer  from 
disturbances  of  speech  usually  show  also  some  degree  of  apraxia 
or  asymbolia,  and  are  often  mentally  inefficient.  In  most  cases, 
they  are  persons  of  advanced  years,  and  often  with  a  more  or  less 
diffused  disturbance  of  cerebral  circulation,  or  with  other  diseased 

1  Compare,  in  this  connection,  the  case  recorded  by  Liepmann,  in  Journal  /. 
Psychologic  u.  Neurologic,  1907,  IX,  284,  in  which  almost  complete  destruction 
of  Broca 's  region  appeared  on  post-mortem  examination  of  the  brain  of  a  man 
whose  hospital  record  showed  no  evidence  of  aphasia,  but  whose  previous 
history  revealed  a  "shock,"  ten  years  before,  which  had  been  followed  by 
temporary  aphasia.  Cases  like  this  are  likely  to  be  frequent  among  those  which 
are  reported  as  completely  negative. 


260  THE  CEREBRAL  HEMISPHERES 

conditions  which  affect  a  considerable  area  of  the  cortex.  Intel- 
lectual deficiency  in  other  directions,  however,  may  be  slight  in 
comparison  with  the  disturbance  of  speech;  and  in  surgical  cases, 
in  which  local  destruction  has  been  brought  about  in  healthy  brains, 
it  is  not  uncommon  to  find  aphasia  unattended  by  any  marked 
.intellectual  defect.1 

§  28.  The  conclusion  seems  warranted,  then,  that  there  is  a 
group  of  functions,  which  are  indispensable  for  language,  but  not 
equally  essential  for  all  kinds  of  intellectual  performance;  and  that 
these  functions  may  be  disturbed  or  destroyed  by  partial  destruc- 
tion of  the  cerebrum.  This  is  to  express  the  conclusion  very  cau- 
tiously. There  can  be  scarcely  any  doubt,  further,  that  the  part 
of  the  cortex  which  is  most  vulnerable  as  concerns  the  linguistic 
functions  is  "  Wernicke's  region,"  in  close  proximity  to  the  receiving 
station  for  auditory  impulses.  Adolf  Meyer,2  who  has  studied 
the  subject,  both  clinically  and  anatomically,  with  great  thorough- 
ness, concludes  that  the  special  symptom,  word-deafness,  or,  as 
he  prefers  to  call  it,  "  word-imperception,"  is  the  result  of  destruc- 
tion of  the  primary  auditory  receiving  station  in  the  left  hemi- 
sphere; whereas  paraphasia  "jargon-aphasia,"  and  verbal  amnesia 
are  associated  with  destructions  within  the  rest  of  Wernicke's 
region.  This  region  contains  nervous  mechanisms  which  are  es- 
sential to  orderly  speech,  and  which  are  more  closely  related  to 
the  understanding  of  heard  speech  than  to  co-ordination  of  the 
muscles  of  articulation.  We  may. then  properly  speak  of  a  speech 
area  in  the  temporal  lobe,  with  the  understanding  that  the  area  is 
too  broad  to  be  a  unit  in  function;  although  special  localizations 
within  it  are  not  yet  clearly  made  out. 

§  29.  To  return  now  to  the  historic  speech  area  of  Broca,  which 
has  been  assailed  with  such  energy:  Marie's  attempt  to  show  that 
the  injury  which  causes  the  motor  symptoms  of  aphasia  is  located 
in  the  neighborhood  of  the  lenticular  nucleus  has  not  met  with 
much  favor;  and  clear  cases  have  been  brought  forward3  in  which 
the  destruction  involved  Broca's  region,  without  affecting  deeper- 
lying  parts.  Such  negative  cases  are,  indeed,  not  fully  convincing 
(see  p.  259),  because  of  the  possibility  of  recovery  later.  But  the 
fact  of  restitution  of  motor  speech,  after  extensive  or  even  complete 
destruction  of  the  third  left  frontal  gyre,  is  an  undoubted  fact;  and 
it  is  not  exceptional.  Restitution  of  function  is  as  puzzling  a  fact 
here  as  in  the  case  of  injury  to  the  motor  area  (compare  p.  241). 
It  cannot  be  interpreted  as  due  to  the  assumption  of  linguistic  func- 

1  C.  v.  Monakow,  op.  cit.,  p.  395. 

2  Journal  /.  Psychologic  u.  Neurologic,  1908,  XIII,  pp.  203  ff. 

8  See  Liepmann,  Journal  fur  Psychologic  u.  Neurologic,   1907,  X,  280. 


DISCUSSION  OF  BROCAS  SPEECH  CENTRE         261 

tions  by  some  part  of  the  cortex  which  previously  had  nothing  to 
do  with  them;  for  the  recovery  is  often  very  sudden;  and  besides  this 
objection,  such  an  elaborate  system  of  reactions  cannot  be  easily 
learned  at  the  time  of  life  of  most  sufferers  from  aphasia.  Usually, 
after  restitution  in  case  of  injury  to  the  Broca  region,  there  are  some 
residual  defects,  such  as  difficulty  in  pronouncing  long  words,  slow 
and  hesitating  speech,  with  perhaps  stuttering,  spasm,  easy  fa- 
tigue; also  difficulty  in  managing  the  grammatical  constructions.1 
The  amount  of  restitution  depends,  in  part,  on  the  general  condi- 
tion of  the  brain  and  of  the  bodily  health.  In  some  cases,  with  poor 
general  condition,  loss  of  the  Broca  region  alone  has  brought  about 
complete  motor  aphasia,  without  later  recovery.  Also,  there  is 
good  evidence  that  the  Broca  region  is  the  most  vulnerable  part  of 
the  cortex,  as  regards  the  motor  co-ordination  of  speech. 

There  seems,  therefore,  on  the  whole,  to  be  good  ground  for  still 
retaining  Broca's  speech  centre;  while  relieving  it  of  part  of  its 
supposed  duties.  It  is  probable  that  the  rearmost  part  of  the  third 
frontal  gyre,  in  the  left  hemisphere,  is  intimately  connected  with  the 
adjoining  part  of  the  precentral  gyre,  from  which  issues  the  motor 
pathway  to  the  muscles  of  speech.  It  is  further  probable  that  this 
part  of  the  third  frontal  has  something  to  do  with  the  combination 
of  elementary  movements  of  the  organs  of  speech  into  those  highly 
practised  sequences  which  constitute  words.  Psychological  anal- 
ysis of  the  exceedingly  complex  mental  activities  involved  in  all  the 
uses  of  human  spoken  and  written  language  tends  to  confirm  the 
conjecture  of  such  writers  as  Campbell  and  Von  Monakow,  that 
speech  functions  cover  a  somewhat  wider  area  in  the  frontal  lobe. 
Among  such  functions  might  be  those  combinations  which  are  of 
a  higher  order  than  that  of  elementary  vocal  movements  into  spoken 
words,  such  as  phrasing,  inflection  of  the  voice,  syntactical  forms, 
etc.,  etc.  Indeed,  the  entire  cerebrum  would  seem  to  be,  of  neces- 
sity, involved  in  man's  linguistic  attainments  and  uses. 

§  30.  A  writing  centre  in  the  second  frontal  gyre,  in  close  prox- 
imity to  the  motor  area  for  the  hand,  has  been  asserted  as  the  teach- 
ing of  a  few  strikingly  positive  cases;  but  the  negative  and  mixed 
cases  have  sufficient  weight  to  prevent  a  general  acceptance  of  this 
localization.  Since  the  neighborhood  of  each  sensory  receiving 
station  is  concerned  with  the  impressions  received  at  that  station 
from  its  sense-organ,  it  is  a  tempting  conjecture  that  the  writing  area 
lies  close  to  the  motor  area  of  the  hand.  This  conjecture  is  fortified 
by  the  analogous  case  of  the  localization  of  the  organs  of  speech.  And 
there  is  some,  but  not  as  yet  conclusive,  evidence  that  the  frontal 

1  Von  Monakow,  op.  cit.,  p.  528. 


262  THE  CEREBRAL  HEMISPHERES 

region,  just  forward  of  the  motor  area,  in  the  left  hemisphere,  is 
of  special  importance  in  all  learned  and  skilled  movements. 

§  31.  Even  after  adding  to  the  primary  sensory  areas  those  neigh- 
boring areas  that  are  probably  concerned  in  the  utilization  of  sen- 
sory data,  and  allowing  a  similar  bordering  zone  along  the  motor 
area  for  the  combination  of  elementary  movements  into  learned 
actions,  there  remains  a  considerable  part  of  the  cortex  of  the  human 

in  without  known  relationship  to  any  special  function.  Con- 
siderable portions  of  the  parietal  and  temporal  lobes  are  still  un- 
accounted for;  also  the  island;  and,  especially,  a  large  part  of  the 
frontal  lobes.  The  frontal  lobes,  because  of  their  superior  develop- 
ment in  man,  have  long  been  regarded  with  special  interest.  They 
have  been  by  many  suspected  of  being  the  great  intellectual  cen- 
tres. Statistics  of  the  symptoms  accompanying  brain  tumors 
in  different  regions1  have  shown  intellectual  disturbances  in  80 
per  cent,  of  the  cases,  when  the  tumor  was  in  the  frontal  lobes,  as 
compared  with  54  to  66  per  cent,  when  the  tumor  was  located 
elsewhere  on  the  cortex.  Bolton,2  after  examining  many  cases  of 
dementia  and  idiocy,  found  that  the  cortex  was  usually  thinned  out, 
especially  in  the  frontal  lobes;  and  from  this  fact  he  concluded  that 
the  frontal  region  is  the  organ  of  attention  and  general  "orderly 
co-ordination  of  psychic  processes."  Loss  of  power  of  attention 
and  of  inhibition  has  been  claimed  by  several  observers  to  be  the 
result  of  injuries  to  this  region.  "  Witzelsucht,"  or  an  addiction  to 
practical  jokes  of  a  weak  order,  with  lack  of  respect  for  propriety 
or  the  rights  of  others,  has  been  frequently  observed.  Some  au- 
thors use  terms  as  broad  and  vague  as  "loss  of  character"  or  "of 
personality,"  in  attempting  to  formulate  the  symptoms  of  frontal 
injury.  Indeed,  all  of  the  clinical  observations  are  rather  vaguely 
formulated  as  regards  the  mental  symptoms.  On  the  contrary,  in 
some  remarkable  cases  of  destruction  of  large  parts  of  the  frontal 
lobes,  no  marked  symptoms  whatever  have  appeared.  Injury  of 
the  right  frontal  lobe  has  more  frequently  been  attended  by  no 
marked  mental  symptoms  than  has  injury  of  the  corresponding 
area  in  the  left  hemisphere. 

§  32.  Of  experimental  investigations  of  the  frontal  lobe  in  ani- 
mals, we  may  recall  the  existence  of  an  area,  which  on  excitation 
responded  with  movements  of  eye,  ear,  and  head.  Changes  in  the 
rate  of  breathing  and  of  the  heart  beat,  were  also  sometimes  ob- 
served.3 This  excitable  frontal  area  is  not  yet  fully  understood. 
Of  work  done  by  the  method  of  extirpation,  the  most  promising  is 

1  Compare  Schuster,  Psychische  Storungen  bei  Hirntumoren  (Stuttgart,  1902). 
*  Brain,  1903,  XXVI,  215-241;  Journal  of  Mental  Science,  1905,  1906. 
3  Langelaan  and  Beyerman,  Brain,  1903,  XXVI,  81-93. 


FUNCTIONS  OF  THE  FRONTAL  LOBE  263 

that  of  Franz,  performed  by  a  combination  of  physiological  and  psy- 
chological methods.  Having  first  taught  a  cat  or  monkey  certain 
specialized  acts,  such  as  getting  into  a  cage  by  turning  a  certain  but- 
ton or  pulling  a  certain  string,  Franz1  removed  parts  of  the  frontal 
lobes,  and  later  tested  the  animal  to  see  if  it  retained  the  recently 
learned  act.  In  general  the  act  was  not  retained  after  the  operation. 
The  objection  that  the  shock  of  operation,  or  the  mere  removal 
of  a  certain  quantity  of  brain  substance,  no  matter  from  what  locality, 
was  sufficient  to  explain  the  loss,  was  met  by  operations  in  other 
regions,  which  were  not  followed  by  loss  of  the  learned  act.  Some- 
times removals  of  parts  of  the  frontal  lobe  itself  were  not  followed 
by  such  loss;  and  the  act  could  be  relearned  after  the  frontal  opera- 
tion, the  new  learning  taking  about  the  same  time  as  if  the  previous 
learning  had  not  occurred.  Habits  of  long  standing,  on  the  other 
hand,  were  not  disturbed  by  such  extirpations.  Injury  to  only  one 
of  the  frontal  lobes  caused,  indeed,  a  slowing  of  the  act,  but  not  a 
loss  of  it.  The  author  concludes  that  the  frontal  lobes  are  con- 
cerned in  the  acquisition  of  new  performances  of  the  sort  used;  but 
that  no  one  spot  is  indispensable  for  the  acquisition  of  a  particular 
act;  and  that  long-continued  practice  in  a  performance  reduces  it 
to  an  automatic  or  semi-reflex  condition,  in  which  the  frontal  lobes 
are  no  longer  necessary. 

§  33.  Attention  should  be  called  to  the  predominance  of  the  left 
hemisphere  in  right-handed  persons.  The  various  defects  which 
have  been  described  as  resulting  from  injury  to  different  portions 
of  the  cortex  usually  result  from  injury  to  the  left  hemisphere. 
Simple  paralysis  or  loss  of  sensation  may  result,  indeed,  from  in- 
V  jury  to  either  hemisphere.  But  object-blindness,  word-blindness 
or  word-deafness,  the  various  aphasias  and  apraxias,  usually  result 
from  injury  to  the  left  hemisphere.  From  a  study  of  ninety  cases 
of  hemiplegia  (one-sided  paralysis  due  to  injury  to  the  motor  area 
or  the  white  matter  beneath  it),  Liepmann2  found  that  injuries  to 
the  left  hemisphere  which  caused  paralysis  of  the  right  hand  caused 
also  awkwardness  and  disturbance  of  skilled  movements  in  the  left 
hand.  Injury  of  the  callosum  by  severing  the  connection  between 
the  two  hemispheres  resulted  in  disturbance  of  skilled  movements 
of  the  left  hand,  but  not  of  the  right. 

It  would  almost  seem,  from  the  evidence  obtained,  that  the  left 
hemisphere  so  completely  takes  charge  of  acts  of  skill,  and  of  the  in- 
tellectual processes  concerned  in  them,  as  to  leave  nothing  for  the 
great  bulk  of  the  right  hemisphere  to  do.  Such  a  conclusion  is, 

1  "The  Frontal  Lobes,"  Archives  of  Psychology,  1907,  No.  2. 

2  Munchner  medicinische  Wochenschrift,  1905,  reviewed  in  Journal  f.  Psychol. 
u.  Neurol,  1906,  VII,  190. 


264  THE  CEREBRAL  HEMISPHERES 

of  course,  in  itself  extremely  improbable,  especially  in  view  of  the 
nearly  equal  size  and  inner  development  of  the  two  hemispheres; 
but  it  must  be  admitted  that  the  role  of  the  right  hemisphere,  aside 
from  the  simplest  sensory  and  motor  functions,  is  not  at  all  clearly 
made  out.  In  general,  it  may  be  that  of  assistance,  or  in  case  of 
need,  of  substitution,  for  the  more  constant  and  important  functions 
of  the  left  hemisphere:  and  for  this  view  there  is  a  considerable 
accumulation  of  evidence.  This  special  culture  of  the  left  hemi- 
sphere— if  we  may  so  express  the  fact — may  well  enough  be  con- 
nected, both  as  cause  and  effect,  with  the  prevalent  right-handedness 
of  the  human  species. 

§  34.  We  have  now  passed  in  review  all  of  the  localizations  of 
function  in  the  cortex  which  have  much  claim  to  attention.  With 
the  exception  of  the  sensory  and  motor  areas,  the  localizations  so 
far  established  are  somewhat  vague,  as  respects  both  their  extent 
on  the  cortex,  and  also  the  exact  function  which  is  performed  by 
any  given  area.  What  is  needed,  before  a  more  satisfactory  ap- 
portionment of  functions  over  the  cortex  can  be  attained,  is,  on  the 
physiological  side,  a  more  detailed  knowledge  of  the  structure  of 
the  cortex  as  a  whole,  and  in  its  different  parts;  and,  on  the  psycho- 
logical side,  a  thorough  analysis  of  such  vague  and  gross  so-called 
functions  as  "speech,"  or  "skilled  movement,"  or  "perception  of 
objects,"  or  "orientation  in  space,"  into  their  elementary  func- 
tional factors.  It  is  highly  probable  that  any  concrete  mental  per- 
formance involves,  physiologically,  a  complex  of  activities  of  various 
parts  of  the  brain;  the  performance  as  a  whole,  therefore,  cannot  be 
localized,  although  the  elementary  functions  may — and,  without 
doubt,  do — each  depend  on  certain  particular  nervous  connections 
that  have  a  definite  location.  It  is  certain,  psychologically,  that 
all  these  mental  activities  involve  a  vast  and  tangled  complex  of 
simpler  factors,  which  have  either  never  come  before  us  for  con- 
scious inspection,  or  have  already  been  quite  lost  out  of  conscious- 
ness, for  purposes  of  such  inspection.  But  to  this  view  of  the  whole 
subject  we  shall  return  again. 

§  35.  On  the  histological  side,  the  mapping  of  the  cortex  into 
areas  of  different  structure  has  recently  made  great  advances. 

The  picture  seen  on  examining  a  cross  section  of  the  cortex  under 
the  microscope  differs  greatly  according  to  the  dye  with  which  the 
tissue  has  been  stained.  Some  dyes  stain  the  cells,  some  the 
fibres.  Prominent,  for  example,  among  the  cell-stains  is  Nissl's 
methylene  blue  stain;  prominent  among  the  fibre-stains  is  the  hema- 
toxylin  of  Weigert.  The  Golgi  stain  shows  cells  with  their  den- 
drites  and  axons,  but  usually  stains  only  a  few  of  the  whole  number 
of  cells  present,  thus,  however,  making  it  easier  to  trace  out  those 


CELL-LAYERS  IN  THE  CORTEX 


265 


that  do  show  themselves.  The  silver-reduction  methods  of  Cajal 
and  of  Bielschowsky  show  the  fibrils  within  the  cells,  and  altogether 
give  the  most  complete  pictures  of  the  intricacy  of  the  cortex.  Each 

of  these  methods  has  its  peculiar  ad-  __ _. 

vantages,  and  a  combination  of  the 
results  of  all  is  necessary  in  order  to 
get  an  adequate  notion  of  the  minuter 
intricacies  of  cortical  structure. 

§  36.  To  begin  with  the  cells,  let 
us  consider  a  picture  like  that  given 
in  Fig.  108.  The  cells  here  are  seen 
to  be  of  several  shapes,  and  of  a 
considerable  range  of  sizes.  Prom- 
inent in  the  cerebral  cortex  are  the 
"pyramidal  cells,"  or  "pyramids," 
cells  of  generally  triangular  appear- 
ance, with  a  long  apical  dendrite 
extending  upward  toward  the  outer 
surface  of  the  cortex  and  several 
basal  dendrites,  extending  horizon- 
tally and  obliquely  downward.  The 
axon  of  a  pyramidal  cell  emerges 
from  the  base  and  usually  passes 
downward  into  the  white  matter, 
giving  off  a  few  collaterals  as  it  goes. 
The  pyramids  differ  in  size  from 
small  to  large.1 

Besides  these  radially  arranged 
cells,  there  are  many  others  of  dif- 
ferent orientation.  Some  send  their 
dendrites  downward  and  their  axon 
upward  into  the  outer  layer;  some 
have  short  axons  which  split  into 
fine  branches  close  to  their  cell; 

Some    present    a    general    horizontal     FIG.  108.—  Section  through  the  Cerebral 

arrangement  of  dendrites  and  axons. 
Examples  of  these  varieties  are 
shown  in  Fig.  109. 

§  37.  For  purposes  of  histological  examination  and  description, 
it  is  convenient  to  divide  the  cortex  into  layers.  The  boundaries 
between  some  of  the  layers  are  not  perfectly  definite,  and  authori- 
ties have  disagreed  both  as  to  the  number  to  be  recognized,  and  also 

1  Cajal,  Studien  iiber  die  Hirnrinde  des  Menschen,  Heft  2,  pp.  27-28  (Leip- 
zig, 1900). 


Cortex,  Stained  by  the  Golgi  Method. 
(Kolliker.)  The  numbers  refer  to 
the  layers  mentioned  on  p.  266. 


266 


THE  CEREBRAL  HEMISPHERES 


as  to  their  names.  A  division  into  six  layers  has  very  high  author- 
ity, and  is  defended  by  Brodmann1  as  being  the  fundamental 
type  throughout  the  neopallium  of  all  orders  of  mammals.  The 

six  layers  are,  he  states, 
clearly  visible  everywhere  at 
an  early  stage  of  develop- 
ment; but  he  admits  that 
they  become  obscured  in 
the  adult  condition.  These 
six  layers,  arranged  in  their 
order  from  the  external  sur- 
face to  the  white  matter, 
may  be  named: 

(1)  The  zonal  or  plexi- 
form  layer. 

(2)  The  outer  granule  or 
small-celled  layer. 

(3)  The  outer  pyramidal 
layer. 

(4)  The    inner    granule 
layer. 

(5)  The  inner  pyramidal 
layer. 

(6)  The  spindle  or  multi- 
form layer. 

The  plexiform  layer, 
though  it  contains  cells,  es- 
pecially the  so-called  "hori- 
zontal cells,"  consists  mostly 
of  fibres.  According  to  Ca- 
jal,2some  of  these,  the  "  tan- 
gential fibres,"  arise  from 
the  horizontal  cells  of  this 
layer,  and  may  be  regarded 
as  association  fibres,  and 
others  are  axons  from  cells 
lying  in  the  deeper  layers,  or 

dendrites  from  the  pyramidal  cells.  The  plexiform  layer  is  therefore 
a  place  in  which  numerous  synaptic  connections  are  formed  be- 
tween the  various  cells  of  the  cortex.  But  the  plexiform  layer  is 
not  the  only  plexus  of  nerve-fibres  in  the  cortex.  Intricate  networks 

1  Vergleichende  Lokalisationslehre  der  Grosshirnrinde  in  ihren  Principien  dar- 
gestellt  auf  Grund  des  Zellenbaus,  pp.  14-42  (Leipzig,  1909). 

2  Op.  cit.,  p.  23. 


FIG.  109.— Different  Types  of  Cells  Found  in  the 
Cortex.    (Starr,  Strong,  and  Learning.) 


CELL-LAYERS  IN  THE  CORTEX 


267 


of  fibres,  medullated  and  unmedullated,  exist  in  the  various  layers. 
When  the  cortex  is  stained  for  medullated  fibres  (Fig.  110),  some 
layers  are  seen  to  contain  many  such  fibres,  and  some  only  a  few. 
In  the  deepest  layers,  near  the  white  matter,  the  network  of  hori- 
zontal and  oblique  fibres  is  specially  dense.  Those  layers  which 
show  but  few  medullated  fibres  are  not,  however,  free  from  a  net- 


FIQ.  110.— Layers  of  the  Cortical  Cells  and  Fibres.     (Vogt.) 

work  of  fibres;  for  on  treating  the  cortex  with  the  Golgi  stain,  dense 
plexuses  are  seen.  Cajal1  describes  what  he  calls  the  sensory 
plexus,  which  is  very  dense  in  the  sensory  and  motor  areas,  and  ex- 
ists also,  with  less  density,  in  the  rest  of  the  cortex.  His  tracing 
of  the  fibres  which  give  rise  to  this  plexus  leads  him  to  believe  that 
they  form  the  path  of  incoming  impulses  to  the  cortex  (see  Fig. 
111).  The  outgoing  path  is,  he  thinks,  provided  by  the  axons  of  the 
pyramidal  cells;  and  the  inner  connections  within  any  minute  portion 
of  the  cortex  are  provided  partly,  but  not  exclusively,  through 

1  Op.  cit.,  p.  83. 


268 


THE  CEREBRAL  HEMISPHERES 


the  plexiform  layer.     Numerous  interweavings  of  fine  dendrites  and 
terminations  of  axons  and  their  collaterals  occur  at  all  levels. 
§  38.  Certain  areas  of  the  cortex — especially  the  precentral  gyre, 


\ 

FIG.  111.— The  Sensory  Plexus  of  the  Cortex.     (Cajal.)      A  is  the  zonal  layer. 

the  calcarine  region,  the  pyriform  lobe,  and  parts  of  the  limbic 
lobe — are  strikingly  peculiar  in  structure  (see  Figs.  112  and  113). 
The  precentral  gyre  is  characterized  by  the  large  size  of  its  pyra- 
mids, some  of  which  are  real  giants,  and  are  so  named.  These  giant 


CELL-LAYERS  IN  THE  CORTEX 


269 


pyramids  are  the  chief  origin  of  the  long  fibres  of  the  cortico-spinal 
tract.  The  calcarine  cortex  is  marked  by  a  very  prominent  stripe, 
which  is  here  called  the  stripe  of  Gennari,  from  the  observer  who 
first  noticed  it  in  this  area.1  The 

1  calcarine   cortex  also  has  a  few 
very  large  pyramids.  Another  of 

2  its  peculiarities  is  a  layer  of  large 
stellate   cells   between   the    two 
layers  of  small  cells  into  which 

the    internal    granule    layer    is      '  ffiSjpM 
split  up.     Cajal2  has  attempted,       -^'<'*^ 

3  in   Golgi  preparations,  to  trace 
the    interrelations    of    the    cells 
and  fibres  in  this  part  of  the  cor- 
tex.   His  results  lead  him  to  con- 
sider it  probable  that  the  sensory 

4  impulses  from  the  retina  impinge 
first  on  the  large  stellate  cells, 
which  would  thus  constitute  the 
primary  receiving  station.     The 
axons  of  these  cells  he  regards 
as   association  fibres  by   which 

J  the  effect  of  the  visual  impres- 
sions is  communicated  to  other 
parts  of  the  cortex;  and  the 
motor  fibres  which  are  known  to 
originate  from  the  calcarine  re- 
gion and  which  give  rise  to  move- 
ments of  the  eyes,  he  believes  to 
arise  from  the  large  pyramidal 
cells.  These  cells  and  fibres 


4a 


4b 


4c 


( 


f  Ni   ry«r 

m- 


Flthen  MotorrtAXreaf  would  thus  account  f  or  the  partic- 
(Lewis,  from  Fer-  ujar  functions  of  the  visual  area. 

Tier's  Functions  of  or.     T     ,         ,  ,  , 

the  Brain.)  §  39.  It  has  long  been  known 

that  the  internal  structure  of  the 


FIG.  113.— Cortex  of  the 
Calcarine  Area.  (Lew- 
is, from  Ferrier's 
Functions  of  the 
Brain.)  The  splitting 
of  layer  4  into  tbree 
layers  is  characteristic 
of  this  area.  The 
fibre-stripe  of  Gennari 
comes  in  layer  4b. 


cortex  differs  in  different  parts;  but  not  till  quite 

recently  has   the   laborious   work  of  examining 

systematically  the    whole   cortex,  and  mapping 

out    the  limits  of  each  variety  of   its    structure,  been   attempted. 

This  has  now  been  done,  independently,  by  different  observers, 

with  results  that  are  in  close  agreement;    and  this  work  is  one 

1  This  stripe  is  visible,  on  close  inspection,  in  most  areas  of  the  cortex,  and 
bears  in  general  the  name  of  Baillarger. 

2  Studien  uber  die  Hirnrinde  des  Menschen,  Heft  1,  pp.  62-70  (Leipzig,  1900). 


270 


THE  CEREBRAL  HEMISPHERES 


of  the  greatest  recent  ad- 
ditions  to  our   knowledge 
of    the    brain.     Campbell 
made  a  complete  examina- 
tion   of   several   human 
brains,    and    also    of    the 
brains    of   the   anthropoid 
ape  and  of  the  dog,  cat, 
and  pig.     His  examination 
was  directed   both    to  the 
cells  and  to  the  medullated 
fibres.1    A  yet  more  exten- 
sive  program   is   now   in 
process    of    execution     in 
Professor   Vogt's   Neuro- 
biological  Institute  at  Ber- 
lin, the  work  being  divided 
among    several    collabora- 
tors, and  extending  to  all 
orders  of  mammals.    Brod- 
mann2  has   published    his 
studies  on  the  cells  of  the 
cortex  in  various  mammals. 
Mauss3  has  issued  a  study 
of  the  medullated  fibres  of 
the  cortex  in  monkeys ;  and 
Vogt4  has  given  a  map  of 
the  frontal  lobes  in  man  as 
divided  on  the  basis  of  the 
medullated  fibres.     The 
maps  of  the  cortex  afforded 
by  these  different   studies 
agree  in  the  main,  although 
Campbell   did   not  subdi- 

1  A.  W.  Campbell,  Histological 
Studies   on   the   Localization   of 
Cerebral  Function    (Cambridge, 
1905). 

2  Vergleichende    Localisations- 
lehre  der   Grosshirnrinde   (Leip- 
zig, 1908),  and  several  papers  in 

FIG.  114.— Medullated  Fibres  in  Two  Regions  of  the  the  Journal  f.  Psvchol. 
Cortex.     (Campbell.)     A  is  from  the  precentral  or        3  jm,rr,ni  t    PoL/,^7     ,, 
motor  area,  which  is  extraordinarily  rich  in  fibres;        ,     fonc   VTTT    o?o   oo^ 

B  is  from  the  pref rental  area,  which  is  compara-  Tol->  iyus>  Alii,  Zb6-625 
tively  poor  in  medullated  fibres.  *  Ibid.,  1910,  XV,  221-232. 


HISTOLOGICAL  MAPPING  OF  CORTEX 


271 


vide  so  minutely  as  did  the  Berlin  observers;  and  among  the  lat- 
ter, Vogt  has  been  able,  by  studying  the  fibres,  to  make  more  sub- 
divisions than  were  recognizable  from  the  study  of  the  cells  alone. 
§  40.  The  principles  that  have  guided  all  these  investigators  in 
marking  the  boundaries  between  the  cerebral  areas  may  be  illus- 
trated by  a  few  examples.  The  precentral  area  is  characterized 
by  the  giant  pyramids,  and  these  stop  abruptly  at  the  bottom  of  the 
central  fissure,  thus  establishing  the  posterior  border  of  this  area; 
its  anterior  border  is  less  abrupt,  but  can  be  determined  within  a 


FIG.  115. — Histological  Map  of  the  Cortex.  Lateral  Surface.  (Campbell.)  The  areas  ex- 
tend down  into  the  fissures:  thus,  the  precentral  and  postcentral  meet  at  the  bottom  of 
the  central  fissure;  and  the  audito-sensory  lies  almost  entirely  within  the  fissure  of  Sylvius. 


millimetre  or  two.  Another  characteristic  of  the  precentral  area 
is  its  lack  of  a  well-defined  inner  granular  layer,  and  the  consequent 
fusion  of  the  outer  and  inner  pyramidal  layers.  The  granule  layer 
appears  promptly  behind  the  central  fissure.  The  thickness  of 
the  cortex  in  the  precentral  area  is  very  great,  and  medullated  fibres 
exist  in  extraordinary  richness;  the  wealth  of  fibres  is  one  of  the  dis- 
tinctive marks  of  this  area. 

The  calcarine  area  is  distinguished  by  a  splitting  of  the  inner 
granular  layer  into  two,  with  a  layer  of  large  stellate  cells  between 
(see  p.  269);  and  in  fibre  preparations,  by  the  distinctness  of  the 
stripe  of  Gennari,  which  can  be  seen  with  the  naked  eye;  hence 
this  area  is  named  also  the  "area  striata."  The  boundaries  of 
this  area  are  perfectly  sharp;  the  stripe  appears  suddenly,  and  the 
subdivision  of  the  granular  layer  is  also  very  sudden. 


272 


THE  CEREBRAL  HEMISPHERES 


There  are  other  instances  of  sudden  transitions  between  one 
type  of  structure  and  the  adjoining  types;  but  as  a  rule,  the 
transitions  are  less  abrupt.  The  distinctions  between  neighbour- 
ing areas  are  based  partly  on  the  fusion  or  subdivision  of  layers, 
and  partly  on  quantitative  grounds,  such  as  the  thickness  of  the 
whole  cortex  and  of  the  separate  layers,  the  size  and  frequency 
of  the  cells,  the  calibre  of  the  single  fibres,  and  the  density  of  the 
fibre  network. 

§  41.  Some  of  the  areas  thus  marked  out  in  terms  of  structure 
by  the  accompanying  histological  maps  (Figs.  115  and  116)  are 


FIG.  116. — The  Same,  Mesial  Surface.     (Campbell.)     Brodmann's  map  differs  from  this  of 
Campbell  in  that  it  subdivides  many  of  the  areas  into  two  or  more. 

already  familiar  in  the  localization  of  function.  The  precentral 
area,  for  example,  coincides  almost  exactly  with  the  motor  area  as 
delimited  by  Grunbaum  and  Sherrington;  the  only  difference  is 
that  the  excitable  area  extends  a  millimetre  or  two  further  forward 
than  the  area  of  giant  pyramids.  The  area  striata  coincides  with 
the  visual  area  as  marked  out  by  the  termination  of  the  optic  radia- 
tion, and  hence  is  named  by  Campbell  the  " visuo-sensory "  area; 
and  there  is  an  area  in  the  temporal  lobe  corresponding  to  the  end- 
station  of  the  auditory  fibres.  This  correspondence  of  differences 
of  structure  with  the  known  differences  in  function  lends  force  to 
the  view  that  the  other  differences  in  structure,  over  parts  of  the 


GROUPING  OF  FUNCTIONS  IN  CORTEX          273 

cortex  whose  function  is  not  known,  may  also  be  taken  as  indica- 
tions of  differences  of  function. 

§  42.  It  is  a  reasonable  conjecture,  then,  that  these  structural  maps 
of  the  cortex  are  functional  maps  as  well.  We  cannot,  indeed,  as  yet 
infer  the  function  from  the  structure;  but  it  is  certainly  significant 
that  the  motor  area  and  the  sensory  areas  present  the  appearance 
of  fuller  development  than  do  most  of  the  other  areas.  These 
other  areas  are  also  later  in  becoming  myelinated  in  the  individual, 
and  later  in  making  their  appearance  in  the  race.  They  are,  there- 
fore, probably  concerned  with  functions  which  are  less  used  and 
less  essential  than  the  elementary  functions  of  sensation  and  move- 
ment. 

Another  important  suggestion  is  afforded  by  the  fact  that  it  is 
possible  to  mark  out  areas  which  possess  a  nearly  or  quite  uniform 
structure.  The  fact  that  a  uniform  structure  exists  over  any  con- 
siderable area  of  the  cortex,  giving  place  at  its  borders  to  areas  of 
other  structure,  would  seem  plainly  to  indicate  that  within  each  area 
the  elements  have  something  in  common  in  the  manner  of  their 
functioning.  Another  fact  may  be  brought  forward,  which  points 
in  the  same  direction.  Rich  provision  seems  to  be  made,  both  in 
the  plexiform  layer  and  by  the  dense  plexuses  in  deeper  layers,  for 
causing  neighboring  cells  to  act  together.  It  seems  impossible, 
then,  that  the  cells  in  these  areas  should  be  thrown  into  action 
singly;  they  must  act  in  groups  or  neighborhoods.  Some  of  these 
neighborhoods  may,  indeed,  be  microscopic  in  size,  much  smaller 
than  the  structural  areas  which  are  mapped  out;  also,  it  is  probable 
that  these  neighborhoods  are  not  sharply  limited  in  extent,  but  that 
the  co-operating  cells,  in  any  given  function,  are  most  active  in  a 
certain  part  of  each  neighborhood,  and  less  and  less  active  the  fur- 
ther we  go  from  this  centre  out.  In  this  case,  the  neighborhoods 
concerned  in  different  allied  functions  would  certainly  overlap. 

If  such  a  conception  as  the  foregoing  is  even  half-way  true,  con- 
tiguous spots  of  the  cortex  must  possess  related,  and  even  over- 
lapping functions;  so  that  the  transition  from  one  function  to  an- 
other would  be  gradual  over  a  certain  area;  and  this  area  would 
accordingly  be  the  seat  of  a  certain  group  or  range  of  related  junc- 
tions. Such  considerations  make  it  seem  highly  probable  that  the 
structural  areas  which  the  histologists  have  mapped  out  are,  indeed, 
areas  for  different  groups  of  functions. 

§  43.  But  what  are  the  "groups  of  functions"  which  we  have  a 
right  to  expect  to  be  thus  localizable?  As  to  this,  little  that  is 
definite  and  satisfactory  can  be  said  at  present.  It  is  a  question 
to  the  answer  of  which  psychology  must  make  the  preliminary, 
directing  if  not  determining,  contribution.  And  the  analysis  of 


274  THE  CEREBRAL  HEMISPHERES 

mental  functions  into  their  elements,  in  a  manner  suitable  for  physi- 
ological use,  has  scarcely  been  begun.  The  so-called  "facul- 
ties" of  memory,  imagination,  attention,  and  reasoning,  are  scarcely 
to  be  thought  of  in  this  connection.  They  are  gross,  unanalyzed 
terms  of  a  popular  psychology,  having  no  claim  to  represent  ele- 
mentary functions  of  the  mental  life.  The  results  of  local  injury 
to  the  cortex  show  that  memory  for  visual  objects  is  dependent 
on  the  integrity  of  the  occipital  lobes;  for  auditory  objects,  on  the 
temporal  lobes,  etc.;  and  the  same  thing  is  true  with  respect  to  the 
imagining  of  these  objects.  As  for  attention,  which  some l  have 
sought  to  localize  in  the  frontal  lobes,  experience  shows  that  any 
sort  of  thing  may,  on  occasion,  claim  the  attention  and  drive  out, 
or  inhibit,  other  claimants  to  control.  In  studying  spinal  inhibi- 
tion (see  pp.  172  f.)  the  fact  was  noted  that  different  reflex  functions 
exert  a  mutually  inhibitory  influence;  any  one  can,  on  occasion,  in- 
hibit the  others.  But  this  is  apparently  the  case  with  mental 
functions,  when  studied  from  the  point  of  view  of  introspection, 
as  well;  any  one  can,  on  occasion,  be  dominant  and  inhibit  others 
which  are  antagonistic  to  it.  No  one  centre  of  attention,  or  of  in- 
hibition, can  therefore  be  expected  to  be  discovered.  In  general, 
instead  of  seeking  to  localize  such  so-called  "  faculties,"  it  is  neces- 
sary to  start  with  complex  and  concrete  acts  and  processes,  and  by 
a  study  of  analysis  discover  the  more  simple  and  elementary  acts 
and  processes  which  have  a  value  for  the  ends  of  physiological 
psychology.  Such  analysis,  however,  is  extraordinarily  difficult; 
and  it  is  probable  that  in  the  future,  as  in  the  past,  the  efforts  of 
normal  psychology  will  need  to  be  supplemented  by  the  study  of 
mutilated  functions,  as  they  are  offered  for  observation  by  disease 
and  injury. 

In  spite  of  the  fact  that  it  has,  of  late,  been  much  decried  and  even 
ridiculed  in  certain  quarters,  we  shall  follow  this  method  of  careful 
and  minute  analysis  in  the  part  of  the  work  to  which  we  now  pro- 
ceed. In  this  part  it  is  our  purpose  to  point  out  certain  more  spe- 
cific forms  of  correlation  which  exist  between  our  psychical  ex- 
periences and  the  functions  of  the  nervous  mechanism,  when  under 
different  kinds  and  degrees  of  external  stimulation  and  variously 
influenced  by  acquired  habits  and  associations.  But,  first,  it  is  in 
place  to  summarize  our  conclusions  as  to  the  mechanical  nature  of 
this  system,  taken  as  a  whole. 

1  Most  prominently  Wundt,  Physiol.  Psychol,  6th  ed.,  1908,  I,  378. 


CHAPTER  XI 
MECHANICAL  THEORY  OF  THE  NERVOUS  SYSTEM 

§  1.  The  nature  of  the  process  by  which  the  nervous  system  is 
developed,  as  well  as  the  nature  of  the  developed  structure  and  its 
functions,  so  far  as  physical  science  can  go  at  all,  points  in  the  di- 
rection of  a  mechanical  theory.  But  in  respect  to  both,  such  a 
theory  is  at  present  in  an  exceedingly  fragmentary  and  uncertain 
condition.  Further  investigations  may  largely  remove  the  present 
limitations.  But  the  most  complete  theory  possible  can  scarcely 
be  more  than  a  statement  of  the  order  and  extent  of  physical  changes, 
the  real  causes  and  meaning  of  which  it  lies  beyond  the  power  of  a 
mechanical  theory  to  give. 

The  impregnated  ovum  does,  indeed,  become  converted  into  the 
developed  organism  by  an  evolution  that,  at  every  step  in  its  course, 
appears  as  an  alteration  in  the  arrangement  of  material  molecules, 
under  conditions  derived  from  the  original  nature  of  the  molecules 
themselves,  from  their  necessary  relations  to  each  other,  and  from 
the  action  of  their  total  environment.  By  division  of  that  which 
was  single  into  several  parts,  by  bending  of  that  which  was  straight, 
by  stretching  in  one  direction  and  compressing  elsewhere,  by  swell- 
ing and  dilating  in  the  various  outlines  under  the  influence  of  press- 
ure, by  folding  and  tucking  in  so  as  to  close  up  an  opening  here 
and  form  another  there,  by  laying  down  cells  of  the  same  kind  in 
right  lines  or  grouping  them  in  masses,  etc. — in  brief,  by  motion 
of  particles  of  matter  in  such  a  way  that  the  motion  of  each  is  con- 
ditioned upon  that  of  the  others,  the  nervous  mechanism  is  built 
up.  What  it  can  accomplish  in  the  way  of  further  molecular  mo- 
tion, after  it  is  thus  built  up,  depends  of  course  in  large  measure 
upon  what  it  is  made  to  be  by  the  very  process  of  building.  How 
far  it  is  possible  even  to  propound  a  mechanical  theory  of  the  build- 
ing process  belongs  to  the  speculations  of  embryologists  to  con- 
sider. It  is  our  problem,  at  present,  to  consider  as  a  whole  the  few 
data  upon  which  it  has  been  thought  possible  to  base  a  mechanical 
theory  of  the  behavior  of  the  nervous  system  after  it  has  once  been 
constructed  as  a  result  of  the  embryonic  process. 

§  2.  The  machine-like  nature  of  much  of  the  structure  and  move- 
ment of  the  human  body  does  not  escape  the  most  ordinary  obser- 

275 


276  THE  NERVOUS  MECHANISM 

vation.  When  this  body,  either  as  a  whole  or  with  respect  to  some 
of  its  parts,  changes  its  position  in  space,  its  various  masses  sup- 
port and  act  upon  each  other  in  essentially  the  same  manner  as 
the  masses  of  matter  which  compose  the  parts  of  any  machine  con- 
structed by  human  skill.  Such  movement  is  possible  for  it,  because 
its  framework  of  bones  has  a  rigidity  sufficient  to  sustain  the  other 
less  rigid  organs;  and  because  the  bones  are  so  divided,  and  yet 
articulated,  that  they  can  assume  different  relations  toward  one  an- 
other in  accordance  with  the  simplest  principles  of  mechanics. 
The  laws  of  the  lever,  of  the  pulley,  the  ball-and-socket  joint,  etc., 
need  no  modification  when  applied  to  this  particular  machine  of  the 
human  body. 

The  action  of  certain  other  of  its  parts,  which  do  not  belong 
to  the  bony  framework  but  which  are  of  muscular  or  epithelial 
structure,  is  also  plainly  of  the  same  machine-like  character.  The 
movement  of  the  heart,  for  example,  is  in  part  to  be  explained  as 
that  of  a  pump  with  chambers  and  valves;  and  the  flow  of  the 
blood  through  the  arteries  as  that  of  a  fluid  pumped  through  con- 
duits, of  unlike  and  changeable  sizes.  So,  too,  the  lungs  may  be, 
with  considerable  propriety,  compared  to  bellows  which  alternately 
suck  in  and  expel  the  surrounding  atmosphere.  The  optics  of  the 
eye  and  the  acoustics  of  the  ear  are  special  only  so  far  as  the  struct- 
ure of  the  organs  makes  necessary  a  special  application  of  the  gen- 
eral laws  of  those  sciences.  Moreover,  the  distribution  of  the  fluids 
among  the  tissues  of  the  body  takes  place — in  part,  at  least,  if  in  part 
only — under  the  laws  which  govern  the  distribution  of  fluids  generally 
when  separated  by  membranes  which  they  can  permeate.  Nor  is 
the  chemistry  of  the  same  tissues  and  fluids  by  any  means  wholly 
unlike  that  with  which  the  experiments  of  the  laboratory  make  us 
familiar.  When,  however,  we  begin  to  speak  of  those  changes  of 
relative  position  which  take  place  at  extremely  minute  distances 
among  the  molecular  elements  of  which  the  larger  masses  of  the 
body  are  composed,  we  seem  compelled  to  drop  the  conception  of 
a  machine  and  to  seek  both  another  conception  and  another  title. 

The  very  attempt,  then,  to  explain  the  motion  of  the  more  purely 
machine-like  parts  of  the  human  body,  leads  us  to  consider  certain 
activities  of  other  parts  for  which  the  word  "mechanism"  is  more 
appropriate.  The  movement  of  none  of  the  more  or  less  rigid  or- 
gans of  the  body  originates  within  these  organs  themselves.  The 
changes  of  relative  position  in  the  parts,  with  which  the  ordinary 
laws  of  mechanics  deal,  imply  antecedent  molecular  changes  in 
other  parts  with  which  these  laws  cannot  deal.  The  motion  which 
finds  its  final  expression  in  the  swing  of  the  arm,  or  of  the  leg,  in 
the  lifting  of  a  weight,  and  even  in  the  contraction  of  the  heart,  or 


MACHINE-LIKE  NATURE  OF  ORGANISM  277 

in  the  rising  and  falling  of  the  chest,  does  not  begin  in  arm,  or 
leg,  or  ribs,  or  diaphragm,  or  cardiac  muscles.  The  change  of  posi- 
tion of  so  considerable  masses  of  matter  is  but  the  summing-up  of 
innumerable  minute  molecular  changes  which  began  within  the 
body,  but  outside  of  the  masses  themselves.  If,  for  example,  we 
inquire  as  to  what  causes  the  bones  to  move — however  strictly  their 
motion,  once  begun,  may  follow  the  laws  of  mechanics — the  answer 
is  to  be  found  in  the  pull  of  the  tendons,  or  cord-like  structures, 
which  are  attached  to  them.  And  if  we  then  inquire,  What  causes 
the  tendons  to  pull  upon  the  bones  by  means  of  their  attachment  ? 
the  answer  must  be,  That  it  is  the  contraction  of  the  muscles  which 
pulls  upon  the  tendons. 

The  next  step  in  following  this  chain  of  causes,  however,  intro- 
duces us  to  a  different  class  of  considerations  from  any  of  the  fore- 
going. For  we  cannot  say  that  the  contraction  of  the  muscles  is 
caused  by  the  pull  of  the  nerves  upon  them.  The  movement  of 
muscular  fibre  in  contraction  is  an  altogether  different  affair  from 
the  movement  of  the  bones  as  they  are  pulled  by  the  muscles;  nor 
do  the  nerves  act  upon  the  muscles  as  the  muscles  act  upon  the 
tendons.  The  elasticity  of  the  muscles  is,  indeed,  a  mechanical 
quality,  like  that  of  which  we  avail  ourselves  in  the  construction  of 
machines.  But  the  quality  of  elasticity  does  not  fully  explain  the 
behavior  of  the  so-called  muscle-nerve  machine  when  its  muscular 
tissue  is  contracting  or  relaxing.  Yet  the  living  muscle,  in  itself 
considered,  may  certainly  be  looked  upon  as  a  molecular  mechan- 
ism. It  is  a  system  of  minute  particles  of  matter  which  act  upon 
each  other  at  indefinitely  small  distances;  and  which,  when  any 
motion  is  set  up  at  one  part  of  it,  propagates  such  motion  accord- 
ing to  laws  that  are  given  in  the  very  constitution  and  arrangement 
of  the  particles  themselves.  This  is  precisely  what  we  understand 
by  a  physical  molecular  mechanism.  The  office  of  the  nerve  with 
respect  to  the  muscle  is  simply,  as  we  know,  to  start  that  molecular 
activity  which  it  is  the  function  of  the  irritated  muscle  itself  to  ex- 
ercise. The  nerve,  however,  cannot  perform  its  office  of  irritating 
the  muscle  without  being  in  a  state  of  molecular  commotion  called 
the  "excitement"  of  the  nerve.  And,  further,  this  excited  condi- 
tion of  that  part  of  the  nerve  which  is  in  immediate  contact  with  the 
muscle  is  itself  a  state  of  the  nerve  which  has  been  propagated  from 
a  distant  point  of  the  nervous  matter.  All  the  machine-like  move- 
ments of  the  masses  of  the  body  require  us,  therefore,  to  look  for 
their  origin  in  minute  molecular  changes  that  originate  in  its  ner- 
vous elements.  And  for  the  further  account  of  these  neural  molec- 
ular changes  we  are  to  look  to  a  mechanical  theory  of  the  nervous 
system. 


278  THE  NERVOUS  MECHANISM 

§  3.  The  basis  for  a  general  view  of  the  nervous  system  as  a 
mechanism  has  been  laid  in  all  the  preceding  examination;  and 
it  cannot  be  denied  that  the  results  of  this  examination  are  such 
as  to  dispose  us  favorably  toward  the  attempt  to  develop  such  a 
view  into  a  complete  mechanical  theory.  Physical  science,  as  a 
matter  of  course,  strives  to  establish  such  a  theory.  It  knows  no 
other  way  of  considering  any  group  of  phenomena  when  brought 
before  it  for  examination.  To  deny  totally  the  application  of  the 
conception  of  a  mechanism  to  the  action  of  the  nervous  system 
would  be  to  refuse  to  apply  to  its  phenomena  the  same  scientific 
treatment  which  we  apply  to  all  other  physical  phenomena.  To 
limit,  a  priori,  such  application  would  be  to  restrict  improperly,  on 
merely  theoretical  grounds,  the  area  of  the  phenomena  with  which 
such  science  is  entitled  to  deal.  The  fact  that  molecular  changes 
here  are  correlated  with  another  class  of  phenomena  which  we  call 
"mental,"  in  no  wise  destroys  the  propriety  of  pushing  our  physical 
science  of  the  nervous  system  to  its  furthest  possible  limits.  The 
movements  of  all  material  bodies,  whether  in  the  elemental  shape 
of  the  molecules,  or  in  the  shape  of  the  same  molecules  when  aggre- 
gated into  masses,  as  well  as  the  laws  under  which  such  bodies  in 
movement  act  and  react  upon  each  other,  constitute  the  legitimate 
sphere  of  physical  science.  But  it  is  to  a  system  of  interacting 
molecules  that  the  conception  of  mechanism  especially  applies. 
The  aim  of  physical  research  with  regard  to  any  given  system  of 
this  kind  is,  therefore,  not  accomplished  until  all  the  movements 
of  its  different  parts  are  explained  in  the  light  of  a  consistent  me- 
chanical theory.  This  general  principle  of  all  physical  science 
neither  needs  nor  permits  a  special  exception  in  the'  case  of  the 
human  nerves,  organs  of  sense,  and  brain. 

On  the  other  hand,  the  very  unsatisfactory  condition  of  the  data 
for  a  mechanical  theory  of  the  human  nervous  system  has  been  im- 
plied in  each  of  the  preceding  chapters.  It  will  appear  all  the 
more  plainly  now  as  we  present  briefly  a  statement  of  two  or  three 
such  theories  in  the  form  in  which  it  has  been  found  possible  for 
different  investigators  to  state  and  to  defend  them.  Nor  can  we 
express  much  confidence  that  physics  and  physiology  combined 
will  ever  be  able  to  point  to  a  complete  theory  of  so  intricate  and 
delicate  a  mechanism  as  this  nervous  system.  Moreover,  we  do  not 
by  any  means  affirm  that  a  purely  mechanical  treatment,  however 
complete,  would  of  itself  suffice  to  furnish  a  satisfactory  understand- 
ing of  all  the  phenomena;  or  even  that  the  phenomena  in  general 
could  by  any  possibility  be  brought  solely  under  the  terms  of  such 
treatment.  We  only  affirm  the  unrestricted  right  of  physical  sci- 
ence to  attempt,  in  the  light  of  the  conception  of  mechanism,  an  ex- 


SIGNIFICANCE  OF  CHEMICAL  CONSTITUTION     279 

planation  of  the  nervous  system  as  well  as  of  all  other  physical 
objects;  and  also  its  right  to  its  persistent  faith  that — So  far  as 
physical  science  can  explain  any  object,  all  the  special  difficul- 
ties of  the  nervous  system  can  be  fitly  considered  only  in  this  way. 

§  4.  The  chemical  constitution  and  structural  form  of  the  ele- 
ments of  nervous  matter  require  that  the  system  which  they  com- 
pose should  be  regarded  in  the  light  of  the  conception  of  mechan- 
ism. It  is  true  that  physical  science  cannot  give  an  accurate  descrip- 
tion of  the  chemical  processes  which  take  place  in  the  formation  of 
the  nerve-fibres  and  nerve-cells,  or  during  their  functional  activity; 
it  cannot  do  so  much  as  this  for  the  living  tissues  generally.  But 
it  finds  here  the  same  chemical  elements  which  exist  elsewhere 
in  nature,  especially  the  four  elements,  oxygen,  hydrogen,  nitrogen, 
and  carbon.  It  nowhere  finds  these  elements  behaving  differently 
in  the  nervous  system  from  the  way  in  which  it  is  their  nature  to 
behave  elsewhere,  under  similar  circumstances.  And  the  fact  that 
precisely  similar  circumstances  do  not  occur  to  induce  the  same 
combination  and  interaction  of  these  elements  outside  of  the  ner- 
vous system,  is  traced  back  to  its  causes  in  a  succession  of  occur- 
rences that  all  have  the  character  belonging  to  the  chemistry  of 
living  tissues.  We  know  of  no  sap  which  is  suitable  for  forming 
organisms  in  general,  but  which  is  itself  a  perfectly  homogeneous 
fluid.  Nucleated  granules  in  the  very  chemical  constituents  which 
give  conditions  to  all  the  subsequent  activity  of  the  molecules,  are 
revealed  by  microscopic  examination  of  those  cells  from  which  the 
whole  body  springs.  This  fact,  together  with  the  character  of  the 
subsequent  process,  may  lead  some  to  insist  that  a  certain  special 
form  of  energy  (called  "vital  force,"  or  by  some  less  obnoxious 
title)  is  marshalling  the  minute  particles  under  its  superior  con- 
trol. But  such  way  of  considering  the  phenomena — whether  ad- 
missible or  inadmissible — does  not  at  all  help  us  to  dispense  with  the 
purely  mechanical  point  of  view.  In  the  original  living  germ  with 
which  the  organism  began,  and  in  all  its  subsequent  development, 
every  chemical  change  in  nervous  matter  is  nothing  more  than  a 
movement  of  physical  molecules,  strictly  under  the  conditions  fur- 
nished by  their  constitution  and  previous  arrangement. 

§  5.  The  general  significance  of  the  form  and  chemical  consti- 
tution of  the  nervous  tissue  is  by  no  means  wholly  obscure.  Es- 
pecially is  this  true  in  regard  to  the  form  of  the  minute  structures 
which  compose  the  system.  The  long  wire-like  form  of  the  nerve- 
fibre  is  understood  by  reference  to  its  function  of  conduction.  Since 
the  nerve-fibre  does  nothing  which  is  directly  beneficial  to  the  body, 
but  simply  serves  to  transmit  "impulses"  which  shall  arouse  ac- 
tivity in  the  muscles  and  other  effective  organs,  the  smaller  the  size 


280  THE  NERVOUS  MECHANISM 

of  the  nerve-fibre,  the  better.  Within  the  central  organs,  the  rami- 
fying of  the  fine  branches  of  the  fibre  and  of  the  cell  seems  fitted  for 
the  function  which  the  centres  are  known  to  exert, — namely  that  of 
providing  for  co-ordinated  action,  and  also  for  action  which  can 
vary  according  to  circumstances.1 

In  regard  to  chemical  composition,  less  is  perhaps  known  that 
affords  an  insight  into  the  mechanism  of  nervous  activity.  The 
protoplasmic  portions  of  the  cell-body,  dendrites  and  axon,  seem  to 
differ  only  in  minor  respects  from  the  protoplasm  of  other  tissues. 
The  lipoids  of  the  sheath  seem  to  play  a  part  as  insulators  or  non- 
conductors. 

§  6.  There  can  be  no  doubt  that  the  arrangement  of  the  nervous 
elements  into  a  system  corresponds  to  the  conception  of  mechanism. 
A  certain  work  of  concatenating  the  different  physical  systems  of 
the  body,  and  of  adjusting  its  relations  to  the  changes  in  its  en- 
vironment, requires  to  be  accomplished.  This  problem  demands  a 
three-fold  exercise  of  function;  it  is  a  problem  in  the  construction 
of  a  mechanism.  The  nervous  system  actually  is  of  threefold  con- 
struction; its  threefold  construction  is  the  answer  which  it  prac- 
tically makes  to  the  above-mentioned  problem.  One  part  of  the 
complex  problem  consists  in  the  conversion  of  certain  of  those 
molecular  motions  which  take  place  in  nature  outside  of  the  living 
organism  into  molecular  motion  within  the  tissues  of  such  organism. 
The  solution  of  this  part  of  the  problem  is  furnished  by  the  end- 
organs  of  the  nervous  system.  The  end-organs  are  those  -special 
mechanisms  which  are  adapted  to  convert  the  molecular  motions 
called  stimuli  into  the  molecular  motions  called  neural  excitation. 
That  by  far  the  larger  portion  of  the  eye  and  ear,  for  example,  acts 
in  a  purely  mechanical  way,  there  is  no  doubt.  It  is  the  office  of 
the  great  mass  of  the  eye  to  transmit  and  refract  the  rays  of  light; 
of  the  ear  to  transmit  and  condense  the  acoustic  waves.  But  when 
the  nervous  elements  of  the  retina  and  of  the  organ  of  Corti  re- 
ceive the  physical  processes  transmitted  to  them,  they  transmute 
these  physical  processes  into  physiological  neural  processes;  in 
doing  this  they  act  as  special  molecular  mechanisms. 

The  second  part  of  the  complex  problem  before  the  nervous  sys- 
tem consists  in  the  conduction  in  all  necessary  directions  of  these 
neural  processes;  only  on  this  condition  can  distant  parts  of  the 
nervous  system  act,  as  it  were,  in  view  of  each  other,  and  thus  the 

1  It  is  true  that  we  cannot  describe  the  peculiar  place  in  the  mechanism,  or  the 
specific  chemistry — if  such  there  be — which  belong  to  the  different  locations  and 
shapes  of  nerve-cells,  bipolar,  multipolar,  stellate,  etc.  But  all  that  we  are  learn- 
ing about  the  minute  structure  and  detailed  functions  of  the  nervous  elements 
seems  to  indicate  that  these  more  obvious  mechanical  characteristics  are  of 
value  in  the  entire  complex  working  of  the  nervous  system. 


ARRANGEMENT  OF  THE  NERVOUS  ELEMENTS    281 

whole  body  be  bound  into  a  living  unity  under  the  influence  of 
changes  in  its  environment,  and  in  the  ideas  and  impulses  of  the 
mind.  The  nerve-fibres  solve  this  part  of  the  problem.  This  they 
do  by  acting  as  mechanisms,  which  have  such  a  molecular  constitu- 
tion and  function  that  a  commotion,  started  at  any  point  in  the 
physical  elements  of  the  system,  spreads  from  molecule  to  molecule, 
in  accordance  with  the  laws  of  the  system. 

The  third  part  of  the  same  complex  problem  requires  for  its  solu- 
tion structures  and  functions  still  more  intricate  and  inexplicable. 
Incoming  molecular  disturbances  must  be  modified  and  redistrib- 
uted so  as  to  give  rise  to  outgoing  molecular  disturbances  along 
definite  tracts,  in  order  that  definite  groups  of  muscles  may  be  made 
to  contract.  Only  in  this  way  can  the  whole  physical  organism,  by 
a  so-called  reflex  activity,  adjust  its  condition,  in  view  of  the  pres- 
ence of  given  kinds  and  degrees  of  stimuli.  Moreover,  the  vital 
functions — the  movements  that  control  respiration,  digestion,  cir- 
culation of  the  blood  and  of  other  fluids,  etc. — must  be  united  so  as 
to  work  to  a  common  end,  and  with  the  modified  forms  and  degrees 
of  their  respective  energies,  which  the  changing  circumstances  re- 
quire. Still  further,  not  only  must  the  neural  processes  set  up  by 
the  end-organs  and  conducted  inward  by  the  afferent  nerves  have 
a  place  of  meeting  in  proximity  with  the  centres  of  origin  for  the 
corresponding  efferent  impulses;  but  all  the  neural  processes  in  this 
place  of  meeting  must  also  be  so  modified  and  made  mutually  de- 
pendent that  they  can  be  correlated,  under  psycho-physical  laws, 
with  the  processes  of  mind.  It  is  the  central  organs  which  alone 
possess  the  molecular  construction  and  functions  necessary  for  such 
wonderful  reflex  and  automatic  activities.  In  their  highest  form — 
the  hemispheres  of  the  human  brain — they  solve  the  problem  of 
providing  a  system  of  molecules,  whose  constitution  and  changes 
may  be  immediately  related  with  the  phenomena  of  mind.  These 
central  organs  are  extremely  intricate  physical  structures.  It  can- 
not be  pretended  that  even  a  beginning  has  been  made  toward  a 
satisfactory  theory  of  their  functional  activity  considered  as  a  special 
case  in  molecular  physics.  But  this  fact  does  not  affect  the  con- 
fidence which  is  based  upon  what  is  known  of  physical  structures 
in  general,  that  in  these  organs,  the  changes  which  take  place  are 
essentially  of  the  same  order  as  are  those  with  which  the  science  of 
molecular  physics  has  elsewhere  to  deal.  They  are  modes  of  motion 
in  which  the  behavior  of  each  molecule,  regarded  as  a  constituent 
element  of  the  system,  is  conditioned  upon  the  constitution  and 
behavior  of  the  other  members  of  the  same  system.  That  is  to 
say,  the  central  organs  must  be  regarded  in  the  light  of  the  con- 
ception of  mechanisin. 


282  THE  NERVOUS  MECHANISM 

§  7.  The  general  office  of  the  nervous  system  may,  then,  be  de- 
scribed in  somewhat  the  following  manner.  The  development  of  a 
rich  and  varied  life,  both  animal  and  intellectual,  requires  a  great 
store  of  sensations  and  of  motions.  The  sensations  are  primarily 
designed  to  serve  as  signs  of  changes  in  the  environment  of  the 
animal  to  which  his  condition  must  be  adapted  by  movement  of 
his  bodily  parts;  but  they  are  also  to  serve  as  a  basis  for  intel- 
lectual attainment  and  development.  The  forces  of  external  nature 
continually  storm  the  peripheral  parts  of  the  animal's  body.  In 
order  that  any  of  these  forces  may  act  as  the  stimuli  of  sensations, 
they  must  be  converted  into  molecular  motions  within  the  tissues 
of  this  body.  In  order,  further,  that  the  masses  of  the  body  may 
constantly  be  readjusted  to  the  external  changes  of  which  the  sen- 
sations are  signs,  the  molecular  motions  must,  in  turn,  be  converted 
into  movements  of  these  masses.  In  other  words,  a  process  of  con- 
stant interchange  must  take  place  between  the  animal  organism 
and  external  nature. 

Disturbances  in  one  part  of  the  body,  by  the  play  upon  it  of  nat- 
ure's energy,  instead  of  becoming  injurious  or  destructive,  are 
thus  made  serviceable  through  inducing  the  needed  disturbances 
of  other  parts  of  the  same  body.  The  equilibrium  on  which  life 
depends  is  maintained.  Moreover,  the  material  necessary  for  self- 
conscious  development,  for  a  growing  knowledge  of  the  so-called 
outside  world,  is  furnished  through  the  conduction  of  these  dis- 
turbances to  their  common  meeting-places  in  the  central  organs. 
The  nervous  system,  especially  in  its  supreme  central  organs,  must 
therefore  be  the  kind  of  mechanism  that  can — so  to  say — be 
"  trained  "  into  co-ordinated  forms  of  action,  so  as  to  serve  as  a  basis 
for  the  mental  activities  involved  in  processes  of  learning. 

§  8.  To  accomplish  the  general  work  of  equilibrating  the  inter- 
action of  the  different  parts  of  the  body,  of  readjusting  its  condition  to 
the  changing  condition  of  its  surroundings,  some  special  construc- 
tion and  arrangement  of  material  molecules  is  necessary.  If  the 
work  is  to  be  done  in  a  highly  elaborate  way,  a  very  intricate  ar- 
rangement of  an  indefinitely  great  number  of  chemically  complex 
molecules  is  necessary.  Such  an  arrangement  is  the  human  ner- 
vous system.  But  just  because  its  arrangement  and  function  are 
of  this  kind,  it  is  a  "mechanism."  As  a  highly  complex  molecular 
mechanism  it  utilizes  the  disturbances  which  arise  from  the  en- 
vironment. It  binds  together  all  the  other  systems  of  the  body  in 
living  reciprocity  of  energies  and  functions.  Its  superficial  parts 
are  so  constructed  that  they  can  be  set  in  motion  by  various  forms 
of  physical  energy — by  light,  heat,  sound,  chemical  change,  etc.; 
they  are  also  adapted  fitly  to  modify  the  impressions  thus  received. 


EQUILIBRATING  OF  DIFFERENT  PARTS  283 

The  molecules  of  its  conducting  nerves  are  so  constituted  and  ar- 
ranged that  they  can  indicate  the  path  along  which  the  disturbance 
thus  occasioned  must  pass;  they  can  dictate  the  conditions  and 
laws  under  which  its  course  must  be  completed.  The  molecules  of 
its  central  organs  are  capable  of  assuming  inconceivably  varied  re- 
lations to  each  other,  of  thus  transmuting  and  redistributing  the 
nerve-commotions  which  reach  them  along  the  incoming  tracts,  and 
even  (it  would  seem)  of  starting  automatically  outgoing  disturb- 
ances in  response  to  self-conscious  sensations  and  ideas. 

But  all  the  foregoing  offices  of  the  nervous  system  are  nothing 
but  the  movements  of  physical  elements,  in  constant  reciprocal  de- 
pendence upon  each  other,  though  in  response  to  excitations  lying 
outside  of  the  system  itself.  To  move  thus  is  the  function  of  a 
molecular  mechanism.  So  far  as  science  can  control  the  different 
parts  of  the  nervous  system  for  experimental  purposes,  it  finds 
them  behaving  in  such  a  manner  as  to  make  a  plain  demand  for  a 
physical  and  mechanical  theory  in  explanation  of  their  behavior. 

§  9.  The  foregoing  description  of  the  nervous  system  as  a  mech- 
anism, like  all  similar  descriptions,  undoubtedly  lacks  scientific 
quality.  It  is  neither  exact  nor  in  such  form  as  to  admit  of  ex- 
perimental verification.  It  is  largely  based  upon  conjectures,  full 
of  gaps  and  assumptions;  and  were  it  pressed  at  every  point  for 
proof,  it  would  be  obliged  to  rely  much  upon  general  principles  in 
mechanics  (the  special  applications  of  which  to  the  case  in  hand  are 
by  no  means  certain  or  obvious),  and  even  to  indulge  in  hopes  and 
promises  with  reference  to  the  future  rather  than  present  demon- 
stration. May  we  not  know  more  precisely  the  nature  of  the  mo- 
lecular changes  which  constitute  the  functions  of  nerve-fibres  and 
nerve-cells  ?  Cannot  physical  science  help  us  to  complete  these  be- 
ginnings of  a  theory? 

§  10.  In  answering  the  question  just  raised,  we  have  to  consider 
separately  the  case  of  the  nerve-fibre  and  that  of  the  nerve-centre. 
The  former  is  much  the  simpler  problem,  and  there  is  more  of  ex- 
perimental knowledge  to  form  the  basis  of  a  theory  for  its  solution. 
In  a  preceding  chapter  (pp.  135  ff.)  we  noted  the  disagreement  be- 
tween physiologists  as  to  whether  the  activity  of  a  nerve  is  to  be 
conceived  of  as  a  chemical  process,  which  involves  the  consumption 
of  fuel,  and  the  setting  free  of  potential  energy;  or  whether,  on  the 
contrary,  the  nerve  impulse  is  a  purely  physical  process,  like  the 
transmission  of  a  wave  over  the  surface  of  water,  or  of  electricity 
along  a  wire.  It  appears  that  in  the  former  case,  the  amount  of 
catabolism  in  the  nerve  must  be  exceedingly  slight,  and  the  recovery 
of  the  fibre  exceedingly  prompt.  This  absence  of  signs  of  chemical 
change  in  the  active  nerve,  we  then  saw,  has  led  a  group  of  physi- 


284  THE  NERVOUS  MECHANISM 

ologists  to  regard  the  nerve-impulse  as  an  electrical  affair,  similar 
to  a  movement  of  ions  along  a  "core  conductor."  In  the  interest 
of  reconciling  these  views  it  was  suggested  (foot-note,  p.  143)  that  a 
mechanical  theory  of  the  nerves  and  their  functions,  which  shall  ac- 
count for  all  the  phenomena,  is  entirely  feasible  and  consistent  with 
what  is  known  of  chemical  change,  as  related  to  electrical  conduction, 
in  other  cases.  It  is  a  reasonable  hope,  then,  that  in  the  not  distant 
future  the  facts  of  "summation/'  "interference,"  "facilitation,"  etc., 
may  be  statable  in  terms  of  such  a  combination  theory.  It  is  sig- 
nificant that  these  very  terms  are  applicable  only  to  phenomena 
which  admit  of  a  mechanical  explanation. 

§  11.  Bound  up  with  this  is  the  further  question,  whether  the 
nerve-impulse  in  all  nerve-fibres  is  of  the  same  quality;  or  whether 
different  nerves  have  different  "specific  energies,"  to  use  a  term 
introduced  by  the  celebrated  physiologist,  Johannes  Miiller.  Miil- 
ler  called  attention1  to  the  fact  that  the  optic  nerve,  whether  excited 
normally  through  the  action  of  light  on  the  retina,  or  artificially  by 
the  passage  of  an  electric  current  through  the  forehead,  or  by  a  blow 
on  the  head,  always  gives  rise  to  a  sensation  of  light.  Similarly, 
each  of  the  sensory  nerves,  however  excited,  gives  rise  to  a  specific 
sensation.  He  accordingly  interpreted  these  facts  to  mean  that 
"Either  the  nerves  themselves  may  communicate  impressions 
different  in  quality  to  the  sensorium" — or  brain — "which  in  every 
instance  remains  the  same;  or  else  the  vibrations  of  the  nervous 
principle  may  in  every  nerve  be  the  same  and  yet  give  rise  to  the 
perception  of  different  sensations  in  the  sensorium,  owing  to  the 
parts  of  the  latter  with  which  the  nerves  are  connected  having  dif- 
ferent properties.  The  proof  of  either  of  these  propositions  I  re- 
gard as  at  present  impossible."  Miiller,  however,  inclined  to  the 
former  opinion — namely,  that  each  sensory  nerve  has  an  action  pe- 
culiar to  it,  a  "specific  energy,"  differing  from  the  energy  of  all 
other  nerves.  With  the  progress  of  the  study  of  the  nerves,  the  ap- 
parent similarity  in  the  action  of  all  of  them  has  led  most  physiolo- 
gists to  the  second  member  of  the  alternative,  according  to  which 
the  specific  quality  is  no  longer  believed  to  inhere  in  the  action  of 
nerve-fibres,  but  rather  in  the  organs,  central  or  peripheral,  with 
which  they  are  connected.  Thus,  in  the  case  of  motor  nerves,  the 
nerve-fibres  which  run  to  the  biceps  muscle  seem  not  to  differ  in 
their  mode  of  action  from  the  fibres  running  to  the  triceps,  or  to 
any  other  muscle.  These  nerves  are,  accordingly,  regarded  as  in- 
different conductors;  and  the  difference  in  the  motor  results  of  their 
activity  is  believed  to  inhere  simply  in  the  muscles  to  which  they 

1  In  1844,  in  his  Physiology.    See  the  English  translation,  1848,  pp.  1059  ff. 


THEORY  OF  SPECIFIC  ENERGIES  285 

lead,  and  in  the  attachments  of  these  muscles  to  the  bones.  This 
may  be  affirmed  to  be  true,  in  general,  of  motor  nerve-fibres. 

§  12.  Strong  additional  evidence  in  favor  of  this  view  is  obtained 
by  "crossing"  two  nerves.  If,  for  example,  the  nerve  A  leads  to 
the  muscle  a,  and  nerve  B  to  muscle  6,  and  if  both  nerves  are  cut, 
and  the  central  end  of  A  sutured  to  the  peripheral  end  of  B,  and 
vice  versa,  then,  after  time  has  been  allowed  for  the  regeneration 
of  the  peripheral  ends,  nerve  A  excites  muscle  b,  and  nerve  B  mus- 
cle a,  so  that  muscle  a  contracts  when  muscle  b  should  contract,  and 
vice  versa.  That  is  to  say,  the  impulses  issue  from  the  nerve-centres 
in  the  normal  way,  but,  the  nerves  being  crossed,  the  impulses  are 
conveyed  to  the  wrong  muscle.  It  appears,  however,  that,  with 
practice,  a  fair  degree  of  co-ordination  can  be  newly  learned.  But 
the  conclusion  from  the  experiment,  from  our  present  point  of  view, 
is  that  nerves  are  capable  of  exciting  other  muscles  than  those  to 
which  they  normally  lead.  The  motor  nerves  are  thus,  to  a  large 
degree  at  least,  interchangeable,  and  have  no  specific  energies  of 
their  own,  but  differ,  as  was  said,  only  in  their  connections. 

In  regard  to  the  sensory  nerves,  equally  conclusive  evidence  does 
not  seem  to  be  at  hand;  but  the  prevalent  view  is,  probably,  that 
the  action  of  the  fibres  of  the  optic  nerve  differs  in  no  essential 
respect  from  the  action  of  the  fibres  of  the  auditory  nerve,  or  of 
other  sensory  nerves;  the  same  principle,  therefore,  holds  here  as 
in  the  case  of  the  motor  nerves — namely,  that  the  difference  be- 
tween them  lies  in  their  connections.  In  the  case  of  the  sensory 
nerves,  the  connections  to  be  considered  are  central — the  connec- 
tions of  the  incoming  sensory  fibres  with  motor  fibres  or  with  cen- 
tral cells.  The  differences  in  conscious  sensation  between  the  sen- 
sory nerves  would,  accordingly,  be  associated  with  differences,  not 
in  these  nerves  themselves,  but  with  differences  in  the  cortical 
centres  to  which  these  nerves  lead.  Visual  sensations,  that  is, 
would  result  from  the  activity  of  the  visual  area,  no  matter  by  what 
means  it  is  excited;  and  the  same  for  the  other  sensory  areas.  It 
has  been  objected  by  Hering1  that  transferring  the  specific  quality 
from  the  nerves  to  the  sensory  areas  does  not  make  the  differences 
between  sensations  any  more  intelligible,  and  that  our  methods  of 
studying  nerve-fibres  are  much  too  crude  to  detect  fine  differences 
in  the  character  of  their  activity,  should  such  exist.  The  objec- 
tion does  not,  however,  counterbalance  the  evidence  derived  from 
more  recent  cases  of  anastomosis,  where  mixed  nerves  are  involved. 
For  it  has  been  found  that,  when  the  juncture  between  the  cut  ends 
of  the  two  nerves  is  made  perfect,  and  time  is  allowed  for  healing 

1  Zur  Theorie  der  Nerventdtigkeit  (Leipzig,  1899). 


286  THE  NERVOUS  MECHANISM 

and  for  the  effects  of  practice,  both  the  sensory  and  the  motor  func- 
tions of  such  a  nerve  can  be  performed  by  another  nerve,  whose 
normal  connections  commit  it  to  quite  a  different  and  even  distant 
area  of  the  body  (compare  p.  243). 

§  13.  In  regard  to  the  mechanism  of  the  nerve-centres,  a  con- 
siderable change  of  opinion  seems  to  be  taking  place  in  the  minds 
of  students  on  the  subject.  In  the  early  days  after  the  microscope 
had  revealed  the  cell  elements  in  the  gray  matter,  and  after  the 
fact  was  established  that  the  gray  matter  constituted  the  real  cen- 
tral organs  of  the  nervous  system,  it  seemed  almost  self-evident  that 
the  nerve-cells  were  the  essential  structures  of  the  nerve-centres; 
and  that,  therefore,  the  mechanics  of  the  centres  was  the  mechanics 
of  the  nerve-cell.  These  cells  were  regarded  as  exercising  control 
over  the  motor  nerves  and  so  over  the  muscles,  much  as  a  general, 
seated  in  his  tent,  exercises  control  over  an  army.  Co-ordination 
was  supposed  to  be  the  special  function  of  the  nerve-cells.  They 
were  also  believed  to  contain  large  stores  of  potential  energy  which 
they,  at  the  time  of  their  activity,  discharged  along  the  nerve-fibres 
leading  from  them.  The  cells  in  the  sensory  areas  of  the  brain 
were  regarded  as  the  essential  termini  of  the  sensory  nerves,  and  the 
activity  of  these  and  other  cells  in  the  brain  was  supposed  to  be  cor- 
related with  consciousness.  Memories  were  spoken  of,  metaphori- 
cally, as  being  "stored"  in  the  cell; — the  meaning  being  that  modifi- 
cation of  the  cells  by  any  experience  is  the  physical  condition  of 
the  later  revival  of  the  experience.  In  all  respects,  the  nerve-cells 
were  regarded  as  the  organs  of  reflex  and  mental  activities. 

§  14.  With  the  progress  of  histology,  it  became  evident,  however, 
that  the  nerve-cells  are  only  a  part  of  the  gray  matter,  and,  in  bulk, 
a  small  part.  The  branches  of  the  cells  were  seen  to  be  fully  as 
characteristic  a  feature  of  the  gray  matter,  and  to  fill  much  more  of 
the  cranial  space.  As  the  conception  of  the  nerve-fibres  and  their 
branches  as  conductors  gained  precision,  the  view  came  into  prom- 
inence, that  the  connections  established  by  the  fine  branches  of 
cells  in  the  gray  matter  are  the  important  fact.  This  view  assigns 
more  importance  to  the  branches  by  which  the  connections  are  made 
than  to  the  cell-bodies  themselves.  The  neurone  conception,  too 
(see  pp.  110  ff.),  added  force  to  the  same  view;  for  it  holds  that  the 
branches  of  the  different  cells  simply  come  into  contact  or  close 
proximity  with  each  other,  so  that  the  surfaces  of  contact  would  be, 
in  all  probability,  the  most  critical  points  in  the  system  of  nervous 
communication.  Within  any  one  neurone,  all  parts — cell-body, 
axon,  and  dendrites — are  in  continuity,  and  apparently  conduction 
is  free  throughout.  Between  one  neurone  and  another,  conduction 
is  less  free,  because  of  the  lack  of  perfect  continuity.  Nerve-im- 


MECHANISM  OF  THE  NERVE-CENTRES  287 

pulses  must  indeed  pass  from  one  neurone  to  another;  but  the  pas- 
sage would  probably  be  more  difficult  than  from  one  part  to  another 
of  a  single  neurone.  Therefore  the  delay  which  occurs  in  the  trans- 
mission of  a  nerve-impulse  through  a  nerve-centre  is  localized  in 
the  passage  from  one  neurone  to  another.  That  is,  the  delay  oc- 
curs at  the  "synapse." 

§  15.  Now,  delay  in  transmission  is  a  type  of  several  of  the  pe- 
culiarities of  central  nervous  action.  One  of  these  is  the  "blocking" 
of  nerve-impulses.  In  general,  this  seems  due  to  the  fact  that  the 
impulse  can  pass  from  the  terminal  branches  of  an  axon  over  to  the 
dendrites  of  another  cell,  but  not  in  the  reverse  direction.  This 
fact  means  that  the  synapse  is  the  place  at  which  such  a  peculiarity 
of  central  conduction  occurs.  Oftentimes,  also,  an  impulse  is 
blocked  in  the  forward  direction.  This  is  the  case  when  anaes- 
thetics act  on  the  centres;  and  since  nerve-centres  are  more  suscepti- 
ble than  nerves  to  the  action  of  ether,  chloroform,  and  alcohol,  the 
probability  is  that  the  loss  of  function  brought  on  by  anaesthetics 
is  due  to  the  blocking  of  the  synapse.  Still  other  instances  of 
blocking  a  nerve  impulse  on  its  way  through  a  nerve-centre  are  seen 
in  the  phenomena  of  inhibition;  and  here,  again,  the  continuity  of 
structure  between  the  various  parts  of  a  neurone  makes  the  view 
probable  that  all  these  blocks  occur  at  the  place  of  separation  be- 
tween one  neurone  and  another.  In  fine,  it  seems  possible  to  con- 
ceive of  the  action  of  the  nerve-centres  as  a  process  of  the  trans- 
mission of  nerve-impulses  that  is  subject  to  the  peculiarities  of  cen- 
tral conduction.1  Most  of  these  peculiarities  can  be  stated  in  terms 
of  resistance — resistance  in  general  high,  but  variable  with  many  con- 
ditions. If  the  highest  resistance  in  the  path  of  a  nerve-impulse 
through  the  centre  lies  at  the  synapse,  then  this  would  be  the  crit- 
ical and  typical  part  of  the  centre.  Thus,  for  example,  memory 
would  not  consist  so  much  in  a  modification  of  the  cell-bodies  as 
in  the  improvement  of  synaptic  connections  between  different  cells. 

§  16.  Our  reasoning  on  this  subject,  it  is  admitted,  has  about  it 
much  that  is  vague  and  uncertain.  It  is  founded  partly  on  the  ap- 
parent structure  of  the  gray  matter,  and  partly  on  the  conception 
of  nerve-action  as  being  essentially  that  of  conduction.  There 
are,  however,  a  few  experiments  which  throw  additional  light  on 
the  mechanism  of  the  nerve-cells.  If  nerve  function,  even  in  the 
gray  matter,  is  essentially  conduction,  then  the  cell-bodies  might 
be  dispensed  with,  were  it  not  for  the  fact  that  they  are  usually  in- 
terposed between  dendrites  and  axon,  and  so  form  a  link  in  the  chain 
of  conduction;  and  were  it  not  for  the  further  fact,  that  they  are 

1  For  a  fuller  account  of  these  peculiarities,  see  Sherrington,  Integrative  Action 
of  the  Nervous  System,  p.  11. 


288  THE  NERVOUS  MECHANISM 

certainly  necessary  for  the  nutrition  and  continued  life  of  all  parts 
of  the  neurone.  If,  however,  the  cell-bodies  could  be  cut  out  of  a 
nerve-centre  without  interrupting  the  continuity  between  the  den- 
drites  of  each  cell  and  the  axon,  such  an  operation  would  not  neces- 
sarily at  once  destroy  the  function  of  the  nerve-centre.  Even  the 
conception  of  such  an  operation  seems  wild  enough  when  only  the 
vertebrate  cord  and  brain  are  considered;  but  in  some  invertebrates 
the  motor  neurones  are  unipolar — dendrites  and  axon  being  di- 
rectly continuous — while  the  cell-body,  with  its  nucleus,  lies  off  to 
the  side,  connected  with  the  rest  by  a  slender  strand.  In  the  crab, 
for  example,  the  cell-bodies  are  bunched  together  at  the  outside 
of  each  ganglion,  and  can  be  destroyed  without  seriously  injuring 
the  axons  and  dendrites  with  their  fine  branches  and  connections. 
The  experiment  has  been  tried  by  Bethe,1  with  the  very  striking 
result  that  the  reflex  functions  of  a  ganglion  are  retained  after  its 
cell-bodies  have  been  removed.  To  be  sure,  the  function  is  not 
retained  permanently  in  the  absence  of  the  cells,  but  it  is  retained 
for  a  few  days;  and  even  if  it  were  only  retained  for  a  few  hours,  the 
evidence  would  be  conclusive  to  show  that  normal  activity  of  the 
ganglion  can  occur  without  the  presence  of  nerve-cells.  Accordingly 
the  activity  of  a  nerve-centre  is  not  essentially  the  activity  of  the  cell- 
bodies  in  it.  The  evidence  applies  in  the  first  instance  only  to  the 
crab,  but  we  have  nothing  of  a  contrary  teaching  in  regard  to  verte- 
brates. 

§  17.  If,  then,  the  query  should  be  raised  anew  as  to  what  is  the 
function  of  the  nerve-cells,  the  answer  may  be  found  by  recalling 
that  the  condition  of  the  centre  slowly  deteriorates  after  the  removal 
of  the  cell-bodies.  A  nerve-fibre,  too,  when  cut  off  from  its  cell  of 
origin,  undergoes  degeneration.  Similar  facts  are  true  of  other 
than  nervous  tissue.  The  nucleus  of  a  cell  is  necessary  to  the  nu- 
trition and  good  condition  of  the  whole  cell,  and  any  part  severed 
from  the  nucleus  suffers  and  usually  degenerates.  Now  the  cell- 
body  of  a  neurone — the  "cell"  as  we  usually  call  it — is  principally 
distinguished  as  the  part  of  the  neurone  containing  the  nucleus. 
We  may  then  be  sure  that  the  cell-body  has  the  highly  important 
function  of  serving  the  nutrition  of  the  whole  neurone;  it  is  neces- 
sary for  maintaining  the  axon  and  dendrites  in  proper  condition 
for  work,  even  though  it  may  take  no  peculiar  part  in  the  actual 
doing  of  the  work. 

§  18.  If  we  accept  the  view  that  the  synapse  is  the  locus  of  the 
most  important  peculiarities  of  central  function,  our  inquiry  be- 
comes: Is  it  possible  to  form  a  reasonable  conception  of  the  mechanics 

1  "Das  Zentralnervensystem  von  Carcinus  Maenas,"  in  Archiv.  f.  mikroscop. 
Anatomic,  1897,  L,  629  ff. 


SIGNIFICANCE  OF  THE  SYNAPSE  289 

of  the  synapse  ?  In  answer  to  this  question,  a  theory  which  has  the 
merit  of  simplicity  and  tangibility  has  been  put  forward  by  Duval 1 
and  others.  Duval  supposes  that  the  fine  branches  of  axons  and 
dendrites,  which  by  their  close  proximity  to  each  other  form  the 
synapse,  have  the  power  of  motility,  in  much  the  same  way  as  that 
shown  by  the  amoeba  (compare  pp.  14  f.).  As,  then,  the  amoeba 
puts  out  temporary  branches,  but  retracts  them  under  certain  con- 
ditions, so,  according  to  this  theory,  the  fine  branches  of  axons 
and  dendrites  can  be  thrust  out  under  certain  conditions  and  re- 
tracted under  others.  When  they  are  thrust  out  they  come  into 
closer  contact;  when  they  are  retracted  they  separate.  As  the  con- 
duction across  a  synapse  would  naturally  be  better  the  closer  the 
contact  between  the  branches  forming  it,  the  protrusion  of  the 
branches  would  mean  better  conduction  through  the  centre,  and  the 
retraction  of  them  poorer  conduction,  or  even  a  blocking  of  the  path. 
Special  application  of  this  theory  has  been  made  to  the  case  of  un- 
consciousness from  anaesthetics  or  from  fatigue.  It  has  been 
claimed  that  the  retraction  of  the  dendrites  would  block  the  path  of 
impulses,  and  thus  reactions  to  stimuli  would  be  prevented.  The 
promoters  of  this  theory  also  believed  they  had  evidence  that  the 
fine  branches  of  the  dendrites  were  shorter  in  animals  subjected  to 
ether,  than  in  animals  killed  suddenly  without  the  use  of  anaes- 
thetics. In.  general,  however,  the  evidence  is  against  any  power 
of  motility  mtKe  nerve-cell  or  its  branches;  and  the  theory  does 
not  command  the  assent  of  the  best  authorities.  It  may,  however, 
serve  a  useful  purpose  as  giving  a  sort  of  rough  diagram  of  what 
goes  on  in  the  synapse.  There  need  be  no  actual  motion  of  the 
branches  as  wholes;  but  there  may  be  molecular  motions  within 
them,  or  chemical  changes  within  them,  which  would  have  the 
same  effect  of  increasing  or  decreasing  the  conductivity  of  the 
synapse. 

§19.  In  view  of  the  attractiveness  of  the  electrical  theory,  as  ap- 
plied to  the  nerve-fibre,  it  will  be  of  much  interest  to  see  whether 
the  same  theory  can  be  extended  to  give  a  reasonable  account  of  the 
action  of  the  synapse.  This  has  been  attempted  with  considerable 
success  by  Sherrington2.  There  is  a  certain  degree  of  discontinu- 
ity at  the  synapse;  it  is  a  boundary  between  cells,  as  is  seen  in  this 
fact,  among  others,  that  the  degeneration  which  occurs  in  a  nerve- 
fibre  on  being  separated  from  its  cell-body  extends  to  the  fine 
branches  of  that  fibre,  but  does  not  pass  over  the  synapse  into  an- 
other neurone.  The  synapse  is  a  cell  boundary,  or  surface  of  sep- 
aration between  cells.  It  should  have,  therefore,  the  physical 

1  Comptes  rendus  de  la  Societe  de  Biologie,  1895,  p.  74. 

2  Integrative  Action  of  the  Nervous  System,  1906,  pp.  15-18. 


290  THE  NERVOUS  MECHANISM 

properties  of  other  cell  boundaries,  one  of  the  most  important  of 
which  (see  above,  p.  14)  is  the  resistance  interposed  by  it  to  free 
diffusion  and  to  the  passage  of  electricity.  The  position  of  the 
synapse  as  such  a  boundary  would  therefore  account  for  the  funda- 
mental fact  that  conduction  through  a  nerve-centre  is  less  free  than 
along  the  nerve-fibre.  The  allied  facts  mentioned  above  regarding 
the  peculiarities  of  conduction  through  the  nerve-centres  are  sus- 
ceptible of  possible  explanation  in  the  same  terms.  The  strange 
fact  of  the  irreversibility  of  conduction  through  a  nerve-centre  be- 
comes a  little  less  mysterious  when  it  is  recalled  that  some  degree 
of  irreversibility  of  diffusion  is  characteristic  of  cell  boundaries. 
The  fact  that  ether,  chloroform,  etc.,  have  a  powerful  effect  on  con- 
duction through  nerve-centres  is  also  partly  cleared  up  by  the  well- 
known  influence  of  these  chemical  substances  on  cell  membranes. 
Cell  boundaries  can  have  their  permeability  altered  by  many  causes, 
and  their  variability  in  this  respect  may  well  be  brought  forward 
in  explanation  of  the  variability  of  conductivity  through  a  nerve- 
centre. 

The  synapse  or  cell-boundary  theory  of  the  action  of  nerve- 
centres  seems,  therefore,  when  worked  out  in  detail,  to  be  more 
capable  of  giving  an  expression  in  physico-chemical  terms  to  most 
of  the  known  peculiarities  of  central  function  than  any  other  theory 
which  has  been  put  forward. 

§  20.  Of  other  theories,1  the  most  prominent  have  conceived  the 
action  of  nerve-centres  as  essentially  a  chemical  and  metabolic 
process,  and  have  laid  stress  on  assimilation  and  dissimilation,  or 
anabolism  and  catabolism,  as  the  fundamental  processes  involved. 
The  most  thoroughly  worked  out  of  these  metabolic  theories  are 
those  of  Wundt2  and  of  Verworn.3  It  does  not  seem  possible, 
with  present  knowledge,  to  fit  these  theories  to  the  details  of  the 
function  of  nerve-centres  so  nicely  as  has  been  done  for  the  cell 
membrane  theory.  As  affecting  all  these  theories,  the  fundamental 
question  concerns  the  evidence  of  catabolism  in  central  activity. 
This  is  the  same  question  which  was  raised  before  in  regard  to  the 
activity  of  the  nerves  (pp.  135  ff.);  but  the  answer  here  would  seem, 
at  first,  to  be  very  different  from  that  reached  in  the  case  of  the  nerves. 
There,  the  evidence  showed  unmistakably  that  the  catabolism  of 
the  nerve-fibres  is,  at  most,  very  small  in  amount.  In  the  case  of 

1  See  a  critical  discussion  of  the  theories  of  central  function  by  Bethe,  in  Er- 
gebnisse  der  Physiologie,  1906,  V,  250-288. 

2  Untersuchungen  zur  Mechanik  der  Nerven  und  Nervenzentren,  1876;  and  in 
the  successive  editions  of  his  Physiologische  Psychologic. 

3  See  the  chapter,  "Vom  Mechanismus  des  Lebens,"  in  the  various  editions 
of  his  Allgemeine  Physiologie,  and  also  many  special  papers. 


RIVAL  METABOLIC  THEORIES  291 

the  centres,  however,  the  prevailing  view  has  been  that  much  ca- 
tabolism  occurs  during  their  activity.  Perhaps  the  chief  fact  in 
favor  of  this  view  is  the  rich  blood  supply  of  the  brain  and  cord, 
and  the  great  dependence  of  their  functions  on  their  blood  supply. 
Stoppage  of  the  arteries  to  the  brain  results  in  speedy  unconscious- 
ness. The  brain  must  have  blood,  must  have  plenty  of  oxygen. 
And  it  uses  up  this  oxygen,  for  the  blood  returning  from  the  brain 
by  the  veins  has  been  deprived  of  its  oxygen.  The  conclusion  seems 
almost  self-evident  that  this  oxygen  was  used  by  the  brain  in  proc- 
esses of  combustion;  and  that  much  combustion  was  needed  to 
supply  the  energy  consumed  in  brain  activity.  But  the  matter  is 
not  so  simple.  For  much  blood  circulates  through  the  brain  even 
in  conditions  of  mental  quiescence,  sleep,  in  anaesthesia;  and  even 
in  these  conditions  the  blood  returning  from  the  brain  has  lost  much 
of  its  oxygen  and  become  venous.  Respiration  experiments  on 
men  performing  hard  mental  work  have  failed  to  detect  any  in- 
crease in  the  consumption  of  oxygen,  or  in  the  production  of  car- 
bon dioxide,  over  the  amounts  consumed  and  produced,  respec- 
tively, in  conditions  of  rest.1  Other  tests  for  the  consumption  of 
oxygen  by  brain  work  have  yielded  no  conclusive  evidence.  On 
the  whole,  it  seems  that  brain  work  is  not  a  strongly  catabolic 
process,  and  that  the  mechanics  of  brain  activity,  and  of  nerve- 
centre  activity  in  general,  is  of  the  same  general  sort  as  the  mechanics 
of  the  nerves.  All  this  is,  however,  no  conclusive  argument  against 
our  recognizing  the  evidence  for  the  view  that  chemical  processes 
in  the  form  of  catabolism,  are  an  essential  part  of  the  mechanics  of 
the  nervous  system.  And  modern  discoveries  are  constantly  show- 
ing more  clearly  what  enormous  amounts  of  promptly  available 
energy  may  be  stored  in  very  small  amounts  of  material  substance. 
Among  such  kinds  of  substance,  the  structure  of  the  nerve-fibres  and 
cells  seems  to  have  a  distinguished,  if  not  a  pre-eminent  position. 

§  21.  Our  main  contention — namely,  that  science  must  view 
the  structure  of  the  nervous  system  in  the  light  of  a  mechanism, 
and  its  functions  as  a  species  of  mechanics,  in  the  most  vague  and 
general  meaning  of  these  terms — would  not  be  in  the  least  impaired 
or  altered,  if  it  should  continue  forever  impossible  to  explain  the 
phenomena  in  terms  of  pure  chemistry  and  physics.  Let  it  be 
found  necessary  to  revive  the  conception  of  "vital  force,"  or  of  a 
considerable  group  of  so-called  "vital  forces."  This  would  not 
at  all  essentially  change  the  conditions  of  the  problem.  Structural 
changes  in  material  substances,  and  forces  assumed  to  account  for 
the  performances  of  such  substances,  can  be  described  and  explained 

1  Atwater,  Ergebnisse  der  Physiologie,  1904,  III,  part  1,  pp.  609  ff. 


292  THE  NERVOUS  MECHANISM 

only  in  terms  of  a  mechanical  theory.  Considered  as  a  mechanism, 
the  human  brain,  with  its  marvellous  outfit  of  fibres  and  cells,  is 
no  more  worthy  to  be  dubbed  spiritual,  or  have  applied  to  it  terms 
that  are  derived  from  the  phenomena  of  consciousness,  than  are  the 
most  obvious  and  grossest  forms  of  matter. 

§  22.  Our  review  of  the  various  molecular  theories  proposed  to 
account  for  the  nervous  mechanism,  either  as  a  whole  or  in  any  of 
its  parts,  has  made  plain  the  important  fact  that  such  theories  are 
all  obliged  to  assume  the  origin  and  continuance  of  a  peculiar  mo- 
lecular structure  for  this  mechanism.  In  other  words,  no  attempt 
to  explain  how  the  nervous  system  acts  can  avoid  the  conclusion 
that  the  determining  factor  in  the  explanation  must  be  found  in 
what  the  nervous  system  is.  The  physiological  functions  of  the 
nerve  depart  when  the  nerve  dies.  The  nerve  dies  when  it  is  sev- 
ered from  the  ganglion-cell.  Both  cell  and  nerve  must,  therefore, 
constitute  a  living  molecular  unity,  in  order  that  their  normal  phys- 
iological functions  may  be  performed.  The  explanation  of  these 
functions  assumes  the  molecular  constitution  of  the  organs  them- 
selves. But  how  shall  we  explain,  in  accordance  with  the  known 
laws  of  molecular  physics,  the  origin  and  preservation  of  such  a  mo- 
lecular constitution?  It  is  the  business  of  biology  rather  than  of 
physiology  to  attempt  an  answer  to  this  question.  But  the  question 
itself  asks  from  science  the  performance  of  a  task  no  smaller  than 
that  of  framing  a  mechanical  theory  of  life.  Biological  science  can, 
as  yet,  do  little  toward  framing  such  a  theory.  Throughout  our  en- 
tire discussion  of  the  nervous  mechanism  we  have  carefully  avoided 
raising  any  inquiry  as  to  the  nature  of  life,  as  to  the  source  and  con- 
ditions of  that  very  molecular  constitution  which  determines  the 
nature  and  working  of  this  mechanism.  We  have  simply  assumed 
and  argued  that,  taking  the  nervous  system  for  what  it  really  is 
and  really  does,  its  structure  and  functions  admit  of  scientific  ex- 
planation, so  far  as  such  explanation  is  possible  at  all,  only  when 
they  are  regarded  as  belonging  to  a  molecular  mechanism.  The 
question  of  a  mechanical  theory  for  the  origin  and  constitution  of 
living  organisms  in  general  lies  outside  of  the  inquiries  of  Physio- 
logical Psychology. 

§  23.  One  other  important  question  has  also  thus  far  been 
avoided.  What  is  the  relation  of  the  mind  to  the  working  of  the 
nervous  mechanism  ?  Can  the  mind  set  this  molecular  mechanism 
at  work,  or  can  it  in  any  way  determine  the  character  of  its  func- 
tions ?  As  far  as  our  consideration  of  the  nervous  system  has  gone 
hitherto,  all  might  very  well  have  been  the  same  without  the  exist- 
ence of  a  single  act  of  conscious  thought  or  feeling  occurring  in 
any  relation  whatever  to  this  system.  Given  the  molecular  mechan- 


REVIEW  OF  THE  EVIDENCE  293 

ism  as  it  is  constituted  and  conserved  by  the  forces  which  control 
as  long  as  life  continues;  and  given  the  necessary  impact  of  out- 
side forces  upon  the  end-organs,  and  the  proper  changes  of  blood 
within  the  central  organs;  and  it  has  been  assumed  that  this  mechan- 
ism would  exercise  its  functions  in  ways  thus  far  described.  But 
the  consideration  of  another  class  of  phenomena  is  now  to  be  in- 
troduced; these  are  the  phenomena  of  human  consciousness,  the 
phenomena  of  Mind.  The  question  whether  such  phenomena  can 
be  true  causes  of  any  of  the  changes  in  the  molecular  mechanism 
is  a  part  of  the  general  question  as  to  the  correlations  that  exist 
between  two  classes  of  facts.  The  answer  to  such  general  question 
belongs  to  the  following  divisions  of  our  work. 


PART  SECOND 

CORRELATIONS  OF  THE  NERVOUS 

MECHANISM  AND  MENTAL 

PHENOMENA 


CHAPTER  I 
THE  QUALITY  OF  SENSATIONS 

§  1.  A  considerable  change  in  the  point  of  view,  and  a  corre- 
sponding change  in  the  methods  of  investigation,  will  be  found 
necessary  for  effective  treatment  of  the  subjects  which  are  to  occupy 
our  attention  from  this  time  onward.  Thus  far  we  have  endeav- 
ored, as  much  as  the  nature  of  the  subject  made  possible,  to  look 
at  the  nervous  system  from  a  purely  objective  point  of  view,  and 
to  arrive  at  an  understanding  of  its  structure  and  functions  by  em- 
ploying the  methods  of  the  physico-chemical  and  biological  sciences. 
In  a  word,  this  system  has  been  examined  as  a  material  mechanism, 
which  like  everything  known  to  human  minds,  must  be  known  in 
terms  of  the  human  consciousness,  but  which  may  be  known,  as 
other,  material  structures  are  known,  without  any  preconceived 
opinions,  or  preconceived  theories,  as  to  its  special  relations  to  this 
consciousness.  Viewed  in  this  way,  the  nervous  mechanism  is 
an  object — to  have  its  constitution  determined  by  the  dissecting 
knife,  the  microscope,  and  the  various  means  for  physical  and  chem- 
ical analysis;  while  its  functions  consist  of  molecular  changes  and 
chemical  processes,  that  are  in  all  important  respects  assumed  to 
be  like  those  with  which  science  is  familiar  in  other  living  bodies. 

It  can  scarcely  have  escaped  observation,  however,  that  this  prom- 
ise to  keep  clear  of  all  the  more  strictly  psychological  implications 
and  complications  has  not  been  completely  fulfilled.  And,  indeed, 
no  amount  of  painstaking,  or  even  of  distinct  aversion  to  all  that 
has  a  remote  connection  with  psychological  topics,  could  possibly 
have  resulted  in  its  complete  fulfilment.  No  treatise  of  the  human 
nervous  mechanism  is  possible,  that  does  not  admit  somewhat  freely 
implications  derived  from  the  science  which  has  for  its  subject- 
matter  the  mind's  conscious  states;  or  that  does  not  adopt  some  pro- 
visional attitude  toward  a  number  of  complicated  psychological 
problems.  And  in  giving  the  results  of  modern  researches  as  to 
the  functions  of  certain  parts  of  the  nervous  system,  the  language 
which  it  is  found  necessary  to  employ  is,  strictly  considered,  often 
much  more  psychological  than  physiological. 

Examples  illustrating  our  contention  will  readily  occur  to  any 
one  who  passes  before  his  mind  in  review  the  descriptions  given, 

297 


298  THE  QUALITY  OF  SENSATIONS 

theories  favored,  and  laws  demonstrated,  in  Part  First  of  this  book. 
For  instance,  in  tracing  the  development  of  the  nervous  system  in 
the  animal  series,  and  the  phylogenetic  peculiarities  of  different 
species,  it  was  found  that  a  certain  at  least  rough  and  indefinite, 
but  no  less  real,  correlation  must  be  assumed  between  this  develop- 
ment and  the  development  of  what,  from  the  psychologist's  point 
of  view,  we  speak  of  as  the  "mind."  In  studying  the  development 
of  the  nervous  system  in  the  individual  man,  we  found  it  necessary 
to  assume  a  yet  more  strict,  if  not  more  complicated  network  of 
correlations  between  the  two  kinds  of  development.  The  differ- 
ent main  parts  of  this  system  in  man  were  seen  to  be  plainly  adapted 
to  the  performance  of  functions  which,  if  not  employed  in  the  im- 
mediate present,  would  before  long  be  needed  to  serve  as  the 
physical  correlates  of  the  main  classes  of  mental  activities  that  are 
involved  in  the  more  elaborate  processes  of  learning.  Provision 
must  be  made  for  the  individual's  becoming  consciously  aware  of 
the  nature  of  his  environment,  and  for  his  reacting  on  that  environ- 
ment with  different  kinds  and  degrees  of  consciously  directed  activity. 
Moreover,  it  was  found  impossible  to  avoid  the  conclusion  that  the 
complexity  and  size  of  the  nervous  system,  and,  in  a  very  special 
way,  of  the  cerebral  hemispheres,  are  correlated  with  the  complexity 
and  the  extent  of  a  possible  mental  development. 

§  2.  It  was,  however,  when  we  came  to  consider  the  so-called 
"localization  of  cerebral  function,"  that  we  found  ourselves  com- 
pelled to  receive  the  terms,  the  analyses,  the  conclusions,  and  even 
the  conjectures,  of  psychology,  as  derived  from  a  study  of  conscious- 
ness, into  our  fullest  confidence.  Indeed,  the  functions  described 
as  cerebral,  and  located  in  different  parts  of  the  cerebrum,  are, 
properly  speaking,  not  cerebral  at  all.  They  are  different  factors,  or 
phases,  or  aspects,  of  conscious  acts.  They  can  only  be  described 
as  such,  in  a  relatively  satisfactory  manner.  Modern  science,  by  a 
skilful  combination  of  the  methods  of  histology,  pathology,  and  ex- 
perimentation, has  established  in  some  cases,  what  parts  of  the  cere- 
bral areas  are  in  some  manner  concerned  in  furnishing  the  condi- 
tions or  accompaniments  of  these  factors,  phases,  or  aspects,  of 
the  mental  performances;  and  it  has  a  few  rather  uncertain  con- 
jectures as  to  the  physical  and  chemical  changes  in  which  these 
conditions  and  concomitants  consist.  But  it  is  these  latter  alone, 
that  are,  strictly  speaking,  localized  in  the  cerebral  areas.  With- 
out psychology — that  is,  without  a  study  of  the  states  of  conscious- 
ness— we  should  not  even  know  where  to  look  for  any  of  those 
physical  and  chemical  reactions  which  are  the  special  performances 
of  the  nervous  system.  And  did  we  not  see  that  a  more  complete 
and  satisfactory  analysis,  from  the  psychological  point  of  view,  is 


CHANGES  IN  POINTS  OF  VIEW  299 

at  present  a  prime  requisite  for  clearing  up  the  very  puzzling  counter 
evidences  and  contradictions  which  still  cling  to  the  doctrine  of 
cerebral  localization? 

The  same  thing  is  even  more  obviously  true  of  all  our  treatment 
of  those  portions  of  the  nervous  mechanism  which  serve  the  pur- 
poses of  end-organs  of  sense.  It  might  almost  be  said  that  there  is 
no  physiology  proper  of  the  end-organs  of  sense.  There  is  increas- 
ing knowledge  as  to  their  histology;  and  there  is  some  growth  to 
our  knowledge  of  the  physical  and  chemical  changes  which  they 
undergo  when  subjected  to  the  various  forms  of  stimuli  to  which 
they,  specifically,  respond.  But  as  "end-organs  of  sense"  we 
know  them  only  through  our  conscious  sensations  in  dependence 
upon  their  integrity  of  structure  and  normal  ways  of  functioning. 
This  fact  results  in  making  the  physiology  of  the  special  senses, 
too,  very  largely  psychological  rather  than  distinctly  physiological 
— a  description  and  analysis  of  how  we  feel,  rather  than  of  how  a 
piece  of  mechanism  looks  and  acts,  when  examined  in  a  purely  ob- 
jective fashion. 

§  3.  It  will  be  seen,  then,  that  after  all,  the  change  which  is  to 
take  place  in  our  point  of  view,  and  the  corresponding  change  in 
the  methods  of  investigation,  are  by  no  means  absolute  and  com- 
plete. We  have  all  along  been  getting  some  very  decided  and  fixed, 
if  not  always  definite  and  mathematically  accurate,  impressions 
as  to  the  correlations  which  exist  in  fact,  between  the  structure,  func- 
tions, and  development  of  the  nervous  system  in  man,  and  the  nature, 
activities,  and  development  of  man's  mental  life.  But  having 
obtained  a  sufficiently  full  and  clear  notion  as  to  what  sort  of  con- 
trivance this  nervous  mechanism  is,  in  fact,  and  of  how  it  can  oper- 
ate, with  its  three-fold  outfit  of  receptors,  conductors,  and  central 
organs,  we  wish  now  to  set  it  agoing,  so  to  say.  In  this  way  we  can 
study  the  results,  as  they  appear  in  consciousness,  of  the  action 
upon  it  of  various  kinds  and  degrees  of  stimulus,  and  the  laws  of 
mental  life,  as  it  develops  in  dependence,  more  or  less  remote, 
upon  these  results.  It  is  thus  much  of  change  which  has  seemed 
to  us  to  justify  the  beginning  here  of  a  "Part  Second"  of  our  treat- 
ise on  the  one  subject  of  Physiological  Psychology. 

§  4.  A  study  of  the  correlations  which  exist  between  the  ner- 
vous mechanism  and  the  mental  life  in  man,  requires  a  certain 
amount  of  preliminary  analysis  of  experience.  The  first  topic 
to  be  approached  in  this  way  is  our  experience  in  the  use  of  our 
senses. 

The  world  as  known  to  us  by  our  senses  consists  of  a  great  num- 
ber of  so-called  "things"  that  are  believed  to  be  separate  existences, 


300  THE  QUALITY  OF  SENSATIONS 

but  possess  certain  common  characteristics,  and  stand  in  certain 
relations  to  each  other,  of  space,  time,  and  action.  It  is  with  the 
things,  their  common  qualities  and  mutual  relations,  that  unreflect- 
ing practical  life  is  chiefly  concerned.  But  even  without  special  re- 
flection, every  one  learns  that  his  knowledge  of  such  external  objects 
depends  upon  the  kind  and  degree  of  the  effect  they  exercise  upon 
his  consciousness  through  the  senses.  Attention  is  thus  turned  from 
the  things  themselves  to  the  sensations  produced  in  us  by  their  ac- 
tion. The  variety  of  such  sensations,  at  first  bewilderingly  great, 
is  soon  reduced  to  some  order  by  a  classification  which  refers  them 
to  the  different  organs  through  which  they  come.  Thus,  certain 
sensations  are  received  through  the  nose,  others  through  the  mouth, 
the  ear,  the  eye,  or  the  skin — especially  as  covering  that  part  of  the 
body  (the  hand)  which  is  most  active  in  touch.  Smell,  taste,  hear- 
ing, sight,  and  touch  are  the  five  classes  of  sensation,  as  the  group- 
ing is  made  by  the  unprejudiced  judgment  of  all;  and  until  recently 
this  was  considered  sufficient  for  scientific  purposes. 

A  further  rough  and  scientifically  inadequate  classification  takes 
place  among  the  sensations  of  the  same  sense.  Those  of  smell,  in- 
deed, defy  classification,  whether  popular  or  scientific.  Among 
tastes,  the  most  familiar  are  easily  distinguished;  such  are  the 
sweet,  the  sour,  and  the  bitter.  The  two  principal  classes  of  sensa- 
tions of  sound  are  easily  discriminated,  as  either  noises  or  musical 
tones;  the  former  are  further  classified  as  respects  the  character  of 
the  feeling  which  accompanies  them,  and  the  latter  as  high  or  low 
in  pitch.  The  different  more  prominent  colors — including  black 
and  white — are  recognized  by  all  persons  of  normal  vision  as  modes 
of  the  sensations  of  sight;  hence  the  colors  commonly  named,  and 
the  various  so-called  "shades"  of  these  colors.  That  more  than 
one  class  of  sensations  arise  through  the  skin  is  shown  by  the  popu- 
ular  use  of  the  word  to  "feel."  Things  feel  hard  and  soft,  smooth 
and  rough,  as  well  as  warm  and  cold.  But  things  are  also  said 
to  feel  heavy  or  light.  The  feeling  by  which  their  weight  is  esti- 
mated, however,  is  only  ascribed  in  a  very  indefinite  way  to  the 
parts  of  the  body  that  are  chiefly  concerned  in  passively  supporting, 
or  actively  lifting,  or  pushing  against  their  weight.  The  particular 
use  of  tactual  feeling,  as  well  as  the  general  use  of  the  muscular 
sense,  in  gaining  this  class  of  sensations  is  little  noticed  by  ordinary 
reflection. 

§  5.  All  the  sensations  are  also  regarded  as  having  some  place 
in  a  scale  of  degrees  of  sensation;  they  are  either  strong  or  faint, 
or  else  lie  somewhere  between  the  two  extremes.  They  are  also 
habitually  thought  of  as  related  to  time,  and  as  being  connected 
with  the  motion  in  space  of  the  objects  that  occasion  them.  Of 


NEED  OF  FURTHER  ANALYSIS  301 

the  molecular  action  of  their  stimuli  upon  the  end-organs  of  special 
sense;  of  the  hidden  chemical,  electrical,  or  other  processes  con- 
nected with  the  activity  of  the  peripheral  and  central  nervous  sys- 
tem; of  the  physiological,  psycho-physical,  and  psychological  laws 
under  which  the  mind  reacts  in  the  form  of  simple  sensations,  and 
combines  these  sensations  into  the  composite  objects  of  sense;  of 
all  these  and  other  similar  matters,  the  unreflecting  conception  of 
sensation  takes  no  account. 

§  6.  It  is  obvious  that  the  analysis  of  sense-percepts  which  suf- 
fices for  working-day  life  will  in  no  respect  answer  the  demands  of 
science.  Its  " common-sense"  character  is  a  distinct  mark  of  its 
inadequacy.  An  adequate  scientific  treatment  of  this  branch  of 
Physiological  Psychology  requires  at  least  four  things:  (1)  to  dis- 
tinguish the  simple  sensations  from  those  complex  objects  of  experi- 
ence with  which  alone  our  adult  consciousness  is  familiar;  (2)  to 
point  out  the  varieties  of  quality  and  degrees  of  quantity  which  be- 
long to  these  sensations,  and  to  discover  the  laws  which  relate  them 
to  changes  in  the  form  and  intensity  of  their  stimuli;  (3)  to  show 
how  the  simple  sensations  are  constructed  by  the  mind  into  the  so- 
called  "presentations  of  sense"  under  mental  laws  of  time-form 
and  space-form;  and  (4)  to  indicate  how  far,  if  at  all,  the  higher 
mental  activities  of  association,  memory,  will,  and  judgment  may 
be  brought  under  laws  similar  to  those  upon  which  the  formation 
of  these  presentations  of  sense  depends.  It  is  upon  these  four 
heads  of  inquiry  that  modern  psychology,  as  studied  from  the 
psycho-physical  point  of  view,  has  expended  most  of  its  painstak- 
ing researches.  Its  success  has  been  by  no  means  complete.  All 
these  fields  of  inquiry  still  include  many  unanswered  questions;  all 
of  them  present  the  results  of  researches  that  seem  in  various  re- 
spects conflicting.  Yet  it  is  precisely  in  these  fields  that  modern 
psychology  has  achieved  its  most  brilliant  successes. 

§  7.  The  distinctions  with  which  scientific  analysis  begins  are  to 
a  large  extent  received  from  ordinary  experience.  Some  of  the 
most  essential  of  the  distinctions  are  confirmed  by  the  results  of 
this  analysis.  They  all,  however,  require  to  be  carried  farther  and 
to  be  fixed  with  much  more  of  accuracy  than  belongs  to  the  im- 
pressions of  common  life.  New  distinctions  also  have  to  be  intro- 
duced. For  example,  scientific  investigation  maintains  the  differ- 
ence between  sensations  of  smell  and  sensations  of  taste;  but  it 
points  out  what  is  not  ordinarily  apparent — namely,  that  certain 
results  commonly  referred  to  the  latter  sense  really  belong  to  the 
former.  It  also  adds  the  sensations  of  the  muscular  sense  to  the 
classes  popularly  described;  and  it  discriminates  more  clearly  be- 
tween the  several  kinds  of  sensations  that  have  the  skin  for  their 


302  THE  QUALITY  OF  SENSATIONS 

organ.  As  we  have  already  seen,  it  assigns  two  very  different 
classes  of  sensations  to  the  ear. 

Psycho-physical  science,  moreover,  accepts  the  common  distinc- 
tion between  the  quality  and  the  quantity  of  the  different  sensa- 
tions. But  it  describes  with  all  possible  accuracy  the  limits  within 
which  alone  this  distinction  can  be  carried  out.  It  shows  that  the 
quality  and  quantity  of  sensation  are  inseparably  connected;  that, 
as  Lotze  held  (a  view  confirmed  by  von  Kries  and  others),  changes 
in  quality  can  be  distinguished  from  changes  in  intensity,  with 
perfect  confidence,  only  in  the  case  of  sensations  of  hearing.  It  is 
possible  that  even  here  the  distinction  is  largely  made  on  the  basis 
of  complex  experience.  Very  intense  sensations  of  heat  and  cold  so 
far  change  their  specific  character  as  to  tend  to  pass  into  each  other, 
or,  perhaps,  to  become  submerged  in  a  common  tone  of  painful 
feeling.  Minimum  sensations  of  heat  and  pressure  are  difficult  to 
distinguish  from  each  other;  maximum  sensations  of  pressure  are 
likely  to  lose  the  characteristic  quality  of  touch  and  be  displaced 
by  sensations  of  pain.  To  treat  scientifically  of  the  quality  of 
sensations  requires,  then,  a  large  amount  of  the  most  careful  an- 
alysis. 

§  8.  It  is  essential,  in  the  first  place,  to  distinguish  "simple 
sensations"  from  "presentations  of  sense,"  or  those  complex  ob- 
jects of  consciousness  which  result  from  an  act  of  mental  synthesis 
on  the  basis  of  several  simultaneous  affections  of  sense.  As  respects 
developed  experience,  the  simple  sensation  is  a  necessary  fiction  of 
psycho-physical  science.1  Consciousness  is  scarcely  more  able  di- 
rectly to  analyze  a  presentation  of  sense  into  those  factors  out  of 
which  it  originated  than  it  is  to  analyze  a  drop  of  water  into  its 
component  oxygen  and  hydrogen  gases.  Simple  sensations,  there- 
fore, are  not  objects  which  can  be  examined  in  the  direct  light  of 
introspection.  Yet  they  are  factors  which,  as  scientific  analysis 
shows,  actually  enter  into  all  such  objects  as  can  properly  be  spoken 
of  under  the  term  "presentations  of  sense."  Any  sensation  which 
is  absolutely  unanalyzable  with  respect  to  distinctions  of  quality, 
and  which,  therefore,  cannot  be  considered  as  consisting  of  com- 
ponent parts,  is  called  simple.  It  is  distinguished  'as  a  sensation 
from  all  other  elementary  forms  of  feeling  or  knowledge,  by  the 
relation  which  it  sustains  to  the  presentations  of  sense.  A  sensa- 
tion, unlike  the  feeling  of  grief,  of  desire,  or  of  weariness,  etc.,  is  a 

1  All  our  subsequent  work  will  be  completely  misunderstood  unless  this  state- 
ment is  constantly  borne  in  mind.  On  the  one  hand,  the  scientific  study  of 
sense-experience  is  impossible  without  the  analysis  which  employs  this  fiction; 
on  the  other  hand,  to  give  reality  to  the  fiction,  as  though  it  were  an  experienced 
element  in  consciousness,  is  to  favor  an  atomistic  theory  of  mental  life. 


QUESTIONS  REQUIRING  AN  ANSWER  303 

potential  factor  of  a  material  object.  Through  the  senses  we  know 
"things";  not,  indeed,  as  though  they  appeared  before  the  mind 
by  immediate  apprehension  in  the  form  of  exact  copies  of  extra- 
mental  realities.  But  every  sensation  is  an  affection  of  the  mind 
recognized  as  connected  with  an  extra-mental  reality,  through  the 
activity  of  the  senses.  Simple  sensations  are  those  elementary 
factors,  themselves  indecomposable,  out  of  which  the  presentations 
of  sense  are  composed.  The  objects  of  sense,  however,  do  not  have 
the  character  of  mere  compounds  of  simple  sensations.  Sensations 
must  not  only  be  associated  and  compounded,  but  also  localized 
and  projected  without  (that  is,  set  in  systematic  relations  of  space- 
form),  in  order  to  constitute  the  objects  of  sense. 

§  9.  The  foregoing  remarks  suffice  to  indicate,  in  a  preliminary 
way,  what  is  the  nature  and  value  of  the  psycho-physical  investi- 
gation of  sensation.  We  inquire,  in  the  next  two  chapters,  as  to 
the  Quality  of  Sensations.  The  inquiry,  when  conducted  from  the 
psycho-physical  point  of  view,  involves  an  answer  to  three  questions : 
(1)  What  is  the  precise  locality  in  the  organism  where  the  specific 
excitation  which  occasions  each  kind  of  sensation  originates;  and 
what  is  the  nature  of  the  action  of  the  stimulus  in  producing  such 
excitation  ?  (2)  What  are  the  kinds  of  sensations  which  appear  in 
consciousness  as  the  result  of  the  various  excitations  ?  (3)  What  are 
the  laws  by  which  the  quality  of  the  sensations  is  related  to  the 
kinds  of  excitation?  Neither  of  these  three  questions  can  be 
answered  completely.  The  investigation  of  the  first  is  much  re- 
stricted by  our  almost  complete  ignorance  of  those  processes  in 
the  central  organs  that  are  in  all  cases  the  proximate  internal 
stimuli  or  immediate  antecedents  of  the  sensations.uXMoreover,  as 
has  already  been  made  apparent,  our  knowledge  of  the  intimate 
structure  of  the  end-organs  of  sense,  and  of  the  nature  of  the  physi- 
cal processes  which  excite  them,  is  still  very  incomplete.  The  de- 
tection of  obscure  but  important  differences  in  the  qualities  of  con- 
scious states  of  sensation  is  by  no  means  easy;  it  requires  great  skill, 
strict  and  trained  attention,  and  unwearied  repetition  of  experiment. 
But  these  conditions  of  success  have  a  great  effect  in  altering  the 
quality  of  the  sensations  themselves.  Besides  all  this,  remarkable 
idiosyncrasies  not  infrequently  appear;  and  language  can  only  im- 
perfectly describe  even  the  most  common  factors  of  the  varied  and 
living  experiences  with  which  science  tries  to  deal. 

In  investigating  the  laws  that  define  the  relations  between  our 
subjective  experience,  called  sensation,  and  objective  phenomena 
in  the  shape  of  physical  energy  acting  upon  the  nervous  mechanism, 
there  is  often  the  greatest  doubt  as  to  what  manner  of  laws  are  be- 
ing investigated.  They  may  be  considered  as  purely  physiological, 


304  THE  QUALITY  OF  SENSATIONS 

or  as  psycho-physical,  or  as  purely  psychological.  It  is  not  strange, 
therefore,  that  different  theories  exist  for  accounting  for  all  the 
more  important  groups  of  facts,  depending  upon  the  emphasis  laid 
by  different  investigators  upon  the  value  of  each  of  the  three  possi- 
ble modes  of  explanation.  The  truth  is,  that  each  sensation  is  sepa- 
rated by  a  series  of  intricate  physiological  and  psychical  processes 
from  the  application  of  the  stimulus  in  the  gross,  as  it  were,  to  the 
end-organ  of  sense. 

§  10.  What  has  already  been  said  regarding  the  "specific  energy 
of  the  nerves"  (compare  pp.  284  f.)  must  be  assumed  in  discussing  the 
quality  of  sensations.  The  possession  of  common  functions  can- 
not, indeed,  be  denied  to  the  nerve-fibres  in  general.  Conduction 
has  been  seen  to  be  the  one  universal  property  of  all  the  nervous 
elements;  and  even  the  end-organs,  or  receptors,  of  sense-impres- 
sions are  made  up  of  elaborate  and  ingenious  combinations  of  these 
elements.  But  the  phenomena  of  sensation  require  a  further  exten- 
sion of  the  conception  of  specific  functions,  on  whatever  physical  or 
physiological  basis  this  differentiation  is  founded.  Consciously 
made  distinctions  in  the  quality  of  sensations  depend  upon  the  ex- 
citation of  specific  corresponding  elements  of  the  nervous  mechanism. 
Sensations  of  light  and  color  depend  upon  different  species  of  the  ex- 
citation of  the  optic  nerve;  and  similar  specific  quality  cannot  be 
denied  to  the  functional  activity  of  the  nerves  of  smell,  taste,  hear- 
ing, and  touch.  But  the  nature  of  the  evidence  and  the  conclu- 
sions which  must  be  drawn  from  it  will  be  much  better  appreciated 
at  a  later  period  in  the  discussion. 

§  11.  Little  of  a  scientific  character  is  known  concerning  Sensa- 
tions of  Smellt  considered  as  respects  their  specific  quality.  The 
physical  and  nervous  structure  of  the  apparatus  employed  in  ex- 
citing this  species  of  sensations,  and  the  way  in  which  the  stimulus 
is  customarily  applied,  have  already  been  described  (see  p.  176). 
Under  ordinary  conditions  the  stimulus  must  act  in  gaseous  form, 
or  else  be  vaporizable,  with  the  existing  degree  of  temperature. 
The  degree  of  temperature  at  which  different  substances  become 
vapori/able,  and  therefore  odorous,  varies  greatly  according  to  their 
physical  characteristics.  Arsenic,  for  example,  which  at  ordinary 
temperatures  is  inodorous,  when  raised  to  a  dark-red  heat  excites 
intense  sensations  of  smell  by  the  vapor  it  gives  off. 

Whether  an  odorous  substance  must  actually  reach  the  olfactory 
mucous  membrane  in  a  vaporous  form  or  whether  a  solution  will 
in  any  case  excite  sensations  of  smell,  cannot  easily  be  ascertained; 
since  neither  the  positive  nor  the  negative  results  which  have  been 
obtained  by  different  experimenters  can  withstand  all  criticism.  In 
the  case  of  negative  results,  such  as  those  of  \Veber,  who  found  that 


STIMULI  OF  OLFACTORY  SENSATIONS          305 

when  the  head  was  tilted  back  and  the  nostrils  filled  with  a  ten  per 
cent,  solution  of  eau  de  cologne,  the  sense  of  smell  was  not  excited, 
attention  must  be  directed  to  the  fact  that  treatment  of  the  membrane 
by  soaking  it  in  a  liquid  disturbs  the  function  of  the  olfactory  ap- 
paratus for  a  considerable  time.  In  the  case  of  the  positive  re- 
sults, the  difficulty  is  that  of  being  sure  that  the  narrow  chink  in 
which  the  end-organs  are  situated  is  actually  filled  with  the  liquid; 
if  it  were  not,  but  some  air  remained,  then  the  odorous  particles 
might  really  reach  the  olfactory  surface  in  the  form  of  a  vapor. 
On  the  other  side,  it  may  be  argued  that,  since  at  least  a  thin  layer 
of  liquid  must  cover  the  olfactory  membrane,  the  odorous  particles 
must  finally  be  dissolved  or  suspended  in  this  liquid,  and  so  excite 
the  sensations  of  smell. 

§  12.  Whatever  decision  may  be  reached  as  to  the  possibility 
under  highly  abnormal  conditions,  the  ordinary  and  "adequate" 
stimulus  of  smell  is  rightly  assumed  to  consist  in  certain  exceedingly 
minute  particles  contained  in  the  odorous  gas  or  vapor  which  is 
drawn  in  with  the  current  of  air  over  the  mucous  membrane  of 
the  regio  olfactoria.  The  question  is  as  yet  scarcely  decided, 
whether  other  forms  of  stimulus,  besides  these  odorous  particles — 
mechanical,  electrical,  thermic,  or  so-called  subjective — can  ex- 
cite the  sensation  of  smell.  The  older  experimenters  (Volta, 
Pfaff,  Fowler,  and  Humboldt)  failed  to  obtain  any  certain  proof 
that  the  electrical  current  is  an  excitant  of  this  sense.  In  one  place, 
however,  Pfaff  speaks  of  a  sensation  resembling  the  smell  of  sul- 
phur as  caused  by  the  application  of  electricity  to  the  sensory  pas- 
sages of  the  nose.  Ritter  (in  1798)  experimented  by  using  bits  of 
graphite  and  zinc  thrust  into  these  passages,  and  also  by  holding 
one  pole  of  a  battery  in  the  hand  and  placing  the  other  in  the  nos- 
tril. In  the  latter  way  he  thought  that  he  excited  a  genuine  spe- 
cific sensation  of  this  sense.  He  describes  the  positive  pole  in  the 
nostril  as  producing  an  inclination  to  sneeze  and  a  trace  of  a  smell 
like  that  of  ammonia;  the  negative  pole  placed  there  does  away 
with  this  inclination  and  produces  a  kind  of  "sour"  smell.  Such 
phenomena  are  probably,  however,  all  to  be  assigned  to  the  nerves 
of  taste,  touch,  and  common  feeling.  More  recent  investigations 
have  done  little  to  remove  the  reasons  for  doubt.1  The  smell  of 
phosphorus  which  is  developed  by  the  action  of  the  electrical  ma- 
chine is  probably  due  to  the  ozone  set  free;  it  is  not  a  case,  then, 
of  the  direct  excitation  by  electricity  of  the  sensation  of  smell. 
Some  physiologists  (notably  Valentin)  have  observed  that  this  sen- 
sation may  be  awakened  by  mechanical  stimulation,  such  as  strong 
vibration  of  the  nostrils,  violent  sneezing,  etc. ;  others  have  failed 
1  See  W.  Nagel,  Handbuch  d.  Physiologic  des  Menschen,  1905,  III,  602. 


306  THE  QUALITY  OF  SENSATIONS 

to  produce  this  specific  sensory  effect  in  such  ways.  It  does  not 
appear  that  thermic  stimulation  will  excite  the  sensation  of 
smell. 

Experiments  to  prove  that  subjective  sensations  of  smell  may  be 
produced  by  injecting  odorous  substances  into  the  veins  of  animals 
are  very  uncertain.  Human  pathological  cases,  in  spite  of  the  cus- 
tomary indefiniteness  of  the  patient's  testimony  as  to  the  nature  of 
his  sensory  affection,  tend  to  show  that  compression  of  the  olfactory 
nerve  by  tumors,  etc.,  may  produce  sensations  of  smell.  Dis- 
turbances of  the  central  organs,  such  as  occur  in  cases  of  disease, 
may  doubtless  have  the  same  result.  The  powerful  effect  which 
some  odors  have  upon  the  brains  of  some  persons,  so  that  nausea, 
giddiness,  and  other  disturbances  of  feeling  result,  scarcely  needs 
mention;  but  it  cannot  easily  all  be  resolved  into  mental  associations 
connected  with  the  sense  impressions.  Hallucinations  of  smell  are 
among  the  prominent  and  persistent  symptoms  of  certain  forms  of 
insanity. 

§  13.  As  concerns  the  varieties  of  odors,  from  the  psychological 
point  of  view,  the  important  scientific  problem  is  that  of  attempting 
to  reduce  this  manifold  to  order.  In  other  senses,  particularly  in 
sight  and  taste,  such  attempts  have  already  met  with  a  considerable 
degree  of  success.  The  great  variety  of  colors  can  be  ordered  by 
reference  to  a  few  primary  colors;  and  the  tastes  can  be  reduced  to 
mixtures  of  a  few  elementary  tastes.  There  are  some  indications 
that  a  similar  "component  theory"  is  moving  in  the  right  direction 
in  the  case  of  smell  also.  This  theory  would  hold  that  there  is 
a  relatively  small  number  of  primary  odors,  each  aroused  by  a  spe- 
cific physico-chemical  stimulus,  while  the  great  variety  of  other 
odors  have  their  complex  nature  due  to  their  being  aroused  by 
mixed  stimuli.  But  owing  to  the  uncertain  and  vague  character 
of  the  evidence,  a  designation  of  the  primary  sensations  of  smell, 
or  of  their  specific  stimuli,  is  still  very  far  from  an  accomplished 
fact. 

Introspectively,  odors  by  no  means  separate  themselves  into  ele- 
mentary and  compound,  but  all  seem  simple.  It  is,  however,  possi- 
ble to  draw  a  line  between  smell  proper  and  tactile  sensations  of 
the  nasal  organ.  The  interior  of  the  nose  is  supplied  not  only  by 
the  olfactory  nerve  but  also  by  a  branch  of  the  trigeminus;  and  many 
sharp  "odors"  are  in  part  stinging  sensations  from  the  end-organs 
of  the  latter  nerve,  and  persist  in  individuals  who  are,  through  in- 
jury or  congenital  absence  of  the  olfactory  nerve  or  similar  causes, 
completely  anosmic,  or  lacking  in  the  sense  of  smell.  It  is  also 
possible  to  draw  a  line  between  smell  and  the  sensations  of  taste 
which  sometimes  arise  from  vapors,  as  the  sweet  taste  of  inhaled 


ANALYSIS  OF  OLFACTORY  SENSATIONS  307 

chloroform,  which  is  properly  due1  to  excitation  of  taste-buds  on 
the  soft  palate  and  in  the  larynx.  Even  these  distinctions  are  not 
easy  for  introspection;  and  in  common  life  we  ascribe  the  mixture 
of  smell,  taste,  and  touch  sensations  which  may  result  from  sniffing 
a  vaporous  substance,  all  to  the  sense  of  smell. 

§  14.  If  all  these  extraneous  sensations  are  excluded,  the  intro- 
spective analysis  of  the  sensations  of  smell  proper  seems  impossible. 
About  all  that  can  be  done  is  to  notice  resemblances  and  differences 
between  odors,  and  so  to  arrive  at  some  sort  of  classification.  None 
of  the  classifications  of  odors  which  have  been  offered  are  thoroughly 
satisfactory,  however;  nor  do  any  of  them  afford  much  promise 
of  leading  to  the  discovery  of  the  elementary  smell-stimuli.  The 
only  one  which  has  any  claim  to  attention  has  come  down  to  us 
from  Linnaeus,  the  great  naturalist  of  the  eighteenth  century;  it 
has  been  adopted  and  somewhat  developed  by  Zwaardemaker,  who 
ranks  as  the  leader  among  contemporary  students  of  the  sense  of 
smell.  Zwaardemaker's  classification  is  as  follows: 

(1)  Ethereal  odors,  including  the  odors  of  fruits. 

(2)  Aromatic  or  spicy  odors. 

(3)  Fragrant  odors,  including  the  scents  6f  flowers,  and  also 
vanilla,  tea,  balsam,  etc. 

(4)  Ambrosial  odors,  of  which  musk  is  the  most  familiar  example. 

(5)  Alliaceous  odors,  including  onion,  india  rubber,  chlorin,  and 
iodin. 

(6)  Empyreumatic  or  burnt  odors,  including  burnt  foods,  and 
also  tar,  gasoline,  etc. 

(7)  Hircine  or  goaty  odors,  including  cheese,  rancid  butter,  etc. 

(8)  Repulsive  odors,  as  of  certain  insects  and  plants. 

(9)  Nauseous  odors,  as  of  decaying  flesh. 

Some  of  these  classes  seem  broader  than  others;  especially  do 
we  notice  that  numbers  8  and  9  include,  for  many  individuals, 
smells  that  might  be  selected  from  all  the  other  classes;  and  all  are 
capable  of  subdivision  into  subordinate  classes.  More  important 
still  is  the  fact  that  some  odors  do  not  readily  find  a  place  in  the 
scheme  at  all.  If  it  were  possible  to  discover  any  chemical  com- 
munity between  the  members  of  any  class  or  sub-class,  which  should 
set  that  class  off,  as  stimuli,  from  the  other  classes,  a  long  step  would 
be  taken  toward  a  scientific  understanding  of  these  sensations;  but, 
so  far,  there  is  little  sign  of  a  common  chemical  character  among 
resembling  odors.  Perhaps  more  hope  of  advance  is  afforded  by 

1  W.  Nagel,  Handbuch  der  Physiologic,  1905,  III,  611;  Zwaardemaker,  Physi- 
ologie  des  Geruchs,  1895. 


308  THE  QUALITY  OF  SENSATIONS 

the  results  of  examining  the  odors  of  closely  related  groups  of  chem- 
ical substances.  It  is  found,  in  the  first  place,  that  odorous  sub- 
stances, with  few  exceptions,  contain  elements  belonging  to  only 
three  of  Mendelejeff's  groups,  namely  the  fifth  (of  which  nitrogen, 
phosphorus,  and  antimony  are  members),  the  sixth  (including  oxy- 
gen, sulphur,  and  chromium),  and  the  seventh  (including  chlorin, 
bromin,  and  iodin).  When  similar  compounds  of  the  elements  of 
one  of  these  groups  are  arranged  in  the  order  of  their  atomic  weight, 
the  odors  are  found  to  run  along  a  scale,  shading  off  from  one  to 
another.  Thus,  the  odors  of  chlorin,  bromin,  and  iodin  can  be  re- 
garded as  arranged  along  a  linear  scale,  and  the  same  is  true  of  like 
compounds  of  these  three.  Similar  scales  are  found  on  arranging 
the  fatty  acids  in  order,  or  the  monatomic  alcohols,  or  other  like 
series  of  organic  compounds.  The  members  at  each  end  of  such  a 
series  are  often  odorless;  and  the  intensity  as  well  as  the  quality 
of  the  odor  is  likely  to  change  in  a  gradual  manner  along  the 
series.1 

§  15.  On  the  psychological  or  physiological  side,  more  light  can 
be  expected  from  studying  the  results  of  mixture  of  odors,  of  fa- 
tigue of  the  organs,  etc.,  than  from  attempts  to  classify  odors  in- 
trospectively.  Partial  anosmia — a  condition  in  which  the  nose  is 
insensitive  to  certain  odors  but  not  to  all — is  not  infrequent;  and 
the  distinction  draws  a  physiological  line  between  some  odors  and 
others.  It  is  well  known  that  prolonged  action  of  an  odorous  sub- 
stance results  in  a  condition  of  fatigue  (or  adaptation),  in  which 
the  odor  can  no  longer  be  perceived.  In  this  condition,  it  is  found 
that  the  nose  becomes  insensitive  to  certain  other  odors  besides  the 
particular  one  which  has  been  acting,  while  it  remains  sensitive 
to  still  other  odors  of  a  different  character.  The  odors  which  are 
fatigued  together  would  seem  to  belong  together  physiologically, 
and  they  are  also  likely,  in  fact,  to  be  subjectively  similar. 

Somewhat  analogous  results  have  been  obtained  by  studying 
mixtures  of  different  stimuli  which  excite  sensations  of  smell.  Care 
must  be  taken,  in  such  experiments,  to  avoid  the  possibility  of  a 
chemical  union  between  the  two  vapors,  and  this  is  perhaps  best  ac- 
complished by  Zwaardemaker's  device  of  conducting  one  vapor 
to  the  right  nostril  and  the  other  to  the  left,  each  through  a  separate 
tube.  The  effect  of  such  mixed  stimulation  is,  in  some  cases,  the 
production  of  an  intermediate  odor,  but  in  others  the  masking  of 
one  odor  by  the  other,  or  even  the  complete  neutralization  of  each 
by  the  other  so  that  no  odor  is  perceived  from  the  mixture.  Similar 

1  See  Haycraft,  Brain,  1889,  II,  166,  and  in  Schafer's  Textbook  of  Physiology, 
1900,  II,  1254.  The  results  appear  to  be  an  entering  wedge  into  the  tough  prob- 
lem of  the  classification  of  the  stimuli  of  sensations  of  smell. 


STIMULI  OF  GUSTATORY  SENSATIONS  309 

effects  of  the  blending,  or  contrast,  or  opposition  of  sensations  are 
obtained  in  other  classes  of  our  sensory  experience.  It  is  along 
these  lines  that  the  unravelling  of  the  intricacies  of  the  sense  of 
smell  may  be  expected  to  make  most  progress. 

§  16.  The  condition  of  scientific  attainment  as  to  sensations  of 
taste  and  their  stimuli  is  somewhat  better  than  that  as  to  the  allied 
sense  of  smell.  The  adequate  specific  stimulus  for  the  nerves  of 
this  sense  consists  in  certain  tastable  substances;  such  substances, 
however,  do  not  excite  the  end-apparatus  unless  they  act  upon  it 
under  definite  conditions.  Only  fluid  bodies,  or  such  as  are  at 
least  to  some  small  degree  soluble  in  a  fluid  or  menstruum,  excite 
sensations  of  taste;  absolutely  insoluble  bodies  are,  without  excep- 
tion, tasteless.  This  fact  may  be  due  to  the  concealed  position  of  the 
inner  cells  of  the  gustatory  flasks,  which  is  such  that  they  cannot  be 
reached  by  substances  undissolved.  By  no  means  all  soluble  sub- 
stances, however,  have  a  taste.  No  known  law  regulates  the  rela- 
tion between  the  solubility  of  bodies  and  their  power  to  excite  sen- 
sations of  this  class.  The  adequate  stimulus  of  taste  is  thus  chemi- 
cal; and  taste-buds  are  "chemo-ceptors."  Whether  they  can  be 
aroused  by  other  classes  of  stimuli — mechanical,  thermal,  electrical 
— is  not  yet  ascertained  with  certainty.  But  thermal  stimuli,  at 
least,  are  probably  without  any  effect  on  the  taste  end-organs. 
As  to  mechanical  stimuli,  some  good  observers  have  reported  sen- 
sations of  taste  resulting  from  pressing  or  tapping  the  tongue;  but 
others  have  been  unable  to  obtain  the  same  results.  Sensations 
of  this  class  are  certainly  aroused  by  the  passage  of  a  current  of 
electricity  through  the  tongue.  If,  for  example,  the  positive  pole 
or  anode  is  applied  to  the  tongue,  the  sensation  is  sour;  if  the  nega- 
tive pole  or  cathode  is  applied,  the  taste  is  not  so  easy  to  describe, 
but  is  called  sharp,  alkaline,  or  bitter,  by  different  observers.  Wheth- 
er the  sensations  produced  in  this  way  are  the  direct  result  of  the 
action  of  the  current  on  the  end-organs  of  taste  is  a  question  which, 
after  a  hundred  and  fifty  years  of  study,  is  not  yet  settled.  The  in- 
sufficiency of  the  evidence  is  due  to  the  fact  that  the  passage  of  a 
current  sets  up  electrolysis;  and  though  this  may  be  prevented  from 
taking  place  in  the  saliva  of  the  mouth,  there  is  no  way  to  make  sure 
that  electrolysis  is  not  going  on  within  the  taste-buds,  or  at  some 
polarizable  surface  through  which  the  current  has  to  pass.  Now 
electrolysis  of  the  salts  dissolved  in  the  saliva  or  body  fluids  would 
set  free  acid  ions  at  the  anode  and  alkaline  at  the  cathode;  and  thus 
the  tastes  actually  observed  could  be  accounted  for  as  due  to  chemi- 
cal stimuli  generated  by  electrolysis,  rather  than  to  electrical  stimuli. 
This  view  has  considerable  probability  as  an  explanation  of  the 
"electrical  tastes";  it  is  at  least  not  excluded  by  any  results  yet  ob- 


310  THE  QUALITY  OF  SENSATIONS 

tained.1  Apparently,  therefore,  we  have  in  the  taste  end-organs  a 
striking  example  of  the  general  characteristic  of  special  receptors — 
namely,  that,  though  highly  sensitive  to  one  special  stimulus,  they 
are  rather  obtuse  to  all  other  sorts  of  stimuli. 

§  17.  In  attempting  to  delimit,  analyze,  and  order  the  sensations 
of  taste,  we  are  met  with  the  same  difficulty  as  that  which  confronted 
us  in  the  case  of  smell — namely,  that  tastes  usually  occur  in  close 
combination  with  other  sensations.  The  tongue  and  mouth  possess 
the  tactile,  temperature,  pain,  and  muscular  senses  as  well  as  taste; 
and  the  "feeling"  of  food  contributes  largely  to  its  apparent  "taste." 
For  example,  astringent  and  oily  "tastes"  are  really  tactile  sensa- 
tions. It  is  difficult  to  distinguish  between  taste  and  smell,  except 
by  artificially  excluding  smell.  A  simple  experiment  for  accomplish- 
ing this  is  due  to  Chevreul  (1824),  and  consists  in  holding  the  nose 
while  taking  into  the  mouth  substances  of  various  flavor,  but  reduced 
to  like  consistency  so  as  to  avoid  help  in  discrimination  from  the 
side  of  the  tactile  sense.  Under  these  circumstances,  the  surpris- 
ing fact  comes  to  light  that  it  is  impossible  to  distinguish,  by  taste 
alone,  between  coffee  and  quinine,  or  between  apple  and  onion. 
The  conclusion  from  this  experiment,  when  fully  carried  out,  is 
that  most  so-called  tastes  are  really  odors. 

Excluding  the  sense  of  smell  does  not,  however,  interfere  at  all 
in  the  perception  of  the  following  four  tastes:  sweet,  sour,  bitter, 
saline.  These  four  are  universally  recognized  as  simple  tastes, 
and  it  is  very  doubtful  whether  any  others  should  be  added  to  this 
list.  Metallic  and  alkaline  are  added  by  some  authorities,  and  there 
is  still  some  question  regarding  them;  but  it  is  clear  that  they  are 
at  least  partly  composed  of  mixtures  of  sweet,  sour,  bitter,  and  saline, 
along  with  sensations  of  touch  and  smell. 

§  18.  The  following  question  now  arises:  Does  each  of  the  four 
elementary  tastes  have  a  special  end-organ?  Histologically,  no 
difference  appears  between  the  various  taste-buds  which  can  be 
taken  as  an  indication  of  functional  differentiation.  Yet  it  is  clear 
from  experiment  that  the  taste-buds  are  not  all  equivalent.  The 
fact  is  familiar  that  bitter  is  best  tasted  at  the  back  of  the  tongue, 
and  sweet  best  at  the  tip;  while  the  edges  of  the  tongue  are  the  most 
sensitive  to  sour.  Experiment  has  made  these  results  more  pre- 
cise by  sharply  localizing  the  stimulation.  If  weak  solutions  are 
applied  to  individual  fungiform  papillse  by  means  of  a  camers-hair 
brush,  it  is  found  that  the  sensitivity  of  the  papillse  varies  consider- 
ably; although  it  is  impossible  to  localize  the  stimulus  to  single 
taste-buds.  A  few  papillae  are  found  which  respond  only  to  sweet, 

1  See  the  summary  of  evidence  by  Nagel,  in  his  Handbuch  der  Physiologic, 
1905,  III,  630. 


ANALYSIS  OF  GUSTATORY  SENSATIONS  311 

or  only  to  sour,  etc.  Others  respond  to  two,  and  others  to  three, 
some  to  all  four,  classes  of  the  sensation  of  taste;  while  some  do  not 
respond  at  all.1  These  facts,  so  far  as  they  go,  are  favorable  to 
the  hypothesis  that  individual  taste-buds  are  specialized,  each  for 
one  taste. 

The  physiological  relations  maintained  between  the  four  elemen- 
tary tastes  are  not  yet  clearly  made  out.  A  condition  of  partial 
ageusia,  or  lack  of  taste,  can  be  produced  by  chewing  gymnema 
leaves;  the  sense  for  sweet  and  for  bitter  is  thus  temporarily  de- 
stroyed, while  that  for  sour  and  saline  substances  remains.  The 
mixture  of  two  taste-stimuli  does  not  give  rise  to  a  series  of  clear 
intervening  qualities,  as  does,  for  example,  the  mixture  of  red  and 
yellow,  but  the  two  tastes  in  some  cases  remain  side  by  side  and  dis- 
tinct; in  other  cases  they  alternate;  and  in  still  others,  the  two  neu- 
tralize each  other.  Kiesow  has,  however,  found  that  a  mixture  of 
weak  solutions  of  sugar  and  common  salt  gives  a  flat  alkaline  taste, 
weaker  than  that  of  either  of  the  components  of  the  mixture,  and 
not  like  either  of  them.  Contrast  effects,  such  as  the  extreme  sour- 
ness of  an  acid  fruit  after  a  sweet  substance  has  been  eaten,  are  well 
known;  but  it  seems  at  present  impossible  to  utilize  this  experience 
in  explaining  the  dynamic  relations  of  the  four  simple  sensations 
of  taste.  Thare  is  no  clear  indication  that  the  tastes  can  be  ar- 
ranged in  a  linear  scale,  as  the  primary  colors  are,  nor  that  any  taste 
stands  to  any  other  definitely  in  the  relation  of  opposite  or  comple- 
mentary. On  the  whole,  it  appears  as  if  the  four  tastes  were  rather 
isolated  from  each  other,  each  representing  almost  an  independent 
sense.  There  is  much  blending,  to  be  sure;  but  the  amount  is  ap- 
parently no  greater  between  one  taste  and  another  than  between 
tastes  and  odors. 

§  19.  Little  can  be  said  regarding  the  nature  of  the  chemical 
stimulus  which  arouses  any  of  the  sensations  of  taste.  It  is  true 
that  bodies  of  similar  chemical  composition  usually  evoke  similar 
tastes;  though  a  curious  exception  to  this  rule  is  found  in  the  fact 
that  sweet  and  bitter  are  often  evoked  by  bodies  of  very  similar 
composition,  or  even  by  the  same  body.  On  the  other  hand,  sub- 
stances of  very  different  chemical  composition — such  as,  for  ex- 
ample, cane  sugar  and  sugar  of  lead — sometimes  excite  the  same 
sensation  of  taste.  Since  it  is  in  solution  that  substances  act 
on  the  end-organs  of  taste,  the  theory  of  solutions  needs  to  be 
taken  into  account.  It  may  be  that  dissociated  ions  are  the 
essential  stimulus;  and  there  is  some  positive  evidence  that  acid 

1  Ohrwall,  Skandinav.  Archivf.  Physiol,  1890,  II,  1;  Kiesow,  Wundt's  Philosoph. 
Studien,  1898,  XIV,  591. 


312  THE  QUALITY  OF  SENSATIONS 

taste  is  due  to  the  action  of  hydrogen  ions,  and  alkaline  taste  to 
hydroxyl  ions.1 

§  20.  On  passing  to  the  consideration  of  sensations  of  sound  much 
more  help  is  received  from  the  science  of  physics.  But  modern  in- 
vestigations, in  the  form  in  which  they  concern  us,  do  not  go  back 
of  the  great  work  of  Helmholtz,2  who  made  the  entire  field  pecul- 
iarly his  own.  Since  the  first  appearance  of  this  work,  the  subject 
has  also  been  greatly  enriched  by  the  original  researches  of  Oetting- 
en,  Mach,  Preyer,  Hensen,  Stumpf,  and  other  still  later  writers.  In 
speaking  of  the  stimuli  of  these  sensations,  we  are  compelled  to  re- 
fer chiefly  to  the  vibrations  of  air,  which  are  only  remote  excitants 
of  the  end-organs  of  this  sense.  Neither  physics  nor  physiology 
has  yet  been  able  to  fix  the  precise  locality  in  the  organism  (the  ner- 
vous structure  of  the  cochlea)  where  the  immediate  stimulation  of 
the  end-apparatus  takes  place;  or  to  tell  what  is  the  exact  nature 
of  its  action.  We  are  obliged,  then,  to  confine  ourselves  in  the 
main  to  considering  a  relation  between  the  vibratory  energy  of  the 
air  and  certain  states  of  consciousness,  without  attempting  to  ex- 
plain the  many  intermediate  links. 

All  sensations  which  arise  in  the  mind  by  means  of  the  irritation 
of  the  auditory  nerve  are  called  sensations  of  sound.  The  word 
"stnmd"  is  thus  used  by  psychology  for  a  wholly  subjective  affair, 
which  has  no  more  resemblance  to  those  vibrations  which  physics 
designates  by  the  same  word  than  has  the  taste  sweet  to  the  un- 
known physical  properties  that  produce  it.  The  trained  mind,  or 
"trained  ear,"  as  we  say,  has  indeed  the  power  directly  to  analyze 
a  compound  musical  sound  into  its  constituent  elements.  But  each 
of  these  elements  is  purely  a  sensation,  a  subjective  affair.  It  car- 
ries in  itself  no  token  that  it  has  been  produced  by  vibrations  of 
any  kind;  or  that  it  sustains  any  numerical  relation  whatever  to 
the  vibrations  of  which  some  other  sensation  of  sound  is  composed. 
We  know  nothing  directly,  through  sensations,  either  of  the  struct- 
ure of  the  ear  or  of  vibrating  strings  and  particles  of  air,  or  of  the 
mathematics  and  physics  of  music. 

§  21.  Sounds  are  of  two  classes — tones,  or  musical  sounds,  and 
noises.  The  former  are  due  to  periodic  motions  of  sonorous  bodies; 
the  latter  to  non-periodic.  Noises  are  those  sounds  which,  ob- 
jectively considered,  are  wanting  in  the  periodic  regularity  of  stimu- 
lation which  characterizes  all  musical  sounds,  and,  subjectively 
considered,  in  the  peculiar,  pleasant  modification  of  consciousness 

1  Richards,  Amer.  Chem.  Journal,  1898,  XX;  Kastle,  ibid.}  Hober  and  Kiesow, 
Zeitschr.  f.  physicalische  Chemie,  1898,  XXVII. 

2  Die  Lehre  von   d.  Tonempfundungen  als  physiolog.  Grundlage  filr  d.  Theorie 
d.  Musik  (Braunschweig,  1st  ed.,  1862;  and  several  subsequent  editions). 


CHARACTERISTICS  OF  MUSICAL  SOUNDS          313 

which  the  latter  produce.  But  noises  accompany  almost  all  tones; 
and,  conversely,  tones  may  be  detected  by  the  trained  ear  as  mingled 
with  the  noises  of  every-day  life.  No  player  of  the  violin  avoids  all 
noise  of  scraping  from  the  bow;  no  stroke  of  a  workman's  hammer, 
or  slamming  of  a  door,  that  does  not  start  and  catch  up  into  itself 
some  trace  of  musical  tone.  The  interest  of  science  has  hitherto 
been  almost  wholly  concentrated  upon  musical  sounds,  and  little 
has  been  done  by  either  physics  or  physiology  toward  the  analysis 
of  noises.  It  is  characteristic  of  a  noise,  according  to  Helmholtz, 
that  there  is  a  quick  and  irregular  alternation  of  different  kinds  of 
sensation  of  sound.  This  distinctive  character  can  generally  be 
detected  "by  attentive  aural  observation  without  artificial  assist- 
ance." We  can  compound  noises  out  of  musical  tones;  as,  for  ex- 
ample, by  striking  together  all  the  keys  of  an  octave  on  the  piano. 
Hensen1  distinguished  three  "categories  of  unmixed  noises" — the 
"beats"  or  pulsations  which  disturb  the  purity  of  musical  tones; 
the  crackle,  crack,  or  crash;  and  hissing  sounds.  These  three  shade 
into  each  other,  and,  when  mixed  with  different  kinds  and  quantities 
of  musical  sounds,  make  up  the  noises  which  we  hear  on  every  hand. 

It  is  possible,  therefore,  to  analyze  tonal  elements  out  of  most, 
if  not  all,  kinds  of  noises.  And  although  no  one  has  succeeded  in 
completely  analyzing  a  noise  into  such  elements,  success  has  been 
great  enough  to  encourage  the  opinion  that  a  complete  analysis  of 
all  noises  might  break  them  up  into  combinations  of  various  in- 
harmonic tones. 

§  22.  Musical  sounds  differ,  not  only  in  quality,  but  also  in 
quantity  or  intensity  of  sensation  as  dependent  upon  the  ampli- 
tude of  the  vibrations  which  produce  them.  With  respect  to  their 
quality  they  are  distinguished  as  either  simple  or  complex,  accord- 
ing as  they  result  from  one  set  of  regularly  recurrent  (periodic)  vi- 
brations of  a  given  number  in  a  given  unit  of  time,  or  result  from 
a  combination  of  two  or  more  sets  of  such  vibrations.  The  musi- 
cal sounds  of  ordinary  experience  are  complex.  The  blending  of 
the  simple  tones  into  the  complex  tone  is  not  so  complete,  however, 
that  it  cannot  be  at  least  partially  analyzed  directly  by  a  trained 
ear.  The  complex  sound,  which  results  from  this  compounding 
of  the  contrasts  or  coincidences  of  several  simple  musical  sounds, 
may  be  called  by  the  term  "clang" — in  this  meaning  borrowed 
from  the  usage  of  the  German.  The  quality  of  tones  considered  as 
simple  sensations  is  their  pitch,  which  varies  according  to  a  scale  of 
states  of  consciousness  that  are  immediately  apprehended  and  com- 
pared with  each  other,  and  that  are  discovered  by  objective  meth- 

1  In  Hermann's  Handb.  d.  PhysioL,  III,  ii,  pp.  3-142,  and  works  by  the  same 
author  there  referred  to. 


314  THE  QUALITY  OF  SENSATIONS 

ods  to  correspond  to  a  scale  of  changes  in  the  number  of  the  vi- 
brations of  the  waves  which  occasion  them.  The  pitch  of  tones  is 
therefore  spoken  of  as  "high"  or  "low,"  according  to  the  place 
which  we  assign  to  the  resulting  sensations  in  this'  scale.  Such 
place  in  the  scale  may  be  considered  either  with  respect  to  the  re- 
lation of  any  particular  tone  to  the  upper  or  lower  limits  of  the 
scale,  or  with  respect  to  the  relation  of  the  different  tones  to  one 
another.  "Clangs,"  or  complex  tones — the  musical  sounds  with 
which  we  are  made  acquainted  by  all  ordinary  experience — have 
also  a  variable  quality  called  timbre,  or  "tone-color";  the  timbre 
of  the  clang  is  dependent  upon  the  pitch,  number,  and  relative  in- 
tensity of  the  simple  tones  which  compose  it.  Thus  a  note  having 
the  same  place  in  the  musical  scale  (for  example,  a  of  the  once- 
marked  octave— 440  vibrations)  sounds  differently,  as  we  say,  on 
the  piano,  violin,  cornet,  or  when  sung  by  the  human  voice.  The 
pitch  of  the  tone  as  produced  by  all  these  different  methods  is  the 
same;  but  its  tone-color  is  determined  by  the  character  of  the  over- 
tones which  are  blended  with  the  fundamental  tone. 

§  23.  The  pitch  of  tones  depends  upon  the  rapidity  of  the  peri- 
odic vibrations  (the  number  in  a  given  unit  of  time — usually  one 
second)  which  occasion  them,  or — what  is  the  same  thing — upon 
the  length  of  the  sound-waves.  This  class  of  sensations,  however, 
has  both  an  upper  and  a  lower  limit;  that  is  to  say,  vibrations  either 
below  or  above  a  certain  number  per  second,  or — what  is  the 
same  thing — wave-lengths  that  are  either  shorter  or  longer  than  a 
given  limit,  produce  no  sensations  of  musical  sound.  The  difficulty 
of  determining  these  limits  is  great,  because  the  intensity  of  ex- 
tremely low  or  high  tones  has  to  be  enormously  increased  in  order 
that  they  may  be  heard  at  all;  because  the  perceptions  of  the 
acoustic  sense  are  so  very  blunt  near  the  limits  that  the  different 
sensations  are  almost  certain  to  be  confused;  because  distracting 
sensations  of  common  feeling  mingle  in  these  ranges  of  tone  with 
the  sensations  of  sound,  and  because  near  the  lower  limits  the 
over- tones — especially  the  octave  above — become  so  strong  as  to  be 
mistaken  for  the  fundamental  tones.  On  account  of  these  diffi- 
culties the  older  investigators  made  numerous  mistakes. 

Individual  peculiarities  are  also  very  important  in  determining 
sensations  of  pitch.  Some  persons  can  hear  tones  below  or  above 
t  those  audible  to  most  others;  and,  in  general,  the  range  of  audible 
pitch  decreases  somewhat  with  age — beginning  in  some  cases  with 
the  twentieth  year.  This  difference  in  individuals  is  illustrated 
by  the  judgment  of  Helmholtz,  who  thought  that  sensations  of 
tone  begin  to  cease  when  the  vibrations  fall  below  34  per  second; 
some  tuning-forks  of  great  size,  which  vibrated  only  28  times  per 


THE  PITCH  OF  MUSICAL  SOUNDS  315 

second,  seemed  to  him,  however,  to  have  a  trace  of  tone  in  the  form 
of  a  "weak  drone."  But  Preyer1  found  that  while  14  vibrations 
produced  no  tone  that  he  could  hear,  at  16  vibrations  he  was  able 
to  hear  a  tone;  others  could  distinguish  a  musical  sound  only  at  19 
or  23  vibrations.  The  same  observer  experienced  as  a  sensation  of 
musical  sound  more  than  40,000  vibrations  per  second;  Turnbull 
found  that  the  majority  of  those  with  whom  he  experimented 
could  not  hear  more  than  about  20,000  to  22,500  vibrations  per  sec- 
ond, and  only  one — a  musician — heard  30,000;  Despretz  succeeded 
in  producing  with  tuning-forks  audible  tones  that  had  32,000  vi- 
brations. Of  late  years,  the  Galton  whistle  (a  little  organ  pipe,  of 
adjustable  length),  especially  in  the  improved  form  introduced  by 
Edelmann,  has  been  much  used  in  determinations  of  the  highest 
audible  tone.  Edelmann  reports  that  tones  of  50,000  vibrations 
are  sometimes  heard,  while  Bruner,2  who  tested  a  large  number  of 
individuals,  finds  an  average  of  32,000,  with  individuals  of  appar- 
ently normal  hearing  varying  from  22,000  to  43,000. 

Setting  aside  the  evidence  from  more  or  less  exceptional  cases,  it 
appears  that  vibrations  slower  than  28  to  30  per  second  produce  in 
most  ears  only  a  buzzing  or  groaning  sound;  the  more  acute  tones 
are  unpleasant,  or  even  painful,  and  finally  inaudible  to  all  ears. 
These  results  cannot  be  considered  as  very  concordant  or  precise. 
They  show,  however,  that  the  range  of  the  average  human  ear  is 
not  far  from  ten  octaves,  reaching  from  about  A  8  of  the  sub-contra 
octave  (27J  vibrations  per  second)  to  about  c8  of  the  eight-times- 
marked  octave  (33,792  vibrations  per  second). 

The  table3  on  next  page  gives  the  pitch  of  all  the  musical  tones  au- 
dible to  the  human  ear,  in  the  key  of  C  major,  on  a  scale  in  which 
a1  is  fixed  at  440  vibrations.  Only  about  seven  of  the  rather  more 
than  eleven  octaves  of  the  table  are,  however,  usable  in  music; 
these  seven  reach  upward  from  C^  of  the  contra,  or  from  A2  of  the 
subcontra  octave,  to  b4 — namely,  the  seven  or  seven  and  a  half 
octaves  of  the  modern  piano. 

§  24.  The  sensitiveness  of  the  ear  to  differences  of  pitch  also 
varies  greatly  with  different  individuals,  and  for  the  different  oc- 
taves of  the  musical  scale.  Preyer  found  that  unpractised  persons, 
within  the  octaves  from  c  to  c3  (132-1,056  vibrations  by  the  table, 
but  128-1,024  by  the  scale  adopted  for  his  experiments),  distinguish 

1  Grenzen  d.  Tonwahrnehmung,  p.  23  f. 

3  Archives  of  PsychoL,  1908,  No.  11,  p.  11;  this  work  contains  references  to  the 
older  authorities. 

3  Taken  from  Stumpf,  Tonpsychologie,  I,  p.  xiv,  and  giving  the  German  scale; 
for  the  purposes  of  physics  and  psychology,  C,1  or  "middle  C,"  is  usually  fixed 
at  256  vibrations. 


316 


THE  QUALITY  OF  SENSATIONS 


a  difference  of  from  8  to  16  vibrations  as  producing  a  distinct  dif- 
ference in  the  sensation  of  pitch.  Extreme  cases  of  deafness  to 
differences  in  pitch  are  recorded;  as,  for  example,  that  of  the  man 
who,  in  the  middle  part  of  the  scale,  could  not  distinguish  an  in- 
terval of  less  than  a  third,  and,  in  the  higher  and  lower  parts,  of 
less  than  a  seventh.  Persons  insensitive  to  differences  of  a  tone  or 
half-tone,  who  are  sometimes  said  "not  to  know  one  note  from 


C 

D 

E 

F 

G 

A 

B 

son 

611 

123f 
247^ 
495 
990 
1,980 
3,960 
7,920 
15,840 
31,680 

Subcontra  octave 

16* 
33 
66 
132 
264 
528 
1,056 
2,112 
4,224 
8,448 
16,896 
33,792 

18i9ff 
37J 
74i 
148J 
297 
594 
1,188 
2,376 
4,752 
9,504 
19,008 
38,016 

20| 
41i 
82i 
165 
330 
660 
1,320 
2,640 
5,280 
10,560 
21,120 
42,240 

22 
44 
88 
176 
352 
704 
1,408 
2,816 
5,632 
11,264 
22,528 

24| 
49i 
99 
198 
396 
792 
1,584 
3,168 
6,336 
12,672 
25,344 

27i 
55 
110 
220 
440 
880 
1,760 
3,520 
7,040 
14,080 
28,160 

Cz,  Da,  etc. 
Clt  D^etc. 
C,  D,  etc. 
c,  d,  etc. 
c1,  d1,  etc. 
c2,  d2,  etc. 
c3,  d»,  etc. 
c4,  d4,  etc. 
c5,  d5,  etc. 
c6,  d6,  etc. 
tf,  d?,  etc. 

Contra  octave  

Great  octave  

Small  octave 

Once-marked  octave 

Twice-marked  octave  .... 
Thrice-marked  octave.  .  .  . 
Four-times-marked  octave 
Five-times-marked  octave 
Six  -times-marked  octave  . 
Seven-times-marked  octave 
Eight-times-marked  octave 

another,"  are  by  no  means  infrequently  met.  Sensitiveness  to 
pitch  is,  however,  generally  capable  of  rapid  cultivation,  and  may 
reach  a  high  degree  of  perfection  in  persons  who  have  what  is  called 
"a  good  natural  ear"  for  musical  tones,  if  the  ear  be  also  highly 
trained.  Such  persons  may  become  able  to  discriminate  differ- 
ences in  the  sensations  caused  by  changing  the  number  of  vibra- 
tions not  more  than  a  third  of  a  single  vibration  per  second,  in  the 
region  of  the  scale  between  c  and  g2.  In  the  octave  from  a1  to  a2 
more  than  1,200  tones  are  distinguishable.  But  above  and  below 
this  region  the  distinctions  possible  are  less  fine;  above  c5  even  well- 
trained  ears  commit  errors  in  identifying  two  notes  that  differ  by 
100  or  even  by  1,000  vibrations.  Not  only  the  musical  quality  of 
tones,  but  also  the  power  of  distinguishing  differences  in  them, 
diminishes  rapidly  as  we  approach  the  upper  limits  of  the  scale. 

It  appears,  therefore,  that  sensitiveness  to  pitch  is,  in  general, 
greatest  in  the  most  used  part  of  the  musical  scale;  or,  in  other 
words,  in  that  part  which  falls  within  the  common  range  of  human 
voices.  Within  this  range,  the  least  perceptible  difference  is  a  con- 
stant quantity,  when  expressed  in  number  of  vibrations.1 

§  25.  The  fineness  of  the  possible  distinctions  of  purity  of  in- 
terval also  differs  for  different  individuals  and  for  different  inter- 

1  On  this  whole  subject  compare  the  lengthy  and  interesting  discussion  on 
"  Individuality  des  Sinnes  und  Gediichtnisses  fur  Tonqualitaten, "  in  Stumpf, 
Tonpsychologie,  I,  pp.  262  ff. 


THE  PURITY  OF  MUSICAL  INTERVALS  317 

vals.  The  results  of  experiment  on  this  point  have  been  tabulated 
by  Hensen  as  derived  from  data  drawn  from  Preyer's  investiga- 
tions. This  investigator  found  the  degree  of  sensitiveness  to  be 
greatest  in  the  case  of  the  octave,  next  of  the  fifth,  next  the  whole 
tone,  and  so  on  to  the  minor  third,  where  it  is  least  of  all.  The 
figures  which  give  the  number  of  vibrations  off  from  the  pure  in- 
terval which  is  distinguishable,  for  the  above  intervals,  are  respect- 
ively, 0.13;  0.23;  0.85,  and  1.90.  Other  investigators  have  ob- 
tained somewhat  different  results. 

Immediate  judgment  of  absolute  tone  (as  the  a1  carried  in  mind 
by  musicians)  is  possible;  judgment  between  two  tones  as  to  which 
is  higher  or  lower  in  pitch  is  also  immediate,  and  may  be  exer- 
cised independently  of  everything  except  the  two  sensations  them- 
selves. The  latter  judgment  is  the  common  power  of  mind  be- 
longing to  this  sense;  the  former  is,  as  a  rule,  exercised  only  by 
skilled  persons,  and  by  them  only  very  imperfectly.  Experiments 
of  Stumpf,1  upon  himself  and  three  other  musicians,  showed  that 
the  mistakes  in  judgment  of  absolute  tone  amounted,  in  the  lower 
region  of  the  scale  (from  C:  to  B^),  to  15%-100%  of  the  trials;  in 
the  middle  region  (from  a-gl,  or  from  g-e2),  to  0%-70%;  in  the 
upper  region  (from  ^-/4,  or  from  f-a4),  to  7%-80%.  Only  one  of 
the  four  persons  experimented  upon  seemed  to  approach  the  point 
of  infallibility.  Judgment  of  absolute  tone  is,  therefore,  a  different 
matter  from  that  which  makes  distinctions  in  intervals  or  in  the 
least  observable  differences  of  pitch,  and  is  much  more  precarious. 

§  26.  Those  psychologists  appear  to  be  in  the  right  who  claim 
that  some  power  of  the  mind  immediately  to  judge  differences  of 
quality  in  pitch,  purely  as  such,  must  be  assumed  in  order  to  ac- 
count for  the  foregoing  phenomena.2  Such  judgment,  however, 
may  be,  and  ordinarily  is,  much  assisted  by  auxiliary  discrimina- 
tions of  other  sensations  which  blend  with  those  of  musical  tone. 
Among  such  secondary  helps  the  most  important  are  the  muscular 
sensations  which  accompany  the  innervation  of  the  larynx  and  other 
organs  used  in  producing  musical  tones.  For  we  ordinarily  inner- 
vate these  organs  (at  least  in  an  inchoate  and  partial  way) — that  is, 
we  sound  the  tone  to  ourselves — when  trying  carefully  to  judge  of 
its  pitch.  But  the  niceness  of  these  muscular  sensations  is  not 
great  enough,  even  when  most  highly  trained,  to  account  for  the 
discriminations  of  the  "good  ear."  The  trained  musician  can  de- 

1  Tonpsychologie,  I,  pp.  305  ff. 

2  On  this  subject  compare  Lotze,  Medicin.  Psychologic,  pp.  265  ff.,  480  f.; 
Strieker,  Studieniiber  d.  Association  d.  Vorstellungen,  1883,  pp.  2  f.;  G.  E.  Miiller, 
Zur  Grundlegung  d.  Psychophysik,  pp.  276  ff.,  Berlin,  1878;  and  Stumpf,  Ton- 
psychologie, I,  pp.  134  ff. 


318  THE  QUALITY  OF  SENSATIONS 

tect  by  ear  a  difference  in  quality  between  two  tones  of  400  and 
400-J  vibrations  per  second;  but  the  most  skilful  singer— Jenny 
Lind,  for  example — scarcely  succeeds  in  singing  in  quarter-tones. 
Fairly  good  singers  often  err  by  5-10  vibrations  in  striking  a  note, 
and  cannot  prevent  the  voice  from  varying  by  as  much  as  this  in 
attempting  to  sustain  the  same  tone  for  a  second  or  two.1  More- 
over, the  relative  powers  of  larynx  and  ear  by  no  means  keep 
pace  with  each  other  in  the  same  person.  It  should  also  be  re- 
membered that  all  our  ordinary  discriminations  of  musical  sound 
apply  to  composite  tones,  or  "clangs";  in  discriminating  these  we 
are  aided  by  the  tone-color,  or  tone-feeling,  which  belongs  to  each 
note  as  sounded  by  some  sonorous  body  with  whose  peculiarities 
we  are  previously  more  or  less  acquainted. 

It  follows,  then,  that  the  judgment  is  supplied,  by  the  varying 
qualities  of  musical  tones,  with  the  means  for  arranging  them  in  a 
continuous  series  which  may  be  symbolized  by  different  positions 
assigned  along  an  uninterrupted  straight  line.  Of  any  three  un- 
like tones,  one  must  be,  and  only  one  can  be,  arranged  as  respects 
pitch  between  the  other  two.  And  whenever  any  two  tones,  as  m 
and  n,  are  given,  another  sliding  tone,  which  begins  with  m  and 
ends  with  n,  is  possible.  Moreover,  within  the  bounds  of  our  ex- 
perience of  tones,  as  we  advance  along  the  scale  toward  either  the 
upper  or  the  lower  limit,  we  see  no  tendency  in  the  qualities  of 
the  sejisations  to  approach  each  other.  In  this  respect  the  scale 
of  sound-tones  is  wholly  different  from  that  of  color-tones.  There 
are  not  two  ways,  for  example,  of  getting  from  a1  to  c3  (one  through 
61,  c2,  etc.,  and  the  other  through  gl,  fl,  etc.,  around  to  03,  d3,  and 
then  c3),  as  there  are  two  ways  of  going  from  yellow  to  blue  (i.  e., 
through  green  and  blue-green,  or  through  orange,  red,  and  violet). 
We  speak,  then,  of  the  series  of  tones  as  a  constant  and  infinite 
series;  although,  of  course,  no  series  of  states  of  consciousness  is 
really  infinite,  and  although  the  upper  and  lower  limits  of  the 
musical  scale,  as  well  as  the  limits  of  the  least  observable  differ- 
ences between  two  tones,  are  not  constant  but  variable  for  differ- 
ent individuals. 

The  symbolism  taken  from  relations  of  space,  which  we  employ 
when  we  speak  of  certain  acoustic  sensations  as  "high"  and  of 
others  as  "low"  in  pitch,  or  when  we  distinguish  so-called  "in- 
tervals" between  the  tones  as  large  and  small,  is  strictly  applicable 
only  to  the  complex  tactual,  visual,  and  muscular  sensations  that 
accompany  the  acoustic.  In  sounding  the  lower  tones  with  the 
voice  the  organs  are  depressed;  in  sounding  the  higher,  they  are 
elevated.  Low  tones  have  a  certain  breadth  and  gravity  which 
1  Cameron,  Psychol  Rev.  Monogr.  Suppl.  No.  34,  1907,  p.  227. 


NATURE  OF  THE  "CLANG"  319 

correspond  to  the  foundations  of  a  spatial  structure;  as  sensations 
they  require  more  time  to  come  into  and  depart  from  conscious- 
ness, as  it  were.  A  great  intensity  and  slower  tempo  belong  to  the 
bass-viol  than  to  the  violin.  We  read  up  for  the  notes  of  highest 
pitch,  and  down  for  those  of  lowest  pitch,  in  the  written  musical 
scale. 

§  27.  We  have  seen  that  tones,  like  rays  of  light,  come  to  us  as 
compounded  into  "clangs";  these  really  composite  tones  being 
esteemed  as  single  tones  in  ordinary  experience.  The  nature  of 
such  composition  determines  the  so-called  "timbre,"  or  "tone- 
color,"  of  the  compound  tone.  In  a  word,  each  sensation  of  a  clang 
is  a  summing-up  in  consciousness  of  several  absolute  qualities  of 
musical  sound;  the  stimulus  which  occasions  this  complex  subjec- 
tive state  is  a  complex  sound-wave  made  up  of  the  contrasts  and  co- 
incidences of  several  single  waves  that  have  the  character  of  simple 
pendular  vibrations.  The  quality  of  each  clang  depends  upon  the 
combination  of  simple  waves,  of  different  lengths,  within  this  com- 
plex sound-wave. 

In  the  interests  of  psycho-physical  science,  or  the  study  of  the 
various  qualities  and  combinations  of  sensations  of  sound  from 
the  point  of  view  held  by  physiological  psychology,  it  is  not  necessary 
to  consider  with  any  detail  the  mathematics  and  physics  of  the 
waves,  whether  simple  or  in  combination,  which  act  upon  the  organ 
of  hearing  and  so  become  the  stimuli  of  these  sensations.  To  af- 
ford some  explanation  of  those  acoustic  sensations  which  appear 
in  consciousness  as  having  an  sesthetical  value,  and  which,  when 
combined  and  arranged  in  certain  ways,  give  rise  to  the  art  of  music, 
it  is  enough  to  recall  the  physical  fact  that  a  sounding  body,  such 
as  a  piano  string,  gives  forth  when  struck  not  only  the  "funda- 
mental" pitch  to  which  it  is  tuned,  but  also  a  series  of  "overtones," 
the  vibration  rates  of  which  are  2,  3,  4,  5,  6,  7,  8,  9,  10,  etc.,  times 
the  vibration  rate  of  the  fundamental.  The  string  may  be  said  to 
be  vibrating,  not  only  as  a  whole,  but  simultaneously  in  halves, 
thirds,  fourths,  etc.  The  vibrations  impressed  by  the  string  on 
the  air  and  by  the  air  on  the  drum  of  the  ear,  are  therefore  highly 
compound;  and  a  fundamental  fact  in  the  physiology  of  hearing, 
as  noted  in  an  earlier  chapter  (see  p.  206),  is  that  overtones  can, 
with  training  and  attention,  be  heard  out  of  a  clang,  and  that  there- 
fore the  ear  must  act  as  an  analytic  organ.  Ordinarily,  however, 
we  do  not  consciously  analyze  a  clang,  but  experience  it  as  a  whole. 

Of  the  whole  series  of  overtones,  some  are  favored  by  one  instru- 
ment, and  others  by  another  instrument,  and  thus  the  tones  of  the 
instruments  differ.  It  was  an  achievement  of  Helmholtz  to  analyze 
the  clangs  of  different  instruments,  and  to  show  that  it  is  the  differ- 


320 


THE    QUALITY  OF  SENSATIONS 


ence  in  overtones  which  gives  the  peculiar  quality  to  the  tone  of 
each  instrument,  each  human  voice,  each  vowel,  etc.  Instruments 
which  give  off  few  and  weak  overtones  have  a  soft  but  rather  dull 
tone;  those  which  favor  the  lower  overtones  have  a  rich  but  mellow 
tone;  those  which  favor  the  higher  overtones  tend  toward  shrill- 
ness. Besides  the  overtones,  other  elements  contribute  to  the  pe- 
culiar quality  of  an  instrument;  e.  g.,  the  noises  made  in  operating 
it,  the  suddenness  with  which  its  sound  begins  and  ends,  etc. 

§  28.  Of  the  very  numerous  pitches  which  are  distinguishable, 
music  makes  use  of  only  a  small  selection.  It  is  not  so  much  the 
absolute  pitches,  as  the  intervals  between  the  notes  of  the  scale,  that 
determine  the  qualitative  effects  of  the  music.  The  scales  in  use 
among  the  different  peoples  differ  greatly:  Chinese  music,  for  ex- 
ample, uses  quite  a  different  scale  from  European  music;  and  the 
scale  used  in  the  native  Japanese  music  may  be  called  "indefinitely 
penta tonic."  The  scale  of  European  music  has  come  down  from 
the  Greeks,  and  perhaps  from  the  Egyptians;  it  is  said  to  have  been 
reduced  by  the  Pythagoreans  to  a  mathematically  exact  form,  ac- 
cording to  which  the  relative  vibration  rates  of  the  tones  of  the 
diatonic  scale  are  as  follows:  • 


1st 

2d 

3d 

4th 

5th 

6th 

7th 

Octave 

C 

D 

E 

F 

G 

A 

B 

C1 

1 

! 

1 

f 

f 

1 

¥ 

2 

8 

9 

10 

101 

12 

m 

15 

16 

That  is  to  say,  while  the  tone  C  makes  one  vibration,  D  makes 
nine-eighths,  and  E  makes  five-fourths,  etc.;  or  while  C  makes 
8  vibrations  D  makes  9,  E  makes  10,  etc.  Of  these  relations  in 
the  number  of  vibrations,  the  simplest  is,  of  course,  that  of  the 
octave,  1:2;  and  the  octave  seems  to  be  present  in  almost,  or  quite, 
all  musical  scales;  whereas  the  other  intervals  are  less  universal. 
Even  in  European  music,  the  practical  necessities  of  instruments 
with  keyboards  (the  piano,  etc.)  have  led  to  the  abandonment  of 
the  strict  mathematical  ratios,  and  the  substitution  of  the  "equally 
tempered"  scale,  in  which  the  octave  is  first  divided  into  twelve 
equal  semitones,  and  the  different  intervals  of  the  scale  then  built 
up  as  well  as  possible  out  of  these  semitones.  The  result  is  that 
all  the  intervals  except  the  octave  are  slightly  mistuned  by  compari- 
son with  the  Pythagorean  relations;  yet  the  effect  is  satisfactory 
to  even  musical  ears  which  have  become  accustomed  to  this  scale. 


THEORY  OF  CONSONANCE  AND  DISSONANCE     321 

Indeed,  European  ears,  to  which  Oriental  music  seems  at  first  de- 
void of  musical  quality,  are  able,  after  practice  with  the  Oriental 
scale,  to  find  beauty  in  it;  and  this  fact,  as  well  as  the  fact  of  the 
equally  tempered  scale,  suggests  that  the  use  of  a  particular  scale 
is  partly  a  matter  of  custom.  It  does  not  seem  possible,  in  view  of 
all  the  facts,  to  lay  so  much  stress  on  "simple  ratios"  as  was  done 
by  the  Pythagoreans  and  many  later  theorists;  yet,  on  the  other 
hand,  it  is  not  possible,  at  present,  to  substitute  any  other  theory 
of  the  psychology  of  scale  formation  which  would  meet  with  the 
general  approval  of  authorities. 

§  29.  Modern  European  music  makes  great  use  of  the  simul- 
taneous sounding  of  two  or  more  tones,  forming  chords  and  dis- 
cords. A  chord  or  consonance  may  be  described  as  a  combina- 
tion of  tones  which  gives  a  smooth  and  agreeable  impression;  a 
discord  or  dissonance  as  a  combination  which  gives  a  rough  and  dis- 
agreeable impression.  Among  the  combinations  of  two  tones,  the 
most  perfect  consonance  is  no  doubt  presented  by  the  octave,  with 
vibration  rates  of  2:1;  other  recognized  consonant  pairs  are  the 
Fifth  (theoretical  ratio  3  : 2),  the  Fourth  (4  : 3),  the  Major  Third 
(5  : 4),  Minor  Third  (6  : 5),  Major  Sixth  (5  : 3),  and  Minor  Sixth 
(8  :  5).  That  custom  or  habituation  has  much  to  do  with  the  agree- 
ableness  or  disagreeableness  of  many  of  these  combinations  is 
evidenced  by  the  history  of  European  music;  for  the  older  music 
rejected  Thirds  and  Sixths  as  dissonant,  whereas  they  are  extremely 
satisfactory  to  modern  ears;  while  recent  music  makes  great  use 
of  Seconds,  Sevenths,  and  Ninths,  in  combination  with  other  notes 
of  the  scale.  It  seems  more  proper,  however,  to  speak  of  these 
dissonances  as  "tolerated,"  and  to  regard  the  sesthetical  pleasure 
as  consisting  in  an  appreciation  of  this  contrast  with  preceding  or 
following  consonance. 

Such  facts  as  the  foregoing  make  it  difficult  to  formulate  a  theory 
of  consonance  and  dissonance — and  yet  consonance  and  dissonance 
certainly  are  phenomena  which  call  for  explanation,  in  view  of 
their  importance  in  music,  which  is,  psychologically  considered,  the 
most  highly  developed  system  of  sensory  qualities,  treated  as  such, 
rather  than  for  their  associations  or  meanings.  A  partial  ex- 
planation of  some  of  the  phenomena  of  consonance  and  dis- 
sonance is  found  in  a  fact  brought  forward  by  Stumpf,1  that 
if  a  tone  and  its  octave  are  sounded  together,  they  not  only 
produce  a  smooth  and  agreeable  impression,  but  fuse  or  blend  to 
such  an  extent  that  even  a  musically  trained  ear  may  be  in  doubt 
whether  one  or  two  tones  are  being  sounded.  If,  on  the  contrary, 

1C.  Stumpf,  "Konsonanz  und  Dissonanz,"  in  his  B&itrage  zur  Akustik  und 
Musikurissentschaft,  1898,  I,  1. 


322  THE  QUALITY  OF  SENSATIONS 

a  tone  and  its  Second  or  Seventh  are  sounded,  the  fact  that  two 
tones  are  being  sounded  is  evident,  usually,  even  to  an  untrained 
ear.  In  general,  the  degree  of  fusion  corresponds  to  the  perfec- 
tion of  the  consonance;  Stumpf,  therefore,  suggests  that  the  fusion 
is  the  source  of  the  consonance,  or,  in  other  words,  that  the  unity 
of  the  impression  is  the  measure  of  its  harmoniousness.  The 
physiological  basis  for  the  blending  of  certain  tones  and  the  com- 
parative refusal  of  others  to  blend,  remains  to  be  worked  out. 

§  30.  It  is  important  in  this  connection  to  bear  in  mind  the  com- 
plexity of  the  stimulus  when  two  or  more  tones  are  sounded  to- 
gether. Each  single  tone  is  a  clang  with  overtones.  The  over- 
tones of  one  of  two  complex  tones  may  coincide,  to  a  greater  or 
less  extent,  with  the  overtones  of  the  other.  In  the  case  of  a  funda- 
mental tone  and  its  octave,  all  the  overtones  of  the  upper  tone  co- 
incide with  overtones  of  the  lower;  but  in  case  of  the  smaller  in- 
tervals, the  correspondence  of  overtones  is  less  complete.  Thus  the 
degree  of  consonance  runs  parallel,  to  a  considerable  extent,  with 
the  amount  of  coincidence  of  overtones;  and  Helmholtz  and  others 
have  laid  stress  on  this  point  in  their  theories  of  consonance,  and 
of  the  relatedness  which  is  felt  between  the  various  tones  of  the 
scale. 

Now  we  know  that  when  two  sounding  bodies,  of  different  vi- 
bration rates,  act  on  the  same  body  of  air,  the  waves  from  the  one 
periodically  strengthen  and  weaken  the  waves  from  the  other;  and 
the  number  of  such  "interferences"  per  second  is  equal  to  the  dif- 
ference between  the  two  vibration  rates.  Thus,  if  tones  of  200  and 
201  vibrations  are  sounded  together,  the  compound  tonal  effect 
will  wax  and  wane  once  a  second.  This  change  in  intensity  can  be 
distinguished,  and  is  called  a  beat.  As  the  difference  between  the 
vibration  rates  becomes  greater,  the  beats  become  more  frequent, 
and  also  rougher  and  more  unpleasant,  reaching  the  maximum  of 
unpleasantness  at  about  30  per  second.  Beyond  this  point,  the 
effect  of  beats  becomes  less  marked,  and  disappears  at  a  difference  in 
the  vibration  rate  of  about  50-60.  Rough,  unpleasant  beats  are 
inconsistent  with  consonance;  this  fact,  therefore,  forms  the  basis 
of  a  negative  theory  of  consonance,  which  defines  dissonance  as  the 
presence  of  beats,  and  consonance  as  the  absence  of  dissonance. 
Helmholtz,  who  bases  his  theory  of  consonance  in  this  way,  calls 
attention  to  the  disagreeableness  of  other  intermittent  sensations, 
such  as  a  flickering  light,  for  example.  It  is,  however,  not  only  the 
beats  of  the  fundamental  tones  which  have  to  be  considered;  for,  even 
if  the  fundamentals  do  not  beat,  their  overtones  may  do  so;  and  in 
general,  the  consonances  which  appear  the  most  perfect  are  those 
in  which  the  overtones  do  not  beat  observably,  whereas  the  Minor 


THE  DIFFERENCE-TONE  323 

Third  and  Minor  Sixth,  for  example,  have  overtones  which  produce 
audible  beats. 

§  31.  Another  element  of  the  complex  mass  produced  by  the 
simultaneous  sounding  of  two  tones  is  the  difference-tone.  If  two 
preferably  high  tones  are  sounded  together,  attentive  observation 
reveals  a  low  tone  sounding  with  them;  this  low  tone  is  not  heard 
when  either  of  the  high  tones  is  sounded  alone,  but  only  when  the 
two  sound  together.  The  pitch  of  this  additional  tone  is  equal  to 
the  difference  in  vibration  rate  of  the  two  inducing  tones.  This  in- 
duced tone  is  called  the  first  difference-tone,  to  distinguish  it  from 
others  of  like  character  but  of  lower  or  higher  pitch.  The  external 
stimulus  which  gives  rise  to  the  difference-tone  is  the  same  as  that 
which  gives  rise  to  beats — namely,  the  periodic  interference  between 
two  sets  of  waves  of  different  vibration  rates;  but  the  starting-point 
of  the  difference-tone,  as  a  separate  series  of  vibrations,  is  apparently 
in  the  ear,  either  in  the  tympanic  membrane  or  in  the  fenestra  ro- 
tunda or  ovalis.  Difference-tones  are,  therefore,  of  importance  in 
connection  with  theories  of  hearing;  and  also  in  connection  with 
theories  of  consonance,  especially  in  the  explanation  of  the  discordant 
effect  of  slightly  mistuned  intervals.  In  such  intervals,  the  differ- 
ence-tones of  different  order  are  likely  to  beat  with  each  other.1 

When  we  consider  the  great  complexity  of  the  acoustic  effect 
of  a  chord  of  even  two  tones,  with  its  overtones,  beats,  difference- 
tones,  and  the  degree  of  fusion  of  all  these  elements,  it  is  not  to  be 
wondered  at  that  students  of  this  branch  of  psychology  have  not 
yet  been  able  to  come  to  a  generally  acceptable  theory  of  the  cause 
of  the  harmonious  or  inharmonious  effect  of  the  chord. 

'See  F.  Krueger,  "  Beobachtungen  iiber  Zweiklangen,"  in  Wundt's  Philo- 
sophische  Studien,  1900,  XVI,  307,  568. 


CHAPTER  II 
THE  QUALITY  OF  SENSATIONS  (CONTINUED) 

§  1.  The  analysis  of  the  qualities  of  different  Sensations  of  Sight 
is  much  more  intricate  than  that  of  any  of  the  other  senses.  They 
may  all  be  described  as  sensations  of  color  and  light;  but  an  in- 
definite number  of  colors  is  known  to  experience,  and  many  de- 
grees of  the  sensation  of  light.  Moreover  the  quantity  of  the  white 
light  which  acts  as  stimulus  upon  the  eye  has  an  important  effect 
upon  the  quality  of  the  resulting  color-sensation;  in  other  words, 
the  tone  of  the  color  is  dependent  upon  the  amount  of  white  light 
which  is  mixed  with  the  "saturated"  spectral  color.  The  size  of 
the  colored  object  and  the  resulting  breadth  of  the  sensation,  as 
well  as  the  intensity  of  the  stimulus  and  the  time  during  which  it 
acts,  also  affect  the  quality  of  the  sensation.  Still  further,  the  same 
stimulus  produces  different  sensations  as  it  falls  upon  different  por- 
tions of  a  normal  retina;  while  a  considerable  class  of  persons  are 
color-blind,  or  incapable  of  certain  kinds  of  color-sensations.  The 
previous  condition  of  the  retina,  and  the  relations  between  the  con- 
tiguous portions  when  any  considerable  area  of  it  is  under  stimu- 
lation, must  also  be  taken  into  account.  The  fundamental  laws 
governing  sensations  of  sight  can,  therefore,  be  discovered  only  by 
excluding  for  the  time  many  of  those  variable  elements  which,  in 
fact,  always  enter  into  the  determination  of  the  exact  character  of 
our  experiences  in  the  use  of  our  eyes.  Thus  defining  the  first  prob- 
lem before  us,  we  find  that  it  may  be  stated  in  the  following  terms. 
What  sensations  result  from  the  stimulation  of  a  sufficiently  small, 
but  not  too  small,  area  of  the  most  central  part  of  the  normal  retina, 
for  a  given  time,  when  it  is  not  fatigued  and  the  eye  is  at  rest,  and 
with  neither  too  great  nor  too  small  intensity  of  a  given  kind  of 
light?  Such  sensations  may  be  called  (though  somewhat  ineptly) 
normal  sensations  of  color.  When  the  foregoing  question  is  an- 
swered we  may  go  on  to  consider  the  most  important  variations 
possible  on  account  of  various  forms  of  departure  from  the  so-called 
normal  conditions  of  sensation. 

§  2.  The  ordinary  stimulus,  the  application  of  which  to  the  eye 
gives  rise  to  the  sensations  of  sight,  is  light — or  certain  exceedingly 
rapid  oscillations  of  luminiferous  ether.  Some  forms  of  mechani- 

324 


STIMULUS  OF  VISUAL  SENSATIONS  325 

cal  and  electrical  stimuli  also  produce  the  same  sensations.  Any 
violent  shock  to  the  eye,  such  as  a  blow  upon  the  back  of  the  head, 
may  fill  the  whole  field  of  vision  with  an  intense  light.  The  action 
of  mechanical  pressure  of  moderate  intensity  upon  a  limited  part 
of  the  retinal  elements  may  be  studied  by  rolling  the  eyeball  in- 
ward and  using  the  fingernail,  or  a  small,  blunted  stick,  upon  the 
outer  surface  of  the  closed  lids.  By  such  stimulation  disks  of 
light  (called  phosphenes),  with  darkly  colored  edges,  are  produced 
in  the  field  of  vision  of  the  closed  eye.  Some  observers  have 
claimed  that  very  strenuous  exertion  of  the  apparatus  for  accom- 
modation occasioned  in  their  eyes  similar  phenomena  ("phos- 
phenes of  accommodation").  On  making  or  breaking  a  weak  elec- 
trical current  sent  through  the  eye,  the  entire  field  of  vision  is 
lighted  up;  the  constant  current  also  seems  to  excite  the  optic 
nerve.  The  quality  of  the  sensations  thus  excited  is  found  to  de- 
pend upon  the  direction  of  the  current  through  the  nerve.  When 
the  current  is  ascending,  the  place  where  the  nerve  enters  the  ret- 
ina appears  as  a  dark  disk  upon  a  field  of  vision  that  is  brighter 
than  it,  and  of  pale  violet  color;  when  it  is  descending,  as  a  bright 
bluish  disk  on  a  field  of  dark  or  reddish-yellow  color.  The  retina 
has  also  a  "light  of  its  own"  (Eigenlicht)',  for  its  nervous  elements 
are  rarely  or  never  inactive,  but  have  a  continuous  tonic  excitation. 
Hence  some  persons  see  the  most  gorgeous  and  varied  coloring, 
when  the  eyes  are  closed  in  a  darkened  room.  This  normal  light 
of  the  retina  is  not  constant  either  in  degree  or  in  quality;  both 
the  form  and  the  color  of  the  different  minute  parts  of  the  field 
of  vision,  as  lighted  by  it,  are  very  changeable.  It  may  be  said 
to  have  the  rhythmic  movement  of  all  tonic  excitation.  Such  ex- 
citation is  supposed  to  be  due  to  chemical  effects,  wrought  by  the 
changing  supply  of  blood,  upon  the  nervous  elements  of  the  retina 
and  (perhaps,  also)  of  the  central  organs  of  the  brain.  The  pe- 
culiar action  of  the  ascending  and  descending  electrical  current 
has  been  thought  by  some1  to  be  due  to  its  catelectrotonic  or  an- 
electrotonic  effect  upon  the  central  organs  by  way  of  the  optic 
nerve.  Aubert  has  estimated  the  retina's  own  light  to  be  about 
equal  (in  his  case)  to  half  the  brightness  of  a  sheet  of  white  paper 
when  seen  in  the  full  light  of  the  planet  Venus. 

§  3.  The  place  where  the  light  acts  (and  here,  as  is  supposed,  only 
indirectly  through  photo-chemical — and  perhaps  electro-motive — 
changes  in  the  pigments  of  the  eye)  upon  the  end-organs  of  vision 
must  be  located  at  the  back  of  the  retina  in  the  rods  and  cones. 
The  argument  by  which  we  have  already  (see  p.  194)  connected 

^ee  Fick,  "Physiolog.  Optik,"  in  Hermann's  Handb.  d.  Physiol,  III,  i,  p. 
230. 


326 


THE  QUALITY  OF  SENSATIONS 


the  analytic  power  of  vision  with  the  structure  of  this  nervous 
layer  may  be  carried  yet  further  into  details.  It  appears  likely 
that  each  element  of  the  structure — at  least  in  some  parts  of  the 
retina — should  be  regarded  as  an  isolated  sensitive  spot,  which 
corresponds  on  the  one  side  to  definite  excitations  from  the  appro- 
priate stimuli,  and  on  the  other  side  to  the  smallest  localized  sen- 
sations of  color  and  light.  Accordingly,  in  order  that  two  visual 
sensations  may  be  seen  as  separate,  yet  side  by  side,  in  an  object, 
two  neighboring  retinal  elements  must  be  excited  by  the  stimulus. 
This  implies  that  the  breadth  of  retinal  surface  stimulated  must  be, 
at  least,  about  that  of  the  distance  between  two  such  elements. 

With  this  hypothesis  the  facts  of  his- 
tology and  experimental  physiology 
agree  fairly  well. 

The  degree  of  accuracy  which  sight 
can  attain  seems  to  be  related  to  the 
size  of  the  retinal  elements  directly 
affected  by  the  light.  Certain  indi- 
vidual differences,  either  native  or 
due  to  training,  in  the  fineness  of  vis- 
ual discriminations  must,  apparently, 
be  allowed.  Hooke  observed  that  no 
one  can  distinguish  two  stars  as  two, 
unless  they  are  apart  at  least  30" ; 
few,  indeed,  can  distinguish  them 

when  less  distant  from  each  other  than  60".  E.  H.  Weber  could 
not  perceive  as  separate  two  lines  whose  distance  did  not  cover 
at  least  73"  of  the  angle  of  vision;  Helmholtz  puts  the  limit  of  his 
sharpness  of  vision  at  64".  The  numbers  60",  64",  and  73",  in 
the  angle  of  vision,  correspond  to  a  size  of  the  retinal  elements 
varying  from  0.00438  mm.  to  0.00526  mm.;  and  this  agrees  very 
closely  with  the  calculated  breadth  (by  Kolliker)  of  the  thickness 
of  the  cones  in  the  yellow-spot — namely,  0.0045  mm.  to  0.0055 
mm.  (0.000177  in.  to  0.0002165  in.).  If  white  lines  be  drawn  on 
a  dark  ground  so  closely  together  as  to  approximate  this  limit  of 
vision,  they  will  appear,  not  straight,  but  knotted  and  nicked.  This 
fact  is  due  to  the  action  of  the  stimulus  on  the  mosaic  of  rods  and 
cones,  as  seen  by  the  accompanying  figure  (No.  117).  The  di- 
minishing sharpness  of  vision  as  we  move  away  on  the  surface  of 
the  retina  from  its  most  central  area  corresponds  to  the  compar- 
ative paucity  of  the  nervous  elements,  especially  of  the  cones,  which 
enter  into  the  structure  of  the  peripheral  parts. 

§  4.  Excluding  consideration  of  those  changes  in  the  quantity, 
as  such,  of  visual  sensations  which  are  produced  by  changes  in  in- 


B 


11 
*«* 


FIG.  117.—  A  shows  the  appearance  of 
lines  drawn  very  closely  together, 
which  is  supposed  to  be  due  to  their 
falling  upon  the  nervous  elements 
of  the  retina  in  the  manner  shown 
by  B. 


THE  SPECTRAL  COLOR-TONES  327 

tensity  of  the  light,  and  confining  our  attention  to  what  has  already 
been  defined  as  the  normal  action  of  the  eye,  we  treat  scientifically 
all  the  different  sensations  of  sight  when  we  describe  (1)  the  wave- 
lengths of  the  different  kinds  of  colored  light,  or  pure  color-tones, 
and  (2)  the  relations  in  which  the  different  colors  stand  with  re- 
spect to  the  amounts  of  white  (or  colorless  light)  and  saturated 
light  (or  light  of  pure  color-tone)  which  enter  into  them.  The  fore- 
going distinctions  in  the  quality  of  our  color-sensations  may  be  con- 
firmed by  an  appeal  to  experience.  Red  is  unlike  yellow  in  "color- 
tone,"  and  both  are  unlike  blue;  but  orange  is  more  like  either  red 
or  yellow  than  it  is  like  blue,  while  violet  is  more  like  blue  than  it 
is  like  either  yellow  or  red.  Yet  we  distinguish  colors  of  the  same 
class  (red,  green,  or  violet)  as  being  like  or  unlike  with  respect 
to  their  "brightness";  and  in  respect  of  brightness,  a  certain  shade 
of  red  may  differ  more  from  another  shade  of  red  than  it  differs 
from  some  shade  of  yellow,  green,  or  blue.  The  brightness  of 
a  color  is,  scientifically  speaking,  dependent  both  upon  the  degree 
of  saturation  which  the  color  possesses  and  upon  the  total  inten- 
sity of  the  light. 

§  5.  A  color-tone  is  said  to  be  "pure"  or  "saturated"  when  it  is 
free  from  all  admixture  of  other  color-tones.  Pure  or  saturated 
color-tones  can  be  obtained  only  by  the  use  of  the  spectrum,  which, 
on  account  of  the  different  refrangibility  of  the  different  colored 
rays  that  compose  it,  analyzes  the  compound  ray  of  white  light  into 
its  constituent  color-tones.  By  stimulating  with  different  simple 
rays  those  nervous  elements  which  have  the  same  local  situation 
at,  or  very  near,  the  pole  of  the  eye,  we  test  the  question  whether 
each  special  color-sensation  corresponds  to  a  special  physical  con- 
struction of  the  stimulus.  It  is  thus  discovered  that  the  compound 
ray  of  sunlight,  so  far  as  it  stimulates  the  human  eye,  is  made  up 
of  components  formed  by  oscillations  varying  all  the  way  between 
about  three  hundred  and  seventy  billions  and  about  nine  hundred 
billions  per  second;  and  that  the  color-tone  of  the  sensation  changes' 
as  the  number  of  these  oscillations  changes.  The  table1  on  next 
page  exhibits  these  facts  on  the  scale  of  Fraunhofer's  lines,  which 
mark  those  portions  of  the  spectrum  where  its  principal  colors  ap- 
pear most  obvious  to  the  normal  eye. 

Rays  of  light  which  have  a  number  of  oscillations  less  than  four 
hundred  and  seventy  billions  per  second,  so  far  as  they  affect  the 
retina  at  all,  occasion  the  sensation  of  Red;  and  this  sensation  does 
not  vary  essentially  in  quality  when  the  oscillations  are  four  hundred 
and  forty  to  four  hundred  and  sixty  billions.  But  when  their  number 

1  Taken  from  Pick,  "  Physiolog.  Optik,"  in  Hermann's  Handb.  d.  Physiolog., 
Ill,  i,  p.  173. 


328 


THE  QUALITY  OF  SENSATIONS 


increases  beyond  four  hundred  and  seventy  billions  (0)  the  quality 
of  the  sensation  changes  rapidly,  takes  on  a  yellow  tone  (Orange- 
yellow),  and  finally,  at  about  five  hundred  and  twenty-six  billions 
(D),  corresponds  to  what  we  definitely  call  Yellow.  This  yellow 
becomes  greenish  as  the  oscillations  increase  in  number,  until  they 
reach  about  five  hundred  and  eighty-nine  billions  (E),  when  Green 
appears.  (Changes  from  yellow  to  green  occupy  only  a  small  zone 


Name  of  the  line 

• 

Number  of  vibra- 
tions per  second 

Wave-length  in  the  air 

B  

Billions. 
450 

Millimetres. 
0  0006878 

C 

472 

0  0006564 

D  

526 

0  0005888 

E  

589 

0  0005260 

F 

640 

0  0004843 

G    ..    .                   .           .           . 

722 

0  0004291 

H  

790 

0.0003928 

in  the  spectrum.)  The  green  in  turn  becomes  bluish;  at  six  hun- 
dred and  forty  billions  (F)  Blue  begins  to  appear.  From  this  point 
to  seven  hundred  and  twenty-two  billions  (F-G)  the  color-tones 
that  lie  between  blue  and  violet  are  run  through;  beyond  the  latter 
number  Violet  comes  to  view. 

The  color-tones  of  the  spectrum  are,  therefore,  not  sharply  sepa- 
rated, but  pass  gradually  into  each  other.  The  nearer  together  two 
colors  are  situated  in  the  spectrum,  the  more  nearly  do  they  corre- 
spond in  the  quality  of  their  sensations.  Nor  has  the  spectrum  any 
sharply  defined  limit  at  either  end,  but  passes  gradually  into  black 
— more  gradually  at  the  violet  than  at  the  red  end.  The  energy 
of  the  ultra-red  rays,  as  measured  by  their  physical  and  chemical 
action,  is  greater  than  that  of  the  more  highly  refrangible  rays. 
The  fact  that  these  rays  do  not  excite  visual  sensations  must,  then, 
be  due  to  the  structure  of  the  retina.  The  ^ra-violet  end  of  the 
spectrum  has  been  made  visible  for  a  certain  extent  by  experiment;1 
it  produces  the  sensation  of  a  glimmer  of  lavender-gray  color.  Our 
inability  to  perceive  these  ultra-red  and  ultra-violet  rays  is  not  to 
be  considered  an  imperfection  of  the  eye,  as  Tyndall  thought.  It 
is  rather  purposeful,  and  of  the  greatest  importance  for  vision; 
since,  if  these  ultra  rays  were  visible,  the  clearness  of  objects  would 
be  much  disturbed  by  the  chromatic  aberration  of  the  refracting 
apparatus  of  the  eye. 

1  See  Helmholtz,  Physiolog.  Optik,  pp.  232  f. 


RELATIVE  BRIGHTNESS  OF  COLOR-TONES 


329 


§  6.  Besides  the  foregoing  distinctions  of  color-tones,  the  im- 
pression made  by  the  green-yellow  of  the  spectrum  (D-E,  and  im- 
mediately about  D)  is  by  far  the  strongest;  or,  as  we  should  say, 
this  color  is  naturally  the  "brightest"  of  the  spectral  colors.  From 


. 


FIG.  118. — (From  Fick.)  The  letters  on  the  horizontal  line  stand  for  Fraunhofer's  lines. 
The  ordinates  of  the  interrupted  curved  line  show  the  brightness  of  rays  as  seen;  the  ordi- 
nates  of  the  dark  curved  line,  the  intensity  of  the  rays  as  measured  by  calorific  effect. 

the  region  immediately  around  D,  the  brightness  of  the  color-tones 
diminishes  toward  both  the  red  and  the  violet  ends  of  the  spectrum 
— at  first  slowly,  then  more  quickly,  and  then  more  slowly  again. 
Such  a  relation  cannot  be  due  to  the  spectrum  as  an  objective 
affair;  for  if  we  measure  by  other  physical  means  the  amount  of 
energy  belonging  to  its  different  regions,  we  find  that  of  the  red 
rays  (which  are  by  no  means  brightest)  to  be  strongest.  We  must, 
then,  seek  an  explanation  in  the  structure  of  the  retina,  and  conclude 
that  it  is  peculiarly  sensitive  to  stimulations  by  oscillations  of  about 
five  hundred  and  fifty  billions  per  second  (compare  Fig.  118).  The 
sensitiveness  of  the  retina  to  slight  variations  in  color-tone,  as  de- 
pendent upon  differences  in  the  wave-lengths  of  the  stimulus,  is  also 
different  at  different  portions  of  the  spectrum.  It  is  greatest  in  the 
green  and  blue-green  regions  (D  and  F). 

The  following  table  represents  both  the  foregoing  laws.     The 
numbers  of  the  second  and  third  columns  show  the  relative  bright- 


Fraunhofer 

Vierordt 

Mandelstamm  and  Dobn 

>wolsky 

Red  B   ... 

32 

22 

B  

lilT 

Orange,  C  

94 

128 

C  

T^T 

Reddish-yellow  D 

640 

780 

C-D 

tir 

Yellow,  D-E  

1,000 

1,000 

D..               ... 

jfa 

Green,  E  

480 

370 

D-E  

•sis 

Blue-green,  F 

170 

128 

E 

5^ 

Blue,  G  

31 

8 

E-F  

eh 

Violet  H 

56 

07 

P 

-W 

G  . 

H 

yiir 

ness  with  which  the  different  colors  of  the  spectrum  appear  to  the 
eye,  as  calculated  by  different  methods  and  by  two  observers.     It 


330  THE  QUALITY  OF  SENSATIONS 

will  be  seen  that  the  results  agree  substantially,  though  by  no 
means  perfectly.  In  the  last  two  columns  the  letters  stand  for 
Fraunhofer's  lines,  and  the  figures  give  the  fractional  variation  in 
the  wave-lengths  which  produces  an  observable  variation  in  the 
color-tone  for  different  regions  of  the  spectrum. 

§  7.  The  colors  of  every-day  experience,  like  its  musical  tones,  are 
not  simple  and  pure  color-tones,  such  as  are  obtained  by  spectral 
analysis;  they  are  composite.  Inquiry  must  therefore  be  raised  as 
to  the  effect  produced  in  sensation  from  the  co-working  of  two 
homogeneous  rays  of  light  upon  the  same  elements  of  the  retina 
under  all  the  normal  conditions  to  which  reference  was  previously 
made.  In  pursuing  this  inquiry  no  direct  assistance  can  be  ob- 
tained from  the  discriminations  of  consciousness;  for  sensations  of 
color,  unlike  those  of  musical  clang,  cannot  be  mentally  analyzed 

'  into  their  constituent  elements.  The  science  of  optics  makes  us 
acquainted,  however,  with  the  following  facts:  When  the  wave- 

1  lengths  of  the  two  colors  mixed  vary  but  slightly  (a  few  billions  of 
oscillations  in  a  second)  from  each  other,  the  color  resulting  from 
the  mixture  lies  between,  and  may  be  recognized  as  a  "shade"  of, 
the  colors  mixed.  By  selecting  for  mixture  color-tones  that  lie 
apart  at  the  various  distances  possible  along  the  spectrum,  an  in- 
definite number  of  impressions  of  color  may  be  obtained,  which  all 
differ  from  those  obtained  by  the  homogeneous  colors.  These 
mixed  color-impressions  classify  in  such  a  way,  however,  that  the 
number  of  the  qualities  of  resulting  sensations  is  far  less  than  that 
.of  the  compound  physical  processes  which  stimulate  the  retina. 
J  The  character  of  the  colors  making  up  our  ordinary  visual  ex- 
perience depends  both  upon  the  _place  in  the  spectrum  from  which 

.  the  simple  color-tones  are  selected  for  mixture,  and  also  upon  the 
relative  intensity  of  the  ones  selected.  For  example,  if  a  ray  of 
four  hundred  and  fifty  billions  of  oscillations  per  second  (red)  be 
mixed  with  one  of  seven  hundred  and  ninety  billions  (violet),  a  new 
series  of  impressions  of  color  (the  purples)  is  attained  by  varying 
the  intensities  of  the  two.  These  impressions  are  more  or  less  like 
red  or  like  violet,  according  to  the  relative  amounts  of  the  rays  of 
four  hundred  and  fifty  billions  and  of  seven  hundred  and  ninety 
billions  which  enter  into  the  mixture.  Moreover,  there  are  found 
to  be  two  ways  of  advancing  by  this  process  of  mixing  color-tones 
toward  any  one  of  the  composite  colors.  Thus,  we  may  pass  from 
yellow  to  blue  either  through  green-yellow,  green,  and  blue-green, 
or  through  orange,  red,  purple,  and  violet.  The  table1  on  next  page 
is  of  interest  in  this  connection.  Where  two  colors  are  given  as 

*Made  according  to  investigations  by  J.  J.  Miiller,  and  taken  from  Fick,  in 
Hermann's  Handb.  d.  PhysioL,  III,  i.,  p.  190. 


NUMBER  OF  COLORS  DISTINGUISHABLE 


331 


resulting  from  the  mixture,  the  variation  is  to  be  understood  as  de- 
pendent upon  the  prevailing  intensity  of  one  of  the  two  compo- 
nents. 

§  8.  The  number  of  colors  distinguishable  by  the  human  eye  is 
not  easily  stated  with  accuracy;  like  the  number  of  musical  tones, 
it  varies  with  different  individuals.  The  usual  number  of  seven 


Components 

Tone  of  the  color  obtained 
by  mixture 

Degree  of 
saturation 

Red  and  Yellow 

Orange 

Spectral. 

Orange  and  Yellow-green. 

Yellow  

Spectral. 

Yellow  and  Green 

Yellow-green 

Whitish 

Yellow-green  and  Blue-green 

Green    

Very  whitish. 

Green  and  Cyanic  Blue   

Blue-green  

Whitish. 

Blue-green  and  Indigo 

Cyanic  Blue 

Spectral. 

Cyanic  Blue  and  Violet 

Indigo            .    .            ... 

Spectral. 

Red  and  Yellow-green  

Orange  or  Yellow  

Spectral. 

Red  and  Green  

Orange  or  Yellow  or  Yellow- 

green            

Whitish. 

Violet  and  Blue-green 

Indigo  or  Cyanic  Blue  

Spectral. 

Violet  and  Green   

Indigo  or  Cyanic  Blue  or  Blue- 

ereen 

Whitish. 

Violet  and  Orange 

Red  

Whitish. 

Red  and  Cyanic  Blue 

Indigo  or  Violet 

Whitish. 

Red  and  Indigo 

Violet         

Slightly  whitish. 

fundamental  colors,  as  fixed  by  Newton,  with  the  intent  of  forming 
an  octave  in  the  scale  of  color-tones,  has  no  sufficient  claim  to 
acceptance.  Six  of  the  seven — namely,  red,  orange,  yellow,  green, 
blue,  violet — are  indeed  names  in  common  use.  But  indigo,  as  an 
intermediate  tone,  or  kind  of  semi-tone,  between  blue  and  violet, 
has  perhaps  no  more  real  right  to  recognition  than  various  other 
intermediate  color-tones.  Bonders  put  the  number  of  color-tones 
distinguishable  in  oil-colors  at  one  hundred;  Von  Kries  the  rec- 
ognizable number  of  spectral  tints  at  about  two  hundred  and  thirty. 
But,  as  has  already  been  said,  colors  also  differ  according  to  the 
degree  of  their  saturation  or  purity,  due  to  freedom  from  admix- 
ture of  white  light.  Another  series  of  variations  of  sensation  must 
be  allowed  for,  which  are  due  to  differences  in  "brightness"  or 
intensity.  Introducing  these  two  variable  elements,  Von  Kries 
calculates  the  number  of  distinctions  of  color-sensations,  possible 
for  all  degrees  of  purity  of  tone  and  intensity  of  light,  at  about  five 
hundred  thousand  to  six  hundred  thousand.  This  number  stands 
midway  between  the  "many  millions"  of  which  Aubert  speaks  and 
the  five  thousand  allowed  by  Bonders.  Herschel  thought  that  the 


332 


THE  QUALITY  OF  SENSATIONS 


workers  on  the  mosaics  of  the  Vatican  must  have  distinguished  at 
least  thirty  thousand  different  colors. 

§  9.  Experiment  also  shows  that  if  certain  color-tones  with  a 
given  intensity  are  united  on  the  retina,  the  result  is  a  sensation 
unlike  that  of  any  other  of  the  colors,  whether  pure  or  mixed. 
This  sensation  we  call  "  white,"  and  the  two  colors  which  by  their 
admixture  produce  it  are  called  "complementary."  Complementary 
colors  may  be  mixed  upon  the  retina  in  various  ways;  either  by  al- 
lowing two  spectral  rays  properly  selected  to  be  superimposed  at 
the  same  spot,  or  by  blending  the  reflected  images  of  two  colored 
wafers,  or  by  blending  the  direct  visual  impressions  of  colored 
surfaces  on  a  swiftly  revolving  top  or  wheel,  etc.  But  however 
mixed,  the  resultant  sensation  is  that  of  a  so-called  "white"  color, 
in  which  all  trace  of  the  constituent  elements  is  lost.  Following  is 
a  table  of  complementary  colors1:  the  wave-lengths  are  given  in 
millionths  of  a  millimetre. 


Color 

Wave-length 

Complementary 
color 

Wave-length 

Relation  of 
wave-lengths 

Red 

656  2 

Green-blue 

492  1 

1  334 

Orange 

607  7 

Blue 

489  7 

240 

Gold-yellow  

585.3 

Blue 

485  4 

206  « 

Gold-yellow  

573.9 

Blue 

482  1 

190 

Yellow 

567  1 

Indisro-blue 

464  5 

221 

Yellow  

564  4 

Indigo-blue 

461  8 

222 

Green-yellow  

563.6 

Violet 

433  land 

301 

'  (  less 

It  will  be  noted  that  no  complementaries  are  assigned  for  the 
colors  between  green-yellow  and  green-blue,  with  wave-lengths 
from  five  hundred  and  sixty-four  to  four  hundred  and  ninety-two. 
In  other  words,  the  central  greens  have  no  complementary  colors 
within  the  limits  of  the  spectrum.  In  the  series  of  color  sensations, 
however,  green  has  a  complementary,  namely  purple,  the  stimulus 
for  which  is  a  mixture  of  long  and  short  wave-lengths. 

§  10.  If  the  foregoing  facts  and  laws  are  held  to  be  true  of  the 
"normal"  connection  between  light  and  visual  sensations,  then 
various  classes  of  circumstances  must  be  taken  account  of  as  "ab- 
normal," which,  nevertheless,  enter  into  all  our  daily  experience 
with  this  sense.  Indeed,  the  connection  between  stimulus  and 
sensation  is  not  the  same  for  different  individuals  who  possess  sub- 
stantially the  same  color-sensations;  frequently  the  complementary 
colors  for  two  different  individuals  are  not  precisely  the  same. 
1  From  Helmholtz,  op.  cit.,  p.  277. 


THEORY  OF  COMPLEMENTARY  COLORS    333 

Even  the  two  eyes  of  the  same  individual  often  differ  perceptibly 
in  this  regard.  Important  changes  in  the  quality  of  the  sensations, 
other  than  those  directly  ascribable  to  changes  in  the  wave-lengths 
of  light,  take  place  when  the  intensity  of  the  light  approaches  either 
a  maximum  or  a  minimum.  At  the  maximum  intensities  of  the 
stimulus  all  sensations  of  color-tone  cease,  and  even  homogeneous 
rays  appear  white.  Previous  to  reaching  this  maximum,  red  and 
yellowish  green  pass  over  into  yellow,  and  bluish  green  and  violet 
into  blue.  At  the  minimum  intensities  of  light  every  color-tone 
except  the  pure  red  of  spectral  saturation  appears  colorless  when  seen 
alone  on  a  perfectly  black  ground. 

In  other  words,  light  of  any  wave-length  can  be  weakened  till 
it  gives  no  sensation  of  color,  while  still  giving  a  sensation  of  light. 
There  is,  accordingly,  a  "photo-chromatic  interval"  between  the 
threshold  for  the  perception  of  light  and  the  threshold  for  the  per- 
ception of  the  color  of  the  light;  but  this  interval  is  extremely  small 
or  absent  in  the  case  of  the  longest  waves  which  give  the  sensation 
of  red.  This  phenomenon  is  closely  connected  with  the  process  of 
dark-adaptation,  which  has  been  described  in  an  earlier  chapter 
(see  p.  195)  and  may  be  restated  as  follows:  With  decreasing  illu- 
mination, the  retina  becomes  "adapted,"  or  gets  into  a  condition 
in  which  it  is  sensitive  to  very  faint  lights,  but  does  not  respond  with 
sensations  of  color  to  these  very  faint  lights.  Moreover,  this  adap- 
tation, or  increase  in  sensitiveness,  varies  greatly  with  the  different 
wave-lengths;  and,  indeed,  scarcely  exists  at  all  with  those  which 
produce  sensations  of  red. 

§  11.  Changes  of  color  also  take  place  when  the  time  of  the  ac- 
tion of  the  light  is  reduced  to  a  minimum.  Sensations  of  satu- 
rated color  can  be  produced  by  instantaneous  illumination  of  the 
spectrum  with  the  electrical  spark.  More  time  is  needed,  however, 
to  produce  these  sensations  with  smaller  intensities  of  the  light. 
The  different  colors,  even  when  of  the  same  brightness,  appear  to  re- 
quire different  amounts  of  time  in  order  to  reach  the  maximum  of 
their  effect— red,  0.0573;  blue,  0.0913;  green,  0.133  of  a  second. 
The  tone  of  the  color  varies  with  the  duration  of  the  impression  as  • 
well  as  with  the  intensity  of  the  light.  Very  minute  objects,  too,  • 
appear  of  a  different  color  on  account  of  their  size.  In  general,  the  • 
larger  the  surface,  the  less  the  intensity  of  the  light  necessary  to 
produce  the  sensation  of  any  particular  color-tone;  the  greater  the 
intensity  of  the  light,  the  smaller  the  surface  which  will  suffice  for 
such  sensation.  Fick1  showed  that  the  color-sensations  derived 
from  small  distinct  points  support  each  other,  as  it  were,  in  the  same 
way  as  the  contiguous  points  of  a  colored  surface.  For  if  we  make 
1  Pfiuger's  Archiv,  XVII,  p.  152. 


334  THE  QUALITY  OF  SENSATIONS 

with  a  fine  needle  a  single  hole  (of  about  0.6  mm.  in  diameter)  in 
a  sheet  of  paper  and  look  through  it  at  colored  paper  distant  some 
six  and  a  half  metres,  the  color  of  the  paper  cannot  be  distinguished. 
But  if  the  number  of  holes  be  as  many  as  sixteen,  the  color  can  be 
distinguished  at  the  same  distance,  even  when  the  holes  through 
which  we  look  are  smaller.  Subsequent  experiment  has  shown  that 
the  smaller  the  distance  between  the  single  perforations,  the  greater 
the  distance  at  which  the  eye  can  recognize  colors  through  them. 
In  general,  then,  two  weak  sensations,  both  of  which  belong  to  one 
eye,  may  fuse  together  into  one  stronger  sensation. 

§  12.  Very  important  changes  in  the  visual  sensations  occur  as 
dependent  on  the  part  of  the  retina  which  is  stimulated.  In  this 
respect  a  great  difference  exists  between  the  central  and  the  pe- 
ripheral parts.  The  entire  field  of  this  organ  may  be  somewhat 
indefinitely  divided  into  three  zones — a  central  or  polar,  a  middle, 
and  an  outer  or  peripheral.  It  is  probably  true  that  the  periph- 
eral parts  of  the  retina  produce  no  sensations  which  cannot  be 
produced  by  stimulating  the  central  zone.  But  it  is  equally  true 
that,  under  the  same  circumstances,  the  same  stimulus  produces 
a  markedly  different  effect  upon  sensation  when  applied  to  differ- 
ent localities  of  the  retina.  Rays  which,  falling  on  the  polar  zone, 
produce  the  impression  of  red,  yellow,  or  green,  all  make  an  im- 
pression of  yellow  when  they  fall  on  the  surrounding  zone  (a  few 
millimetres  from  the  fovea  centralis),  and  this  yellow  is  so  much 
the  paler,  the  greener  the  impression  on  the  polar  zone.  Rays 
which  make  on  the  polar  zone  the  impression  of  blue  or  violet 
make  on  the  outer  zone  the  impression  of  blue;  and  this  blue  is 
so  much  the  paler,  the  nearer  the  impression  on  the  polar  zone  is  to 
green. 

Any  conclusions  which  we  might  be  tempted  to  draw,  as  to  physi- 
ological differences  in  the  different  parts  of  the  retina,  in  dependence 
upon  histological  differences,  are  somewhat  complicated  by  the  fact 
that  a  certain  red,  a  certain  yellow,  a  certain  green,  and  a  certain 
blue  can  be  found,  which  do  not  undergo  this  change  of  color-tone 
in  passing  from  the  polar  to  the  intermediate  zone;  but  the  yellow 
and  the  blue  remain  unchanged,  while  the  red  and  the  green  change 
directly  into  gray,  without  first  changing  to  yellow  or  blue.  These 
four  special  color-tones  are,  therefore,  called  the  stable  colors:  the 
stable  yellow  and  blue  are  about  what  we  should  ordinarily  call  a 
typical  yellow  or  blue;  but  the  stable  red  is  a  somewhat  purplish 
red,  lying  outside  of  the  spectrum;  and  the  stable  green  is  a  some- 
what bluish  green,  as  described  in  customary  terms.  The  stable 
blue  and  yellow  are  complementary,  and  this  is  true  also  of  the 
stable  red  and  green. 


PHENOMENA  OF  COLOR-BLINDNESS  335 

The  so-called  stable  colors  are  clearly  of  great  theoretical  im- 
portance; and  the  more  so  because  of  the  following  curious  fact: 
If  the  stable  yellow  and  blue  are  made  of  such  relative  intensity 
that,  when  mixed,  they  give  a  white  or  gray,  the  limit  at  which  they 
lose  their  color  and  become  gray,  on  being  moved  toward  the  periph- 
ery, is  the  same  for  both;  and,  similarly,  if  the  stable  red  and  green 
are  of  such  relative  intensity  as  to  give  gray  when  mixed,  both 
change  to  gray  at  the  same  limit,  on  passing  from  the  polar  region 
outward.1 

§  13.  A  certain  proportion  of  persons  (from  three  to  five  per  cent, 
of  males,  and  apparently  a  much  smaller  proportion  of  females) 
have  a  defect  of  vision  which  is  known  as  "color-blindness."  There 
are  several  forms  of  color-blindness,  in  one  of  which  (very  rare)  no 
distinctions  of  color  are  possible,  but  only  of  brightness;  and  the 
spectrum  appears  to  such  persons  as  differing,  from  part  to  part, 
only  in  intensity.  Light  of  any  color  appears  to  them  identical 
with  light  of  any  other  color,  provided  the  intensities  be  rightly  pro- 
portioned. In  dim  light,  the  vision  of  these  individuals  is  the  same 
as  that  of  normal  persons,  who  also,  it  will  be  recalled  (see  p.  333), 
lose  color-vision  under  similar  conditions.  With  the  increase  of 
light,  the  normal  eye  begins  to  distinguish  colors,  i.  e.,  to  observe 
qualitative  differences  between  different  parts  of  the  spectrum;  but 
this  the  totally  color-blind  eye  fails  to  do.  The  further  fact  that, 
often  at  least,  the  yellow  spot  of  the  totally  color-blind  eye  yields  no 
visual  sensations  whatever,  suggests  that  such  an  eye  possesses  only 
rod-vision  and  no  cone-vision  (compare  pp.  195  f.). 

Much  more  common,  however,  are  the  cases  of  blindness  to  cer- 
tain colors,  or,  more  correctly  expressed,  of  inability  to  distinguish 
certain  colors  which  are  readily  distinguished  by  the  normal  eye. 
It  should  be  said  that,  though  we  speak  of  the  normal  eye  as  dis- 
tinguished from  the  color-blind,  there  is  no  diseased  condition  pres- 
ent in  the  color-blind  eye.  The  defect  is  inherited,  and  is  perfectly 
consistent  with  normal  vision  in  all  other  respects  except  that  of 
distinguishing  certain  colors.  In  the  form  which  is  called  "red- 
green  blindness,"  the  colors  which  are  most  frequently  confused 
are  shades  of  red  and  green,  or  of  purple  and  greenish  blue.  By 
properly  adjusting  the  intensity  and  saturation,  it  is  possible  to 
produce  confusion,  in  these  persons,  of  any  of  the  color-tones  that 
lie  between  red  and  green,  and  also  of  any  lying  between  purple  and 
greenish  blue.  More  precisely  studied,  the  color-vision  of  these 
individuals  is  found  to  reduce — apart  from  matters  of  intensity  and 

1  See  a  review  of  the  somewhat  conflicting  literature  on  this  point,  and  an  ex- 
perimental research  establishing  the  above  results,  in  Baird's  The  Color  Sensi- 
tivity of  the  Peripheral  Retina  (Washington,  1905). 


336  THE  QUALITY  OF  SENSATIONS 

saturation — to  a  single  qualitative  distinction;  and  this  is  the  dis- 
tinction between  two  colors  that  are  excited,  respectively,  by  the 
long  waves  and  by  the  short  waves  of  light.  From  the  red  end  of 
the  spectrum  to  a  point  in  the  green  (wave-length  about  490-500), 
all  visual  sensations  are  with  such  persons  shadings  of  a  single  color; 
and  any  part  of  the  scale  may  be  matched  with  any  other  provided 
the  intensities  are  rightly  selected.  In  similar  manner,  from  this 
point  in  the  green  to  the  violet  (and  purple),  all  is  a  shading  of  a 
single  color.  Colors  taken  from  opposite  sides  of  this  limiting 
point  in  the  green  are,  however,  clearly  distinguished.  This  bound- 
ary itself  is  indistinguishable  from  gray;  and  the  particular  mixture 
of  long  and  short  wave-lengths,  which  produces  in  the  normal  eye 
the  impression  of  a  purplish  red  complementary  to  this  green,  to 
the  color-blind  eye  is  also  indistinguishable  from  gray. 

What  precisely  are  the  color-sensations  of  the  red-green  blind 
individual — whether  his  white  and  gray  look  the  same  to  him  as 
white  and  gray  look  to  the  normal  individual,  and  whether  the  red 
half  of  the  spectrum  seems  to  him  shaded  in  red,  or  in  yellow,  the 
blue  half  in  blue  or  in  violet — is  a  difficult  question  to  approach; 
since  evidently  normal  and  color-blind  individuals  cannot  "com- 
pare notes"  on  such  a  matter.  There  have  been  examined  one  or 
two  cases  of  color-blindness  confined  to  one  eye,  while  the  other  saw 
normally;  and  the  testimony  of  these  individuals  seems  to  indicate 
that,  to  the  color-blind  eye,  gray  and  white  appear  the  same  as  to 
the  normal  eye,  but  that  the  red  end  of  the  spectrum  is  shaded  in 
yellow,  and  the  blue  end  in  blue. 

Under  the  head  of  red-green  blindness  are  recognized  two  sub- 
classes, sometimes  called  the  red-blind  and  the  green-blind,  the  dif- 
ference being  that  the  red-blind  are  comparatively  insensitive  to 
reds  near  the  end  of  the  spectrum.  A  few  (pathological)  cases 
have  also  been  observed  of  a  different  form  of  color-blindness,  called 
blue  or  yellow-blue  blindness.  This  form  of  color-vision,  like  that 
in  red-green  blindness,  is  dichromatic,  and  apparently  the  two  col- 
ors retained  are  red  and  green.  In  these  cases,  the  neutral  point 
in  the  spectrum  falls  in  the  yellow. 

§  14.  Color-blindness  is  evidently  of  great  importance  for  an 
understanding  of  normal  color-vision ;  we  may  even  say  that  dichro- 
matic vision,  from  its  greater  simplicity,  is  better  understood  than 
the  normal,  polychromatic  fofm.  The  evidence,  so  far  as  it  goes, 
shows  that  the  color-vision  of  the  red-green  blind  agrees  exactly 
with  that  of  the  intermediate  yellow-blue  zone  of  the  normal  ret- 
ina; the  two  agree,  at  any  rate,  in  being  dichromatic. 

§  15.  Important  modifications  of  the  normal  action  of  the  retina 
I  are  also  produced  by  previous  excitation.  The  most  important  of 


.    NEGATIVE  AND  POSITIVE  AFTER-IMAGES  337 

these  changes  is  the  adaptation  to  dark  or  to  light  which  has  already 
been  described.  •  Let  us  assume  that  the  eye  has  been  exposed  for 
some  time  to  light  of  a  moderate  intensity,  and  that  it  is  then  turned 
upon  a  much  brighter  light,  with  prolonged  fixation,  and  the  latter 
will  appear  to  grow  less  bright;  but  if,  after  this,  the  eye  is  turned 
back  on  its  previous  moderate  light,  this  appears  darker  than  before. 
If,  however,  the  eye  is  turned  from  moderate  light  to  a  dark  field, 
the  latter  seems  to  grow  brighter  with  continued  fixation;  and  if 
then  the  eye  is  returned  to  the  moderate  field,  this  appears  brighter 
than  before.  These  effects  can  be  seen  more  strikingly,  if  only  a 
part  of  the  field  of  view  is  bright  or  dark.  After  gazing  steadily, 
for  example,  at  a  bright  patch  on<a  medium  background,  and  then 
turning  to  a  uniform  medium  surface,  a  dark  patch  appears  on 
this,  corresponding  to  the  light  patch  previously  fixated;  and  this 
dark  spot  moves  about  on  the  background,  as  the  eyes  move.  If, 
similarly,  a  black  spot  on  a  gray  ground  is  gazed  at,  and  then  the 
eye  turned  to  a  plain  gray  ground,  the  place  of  the  black  spot  is 
now  taken  by  a  bright  spot.  If  a  figure  containing  both  bright  and 
dark  parts  is  fixated,  and  the  eyes  are  then  turned  to  the  gray  field, 
an  image  of  the  figure  appears,  bright  where  the  figure  was  dark  and 
dark  where  it  was  bright.  This  residual  effect  of  excitation  is 
called  the  negative  after-image. 

Negative  after-images  of  colors  are  also  seen,  the  colors  of  the 
image  being  complementary  to  those  of  the  stimulus.  If,  for  ex- 
ample, a  green  spot  is  steadily  fixated,  it  appears  to  lose  some  of 
its  color,  becoming  less  saturated;  and  if  then  the  eye  is  turned  to  a 
plain  gray  field,  there  appears,  in  place  of  the  green,  its  complemen- 
tary purple.  If  the  eye  is  turned  from  green  to  a  purple  ground, 
the  after-image  of  the  green  is  still  seen,  in  the  form  of  a  spot  of 
still  deeper  purple;  it  is  in  this  way  that  the  most  saturated  color 
effects  possible  can  be  obtained.  These  after-images  can  be  ex- 
plained in  terms  of  adaptation,  or  in  terms  of  fatigue.  A  full  expla- 
nation, however,  would  need  to  take  account  of  many  curious  de- 
tails, one  or  two  of  which  are  worth  mentioning.  When  a  patch  of 
color  is  being  steadily  fixated,  it  not  only  grows  less  saturated,  but 
also  changes  its  color.-tone;  under  such  conditions,  it  seems  to  ap- 
proach either  yellow  or  blue — the  colors  which  are  nearer  to  yel- 
low approaching  yellow,  and  those  which  are  nearer  to  blue  ap- 
proaching blue.  Yellow  and  blue,  themselves,  however,  do  not 
change  their  color-tone  on  prolonged  fixation;  nor  do  two  other 
colors — namely,  a  bluish  green  and  a  purplish  red.  These  four  col- 
ors are  stable  here  as  they  were  in  passing  from  central  to  periph- 
eral vision.1  A  further  detail  is  that  the  color  of  the  after-image, 
1  Voeste,  Zeitschrift  /.  Psychol,  1898,  XVIII,  257. 


338  THE  QUALITY  OF  SENSATIONS 

after  strong  stimulation,  changes  from  moment  to  moment,  and 
passes  through  a  series  of  colors. 

§  16.  Another  difficulty  in  the  way  of  a  simple  conception  of  the 
after-image  is  the  existence  of  positive  after-images,  between  which 
and  the  negative  it  is  not  easy  to  draw  any  valid  distinction.  The 
names  positive  and  negative  are  indeed  justified  by  the  fact  that  the 
positive  after-image  presents  both  color  and  light-and-dark  sen- 
sations as  they  appear  during  the  direct  action  of  the  stimulus; 
whereas  the  negative  after-image  reverses  everything.  In  order  to 
study  the  positive  after-image  we  need  only  to  look  for  a  second  or 
two  at  some  luminous  object,  and  then  close  the  eyes  or  turn  them 
toward  a  dark  ground.  A  bright  image  of  the  luminous  object 
will  be  seen  for  a  few  seconds.  Longer  fixation  favors  the  negative 
image;  when  both  are  obtained,  the  positive  precedes,  though,  in 
case  of  the  prolonged  images  which  follow  looking  at  a  very  bright 
object  (as  the  sun),  changes  occur  from  negative  to  positive  and  back 
again  as  the  background  is  changed  from  light  to  dark.  Thus  the 
whole  series  of  facts  becomes  rather  complicated,  and  this  is  still 
more  the  case  when  minute  examination  is  directed  to  the  immedi- 
ate after-effects  of  the  stimulus.  These  effects  can  be  best  exam- 
ined when  the  stimulus  is  moved  at  a  moderate  rate  over  a  dark 
ground,  while  the  eye  remains  fixed  in  position;  the  after-effects 
then  appear  strung  out  behind  the  moving  light.  Three  images 
appear,  separated  by  dark  spaces;  of  these,  the  first  image  corre- 
sponds to  the  sight  of  the  light  itself;  the  second  is  fairly  sharp,  and 
often  called  the  "ghost";  the  third  is  less  sharp  and  passes  over 
gradually  into  black.  Translating  the  spatial  relations  of  this  ex- 
periment into  terms  of  time,  we  conclude  that,  when  a  light  acts 
on  the  retina  for  an  instant,  the  first,  wholly  positive  effect  outlasts 
the  stimulus  for  a  small  fraction  of  a  second,  and  is  succeeded  by 
a  brief  interval  of  no  effect,  then  by  a  recurrence  of  light  effect,  then 
by  another  interval  and  another  recurrence  of  light  which  gradu- 
ally fades  out  and  gives  place  to  a  negative  effect  (black).  The  three 
images  may  all  be  called  positive,  except  that  the  " ghost"  is  nega- 
tive as  regards  color.1  Many  other  details  have  been  observed,  but 
these  are  sufficient  to  show  something  of  the  intricacy  of  the  reaction 
of  the  retina  to  light,  and  the  difficulty  of  forming  a  clear  concep- 
tion of  what  takes  place.  Individual  peculiarities  appear  to  count 
for  much  in  these,  as  in  all  other  details,  of  our  visual  experience. 

§  17.  The  different  parts  of  the  retina  are  interdependent  in  the 
production  of  sensation;  or — to  employ  the  statement  of  Wundt2 
— "The  sensation  which  arises  through  the  stimulation  of  any  given 

1  See  W.  McDougall,  British  Journal  of  Psychology,  1904,  I,  78. 

2  Quoted  from  the  Grundzuge  d.  physiolog.     Psychologic  (2d  ed.,  I,  p.  439). 


PHENOMENA  OF  CONTRAST  339 

point  of  the  retina  is  also  a  function  of  the  state  of  other  immedi- 
ately contiguous  points."  Hence  arise,  in  part  at  least,  the  phe- 
nomena of  contrast,  which  are  of  two  kinds — contrast  of  bright- 
ness and  contrast  of  color-tone.  The  fundamental  fact  in  the  first 
class  of  contrasts  is  this:  every  bright  object  appears  brighter  with 
surroundings  darker  than  itself,  and  darker  with  surroundings 
brighter  than  itself.  These  phenomena  are  explained  by  Helm- 
holtz1  as  deceptions  of  judgment,  such  as  we  are  accustomed  to  in 
our  estimates  of  distances.  To  this  explanation,  however,  Fick,2 
Hering3,  and  others  oppose  strong  and  apparently  conclusive  ob- 
jections. They  would  explain  the  same  phenomena  by  the  modify- 
ing influence  of  the  excitation  of  one  part  of  the  retina  upon  the 
excitation  of  contiguous  parts. 

When  colored  instead  of  white  light  is  used  in  experimenting 
under  the  law  of  contrast,  phenomena  similar  to  those  of  comple- 
mentary colors  are  obtained  A  small  square  of  white  on  a  sur- 
face of  green,  when  covered  with  a  transparent  sheet  of  tissue-paper, 
appears  as  red  on  a  surrounding  surface  of  a  whitish  hue;  on  a  red 
ground  it  appears  as  green,  on  a  blue  ground  as  yellow,  and  vice 
versa.  More  complicated  illustrations  of  this  principle  of  inter- 
dependence may  be  obtained  in  various  ways.  For  example,  if 
on  a  pale-green  background,  in  size  36  mm.  by  44  mm.,  which  is 
divided  into  squares  of  1.8  mm.  by  lines  of  white  0.4  mm.  in  width, 
a  red  letter  E,  21  mm.  by  34  mm.  in  size,  be  constructed  out  of 
similar  squares,  on  observing  the  figure  for  a  few  seconds  with  a 
fixed  gaze,  some  of  the  red  squares  will  disappear  and  be  replaced 
by  green  squares  similar  to  the  background.4  Or  if  on  sheets  of 
different  colored  paper,  20  cm.  by  30  cm.  in  size,  small  strips, 
1  cm.  by  20  cm.,  of  various  colors  are  laid,  as  on  a  background,  and 
the  whole  then  observed  at  a  distance  of  about  3  m.  with  a  fixed  gaze, 
exceedingly  varied  illusions  of  disappearance  and  substitution  may 
be  obtained.5 

These,  and  similar  phenomena,  of  which  a  great  variety  might 
be  mentioned,  certainly  cannot  be  explained  as  deceptions  of  "judg- 
ments" in  any  justifiable  meaning  of  the  term.  They  do,  however, 
all  seem  to  imply  some  physiological  explanation  which  ascribes 
to  each  part  of  the  retina  an  influence  on  contiguous  parts.  Still 
less  in  accordance  with  the  facts  of  experience  would  the  view  of 
Helmholtz  appear  to  be,  in  the  cases  of  those  individuals  who  can 

1  Physiolog.  Optik,  pp.  388  ff. 

2  Hermann's  Handb.  d.  Physiologic,  III,  i,  231  f. 

3  Sitzgsber.  d.  Wiener  Acad.,  June,  1872;   Dec.,  1873. 

4  Fick's  Lehrb.  d.  Augenheilkunde,  1897,  p.  50. 
6Ladd,  "A  Color  Illusion,"  Yale  Studies,  VI,  1898. 


340  THE  QUALITY  OF  SENSATIONS 

by  an  act  of  will  produce,  with  eyes  closed,  simple  colored  shapes 
in  the  retinal  field,  which  upon  opening  the  eyes  will  cast  their 
complementary  images  upon  a  white  background.1 

§  18.  It  will  by  this  time  readily  be  seen  that  a  theory  which  shall 
satisfactorily  account  for  all  the  complicated  phenomena  of  visual 
sensations  is  difficult  to  establish.  Though  ingenuity  of  the  highest 
order  has  been  employed  in  the  development  of  theories  of  color- 
vision,  not  all  the  phenomena  are  well  explained  by  any  one  theory. 
Among  the  more  significant  facts  which  must  find  a  place  in  any 
adequate  explanation  are  the  following:  (1)  The  colorless  vision 
which  occurs  in  dim  light,  in  the  outer  zone  of  the  retina,  and  over 
the  whole  retina  in  cases  of  total  color-blindness.  The  conception 
of  two  kinds  of  vision,  one  making  no  distinction  of  colors,  and  pro- 
vided for,  probably,  by  the  rods,  the  other  distinguishing  colors 
and  provided  for  by  the  cones,  serves  admirably  to  explain  these 
facts,  and  a  number  of  other  details.  This  theory  is  associated 
principally  with  the  name  of  Von  Kries  (compare  pp.  195  f.).  With 
colorless  vision  thus  explained,  as  the  function  of  the  rods  of  the 
retina,  there  remain  to  be  explained  only  the  facts  of  color-vision. 

(2)  The  principal  facts  of  color  mixture  are,  first,  the  discovery 
of  Newton  that  all  the  colors  of  the  rainbow,  when  mixed  in  proper 
proportion,  produce  the  sensation  of  white;  then  the  fact  of  comple- 
mentary colors,  i.  e.,  that  pairs  of  colors,  suitably  chosen,  mix  to 
produce  the  sensation  of  white;  and  further  the  fact  that  all  color- 
tones  can  be  produced  by  the  mixture  of  three  (or  more,  but  not 
less  than  three)  properly  chosen  colors. 

(3)  To  these  two  classes  of  facts  must  be  added  the  facts  of  after- 
images and  of  contrast;  and  (4)  The  existence  of  dichromatic  vision, 
in  the  intermediate  zone  of  the  retina,  in  red-green  blindness,  in 
intense  illumination,  and  (partially  at  least)  in  the  prolonged  ex- 
posure of  the  eye  to  light  of  a  fixed  color.     In  explanation  of  the 
effect  of  intense  light,  and  of  the  dichromatic  zone  of  the  retina,  we 
know  that  the  two  colors  to  which  vision  is  reduced  are  yellow  and 
blue;  in  the  case  of  color-blindness  we  are  not  so  sure,  but  the  indi- 
cations point  to  the  same  two  colors;  in  the  case  of  prolonged  fix- 
ating of  one  color,  again,  the  color  tends  to  change  toward  yellow  or 
blue,  i.  e.,  there  is  a  tendency  to  dichromatic  vision  with  these  two 
colors  remaining.     In  all  these  cases,  moreover,  special  interest  at- 
taches to  a  certain  bluish  green,  and  its  complementary,  a  purplish 
red,  since   these  are  "stable,"  not  being  reduced   completely  to, 
or  in  the  direction  of,  yellow  and  blue,  but  changing  directly  to  gray 
or  white. 

1  See  Ladd,  "  On  the  Direct  Control  of  the  Retinal  Field,"  Psychol  Rev.,  July, 
1894;  Mar.,  1903. 


THEORIES  OF  COLOR  VISION  341 

(5)  The  psychological  simplicity  of  white,  i.  e.,  the  fact  that  it 
cannot  be  introspectively  analyzed  into  colors,  though  its  stimulus 
is  always  a  complex  of  color  stimuli,  is  also  of  prime  importance. 
Black,  too,  seems  a  positive  and  elementary  sensation.  It  should 
be  noted  that  the  occasion  on  which  this  sensation  arises  is  not,  pre- 
cisely (as  was  formerly  held),  the  absence  of  all  light;  for  prolonged 
absence  of  light  gives  rather  the  sensation  of  gray,  and  deep  black 
comes  always  as  an  effect  of  contrast  or  as  an  after-image.  All  of 
the  colors,  to  many  psychological  analysts,  appear  equally  elemen- 
tary; though  others  assert  that  orange,  for  example,  seems  to  them, 
introspectively  analyzed,  to  consist  of  red  and  yellow  components. 

§  19.  Of  the  theories  which  attempt  to  account  for  the  facts  of 
our  experience  with  color  sensations,  it  will  be  sufficient  to  refer  to 
the  two  most  prominent;  but  this  must  be  done  with  the  understand- 
ing that,  as  neither  of  these  has  yet  proved  itself  perfectly  adequate, 
constant  attempts  are  still  required  to  supplement  or  modify  them 
both,  when  taken  singly  and  together.  One  of  these  theories  is 
called  the  Young-Helmholtz  theory,  having  been  propounded  by 
Thomas  Young,  and  elaborated  and  defended  by  Helmholtz;  the 
other  bears  the  name  of  Ewald  Hering.  The  Young-Helmholtz 
theory  approaches  the  matter  from  the  side  of  physics,  i.  e.,  pri- 
marily from  the  facts  of  color  mixture;  the  Hering  theory  takes  its 
start  rather  from  physiological  and  psychological  facts,  such  as  the 
apparent  simplicity  and  positiveness  of  white  and  black,  the  facts 
of  contrast,  etc. 

Since  all  color-tones  can  be  produced  by  mixing  no  fewer  than 
three  of  the  number,  there  must  apparently  exist  at  least  three  sorts 
of  receptors  or  sensitive  substances  in  the  retina,  each  of  them  at- 
tuned to  ether  vibrations  of  different  wave-lengths.  The  Young- 
Helmholtz  theory  supposes  that  there  are,  in  fact,  three  such  sub- 
stances, one  of  them  attuned  to  the  long  waves  of  the  red  end  of 
the  spectrum,  one  to  the  short  waves  of  the  blue  end,  and  one 
to  waves  of  intermediate  length.  Since,  however,  color-sensations 
arise  from  stimulation  by  waves  of  any  length,  within  the  limits 
of  the  visible  spectrum,  each  of  the  three  substances  must  be  ex- 
citable, not  simply  by  waves  of  one  length,  but  also,  in  diminishing 
degree,  by  waves  differing  from  this  length.  The  relative  sensi- 
tivity of  the  three  substances  to  light-waves  of  different  lengths 
would  be  somewhat  as  indicated  by  the  curves  in  the  accompanying 
diagram,  in  which  the  horizontal  distances  denote  distances  in 
the  spectrum,  the  lettered  lines  indicating  the  prominent  lines  of 
the  spectrum.  The  height  of  the  curve  G  over  any  part  of  the  spec- 
trum, indicates  the  sensitivity  of  the  "green-receiving  substance"  to 
light  from  that  part  of  the  spectrum;  and  the  same  for  the  curves 


342 


THE  QUALITY  OF  SENSATIONS 


R  and  B,  which  show  the  varying  sensitivity  of  the  red  and  of  the 
blue  (or  violet)  receptors.  Light  of  any  wave-length,  according 
to  this  diagram,  would  stimulate  all  three  receptors,  but  in  varying 
degrees  according  to  its  length.  White  light  would  excite  all  of 
them  equally,  we  may  suppose. 

§  20.  The  theory  of  Hering  is  even  more  ingenious.  It  supposes, 
first,  that  white  light  is  not,  physiologically,  a  mixture  of  colors, 
but  is  due  to  the  excitation  of  a  special  "brightness-receptor," 
which  responds,  though  in  unequal  degrees,  to  light  of  any  wave- 


FIG.  119. — Diagram  from  Fick,  illustrating  the  Young- Helmholtz  Theory. 
(For  explanation,  see  the  text.) 

length.  The  whole  white-black  series,  including  all  the  shades  of 
neutral  gray,  is  therefore  due  to  the  activity  of  this  receptor,  which 
is  accordingly  named  the  "white-black  substance."  But  the  sen- 
sation of  black,  according  to  Hering,  is  not  due  to  the  total  inac- 
tivity of  the  white-black  substance,  but  to  a  form  of  activity  op- 
posed to  that  which  occurs  in  response  to  bright  light.  This  one 
receptor  is  supposed  to  have  two  opposite  forms  of  activity,  which 
give  the  sensations  of  the  opposites,  white  and  black.  What  the 
two  forms  of  activity  may  be,  is  not  of  so  great  consequence;  Hering 
supposed  them  to  be  anabolism  or  assimilation,  and  catabolism 
or  dissimilation — the  latter  resulting  from  the  action  of  bright  light, 
and  the  former  from  the  action  of  lights  more  dim  than  that  to 
which  the  retina  is,  at  any  particular  time,  adapted.  Other  op- 
posed chemical  or  electrical  processes  would,  however,  do  as  well. 
Now  if  white  and  gray  are  due  to  the  activity  of  a  special  recep- 
tor, and  not  to  the  mixture  of  color-impressions,  it  follows  that  color 
stimuli,  when  mixed,  as  they  are  in  sunlight  or  in  complementary 
colors,  must  have  the  power  of  neutralizing  each  other;  and  so  al- 
lowing free  play  to  the  white-black  process.  The  so-called  pri- 


THEORIES  OF  COLOR  VISION  343 

mary  colors  must,  therefore,  exist  in  pairs,  the  members  of  each  pair 
being  antagonistic.  Hering  therefore  assumed,  in  addition  to  his 
white-black  substance,  two  other  receptors,  one  for  yellow  and  blue, 
the  other  for  red  and  green.  The  red  must  be  slightly  purplish  and 
the  green  slightly  bluish,  in  order  that  they  may  be  exactly  com- 
plementary and  thus  antagonistic.  Light  of  long  wave-length  ex- 
cites in  the  yellow-blue  receptor  a  process  which  gives  us  the  sen- 
sation of  yellow;  but  the  short  wave-lengths  arouse  in  it  an  opposite 
process,  which  gives  us  the  sensation  of  blue.  If  both  long  and  short 
waves  reach  the  receptor  at  once,  the  one  neutralizes  the  other,  and 
the  receptor  remains  at  rest,  leaving  the  white-black  receptor  to 
act.  Most  colored  lights  excite  simultaneously  all  three  receptors, 
and  the  resulting  sensation  is  compounded  of  so  much  light  (or  dark), 
so  much  yellow  (or  blue),  and  so  much  red  (or  green). 

§  21.  Without  attempting  to  evaluate  these  opposing  theories 
by  applying  them  in  detail  to  the  many  facts  which  have  to  be  ex- 
plained, we  may  say  that  both  are  equally  good  as  applied  to  the 
facts  of  color  mixture;  while  neither  seems  inconsistent  with  the 
main  facts  of  contrast  and  after-images.  The  psychological  an- 
alysis of  colors  as  simple  or  as  positive  is  perhaps  of  no  conclusive 
weight;  but  of  all  the  facts,  the  most  crucial  seem  to  be  those  of 
dichromatic  vision.  The  most  promising  way  of  approaching  an 
explanation  of  these  facts,  with  either  theory,  is  therefore  to  sup- 
pose that  color-blindness  is  due  to  the  absence,  or  lack  of  functional 
power,  of  one  set  of  color  receptors ;  since,  in  either  theory,  the  sub- 
traction of  one  set  of  receptors  would  leave  dichromatic  vision. 
This  same  explanation  could  also  be  applied  to  the  intermediate 
dichromatic  zone  of  the  retina.  The  Young-Helmholtz  theory  has 
a  certain  advantage  in  dealing  with  the  two  classes  of  the  red-green 
blind,  which  it  supposes  to  be  due  to  the  absence,  respectively,  of 
the  red-receptors  and  of  the  green-receptors,  and  accordingly  names 
red-blind  and  green-blind.  But  other  facts  strongly  favor  the  Her- 
ing theory.  For  in  the  intermediate  zone  of  the  retina  it  is  certain, 
and  in  red-green  blindness  rather  probable,  that  dichromatic  vision 
is  ,yellow-blue  vision.  Now  if  yellow  and  blue  are  two  of  the  pri- 
mary colors,  since  they  are  complementary,  no  other  single  primary 
would  suffice;  for  white  light  could  not  be  secured  by  the  mixture  of 
two  complementaries  with  any  other  single  color;  nor  could  all  the 
colors  be  produced  by  the  mixture  of  three  such  colors.  In  fact, 
only  half  the  range  of  color-tones  could  be  produced  by  such  mix- 
tures. The  facts  of  dichromatic  vision,  therefore,  apparently  make 
impossible  any  three-color  theory  for  normal  vision.  There  must, 
accordingly,  be  at  least  four  primary  colors;  and  if  four,  the  two 
besides  yellow  and  blue  must  themselves  be  complementary,  in 


344  THE  QUALITY  OF  SENSATIONS 

order  that  the  mixture  of  all  four  may  give  white.  This  turns 
our  minds  back  to  the  two  stable  colors — colors  which,  each  singly, 
become  gray  in  dichromatic  vision — the  bluish  green  and  its  com- 
plementary purplish  red.  These  are  the  red  and  green  which 
Hering  chose  for  primaries,  and  there  can  be  no  doubt  that  the  pe- 
culiar behavior  of  these  two  particular  colors,  as  revealed  in  the 
studies  of  recent  years,  has  done  much  to  strengthen  the  Hering 
theory,  at  least  as  far  as  concerns  the  selection  of  the  primary  colors. 

§  22.  Much  ingenuity  and  painstaking  have  been  expended  in  de- 
vising some  form  of  symbolism  which  should  represent  to  the  eye  in 
geometrical  relations  the  laws  of  the  sensations  of  light  and  color. 
Obviously  the  sensations  of  this  sense  cannot,  like  those  of  hearing, 
be  symbolized  by  the  relations  of  points  along  a  straight  line. 
Color-tones,  unlike  musical  tones,  form  a  series  of  qualitatively  dif- 
ferent sensations  that,  at  certain  places  in  the  scale,  separate  from 
each  other  with  varying  degrees  of  rapidity,  and  then  toward  the 
broken  ends,  as  it  were,  of  this  scale,  tend  to  approach  each  other 
again.  Such  relations  are,  perhaps,  most  successfully  set  forth  by  a 
triangle  in  which  the  different  color-tones  may  be  regarded  as  lying 
together  along  a  curved  line,  from  red  to  violet,  and  the  difference 
in  any  two  color-tones  as  measured  by  the  angle  which  two  lines 
make  when  drawn  from  a  point  within  the  triangle  through  the  points 

upied  on  the  curve  by  the  two  color-tones. 

§  23.  Of  sensations  arising  from  excitation  of  the  Skin,  intro- 
n  enables  us  readily  to  distinguish  those  of  temperature  as 
different  in  quality  from  those  of  contact  and  pressure;  and  to  sepa- 
rate sensations  of  warmth  as  different  in  quality  from  those  of  cold. 
Further,  the  sensation  of  pain  which  comes  from  pricking  the  skin 
can  scarcely  be  regarded  as  anything  but  a  distinct  quality  of  sensa- 
tion (i.  e.,  in  the  narrower  meaning  of  both  words,  "pain"  and 
"sensation").  These  introspective  distinctions  are  corroborated 
in  a  most  interesting  way  by  experiments  with  minutely  localized 
stimuli  applied  to  the  skin.  Such  experimentation1  reveals  the  ex- 
istence of  what  are- called  "touch-spots,"  "warmth-spots,"  "cold- 
spots,"  and  "pain-spots."  If  a  small  area  of  the  skin,  free  or  freed 
from  hairs,  is  explored,  first  with  a  bristle  or  hair  which  requires 
very  little  force  to  bend  it,  no  sensation  is  felt  at  most  points  on 
pressing  the  end  of  the  hair  against  the  skin,  but  at  certain  points, 

1  Since  it  was  inaugurated  by  Blix  in  1882,  by  Goldscheider  in  1884,  and  by 
Donaldson  in  1885,  this  work  has  been  continued  by  Von  Frey,  Kiesow,  Alrutz, 
and  many  others.  See  Goldscheider's  Gesammelte  Abhandlungen,  Physiologic 
der  Hautsinnesnerven  (Leipzig,  1908),  also  Sherrington  in  Schafer's  Textbook  of 
Physiology,  1900,  II,  920-1001,  and  Thunberg  in  Nagel's  Handbuch  der  Physiol- 
ogic, 1905,  III,  647-733. 


SENSATIONS  OF  THE  SKIN  345 

a  clear  sensation  of  touch  emerges.  These  are  the  touch-spots 
(Fig.  120);  and  if  they  are  marked  for  identification  and  retested 
on  another  day,  careful  exploration  finds  their  arrangement  un- 
changed. If,  next,  the  skin  is  explored  with  a  brass  cone,  cooled 
a  few  degrees  below  the  temperature  of  the  skin,  and  applied  with 
its  (somewhat  rounded)  point  against  the  skin,  no  temperature  sen- 
sation will  be  felt  at  most  points;  but  at  certain  points  a  clear  sensa- 
tion of  cold  appears.  These  cold-spots  are  not  identical  with  the 


ODE 

t 


Fia.  120. — Arrangement  of  Pressure-spots  (Goldscheider).  A,  dorsal  and  radial  surface  of 
the  first  phalanx  of  the  index  finger;  B,  membrane  between  thumb  and  index  finger;  C, 
dorsal  surface  of  forearm;  D,  back;  E,  inner  surface  of  forearm;  F,  back  of  hand. 

touch-spots  (see  Fig.  121).  Exploration  with  a  warm  cone  brings 
out,  though  with  more  difficulty,  the  existence  of  another  series  of 
spots  specifically  sensitive  to  warmth;  and  exploration  with  a  fine- 
pointed  needle  or  stiff  bristle  reveals  numerous  spots  which  give  a 
sharp,  minute,  pricking  sensation.  The  pressure  needed  to  excite 
these  pain-spots  is  much  greater  than  that  needed  for  the  touch- 
spots. 

The  skin  thus  resembles  a  mosaic  of  differently  sensitized  spots; 
it  differs  from  a  mosaic  in  this,  however,  that  there  are  insensitive 
spots  between  those  which  are  sensitive.  No  great  care  is  required 
to  convince  oneself  of  the  general  truth  of  these  observations;  but, 
on  the  other  hand,  great  care  is  necessary  to  reach  precise  results, 
largely  because  of  the  difficulty  of  properly  confining  the  stimulus 
to  the  point  which  it  is  desired,  at  any  moment,  to  examine;  if  the 
pressure,  the  warmth,  or  the  cold,  is  too  great,  it  spreads  quickly 
to  neighboring  regions  and  the  observations  become  uncertain  and 
confused. 


346 


THE  QUALITY  OF  SENSATIONS 


§  24.  The  number  of  spots  of  the  four  varieties  already  mentioned 
is,  in  general,  very  unequal, — the  warmth-spots  being  the  fewest, 
and  the  pain-spots  the  most  numerous.  The  relative  numbers  may 
be  roughly  stated  as  1  warmth-spot  to  10  cold,  10  touch,  and  40 
pain-spots.  This  proportion  varies,  however,  in  different  parts  of 
the  skin.  From  the  cornea,  for  example,  it  seems  that  only  sensations 
of  pain  arise;  while  from  certain  areas  on  the  inner  surface  of  the 
cheek,  over  against  the  gums,  no  sensations  of  pain  can  be  elicited.1 
Hairs  are  organs  of  touch,  and  the  corresponding  sensation  can 
be  aroused  either  by  bending  or  pulling  the  hair,  or  by  pressure  on 

B 


FIG.  121. — Arrangement  of  Temperature-spots.     A,  cold-spots;  and  B,  warmth-spots — 
from  the  palm  of  the  left  hand  (Goldscheider). 

the  skin  over  the  hair  follicle,  on  the  "windward"  side  of  the  point 
of  exit  of  the  hair.  In  general,  the  arrangement  of  the  spots  seems 
to  be  irregular  within  any  small  area. 

Touch-spots  are  excited  by  either  depressing  or  by  pulling  out 
the  skin,  or,  in  general  terms,  by  any  deformation  of  the  skin;  they 
can  also  be  excited  by  electricity.  Pain-spots  can  be  excited  me- 
chanically, electrically,  chemically,  or  thermally;  they  seem  to  be 
adapted  to  receive  any  stimulus  which  is  intense  enough,  or  almost 
intense  enough,  to  injure  the  skin.  Cold-spots  can  be  aroused,  not 
only  by  cold  objects,  but  also  by  warm  objects  ("the  paradoxical 
sensation  of  cold");  warmth-spots  cannot,  perhaps,  be  aroused  by 
anything  but  warmth. 

§  25.  Concerning  the  exact  nature  of  temperature  stimuli,  two 
theories  are  in  the  field.  The  one  (that  of  Weber)  holds  that  the 
warmth  organs  are  excited  whenever  the  temperature  of  the  skin 
is  rising,  and  the  cold  receptors  whenever  the  temperature  of  the 
skin  is  falling.  The  other  theory,  by  Hering,  holds  that  any  portion 
of  the  skin  is,  at  any  moment,  adapted  to  a  certain  temperature, 
which  may  be  called  its  zero  or  indifference  point;2  and  that  tempera- 
tures above  this  zero  point  excite  the  sensation  of  warmth,  and 
temperatures  below  the  sensation  of  cold  or  cool.  The  difference 
between  the  two  theories  will  become  clearer  when  applied  to  the  fol- 

1  Kiesow,  in  Wundt's  Philosophische  Studien,  1898,  XIV,  567. 
2 More  properly  speaking,  not  a  "point,"  but  a  zone  extending  for  about  the 
space  of  one  degree  (Fahrenheit). 


NATURE  OF  TEMPERATURE  STIMULI  347 

lowing  experiment.  Suppose  that  the  temperature  of  the  skin  of 
both  hands  is  80°  Fahrenheit,  and  feels  neither  warm  nor  cool. 
Immerse  one  hand  in  water  at  100°,  the  other  in  water  at  60°,  and, 
after  leaving  them  there  for  a  minute,  plunge  both  into  water  at  80°. 
This  will  now  feel  warm  to  the  hand  which  has  been  in  the  cold 
water,  and  cool  to  the  hand  which  has  been  in  hot  water.  The  ex- 
planation, according  to  the  first  theory,  is  that  the  temperature  of 
the  hand  which  has  been  heated  is  now  lowered,  that  of  the  other 
hand  raised  by  the  same  water.  The  other  theory  says  that  immer- 
sion of  one  hand  in  hot  water  has  adapted  it  to  a  temperature  above 
80°,  i.  e.,  that  its  zero  point  has  been  raised,  so  that  80°  is  now  below 
its  zero  and  feels  cool;  and  the  opposite  for  the  other  hand.  Such 
an  experiment  does  not,  however,  enable  us  to  decide  between  the 
two  theories.  A  more  crucial  instance  is  afforded  by  prolonged  ex- 
posure to  warmth  or  (moderate)  cold.  If  one  is  in  a  hot  bath,  or 
sitting  before  a  fire,  or  has  a  fever,  the  sensation  of  warmth  persists, 
though  perhaps  in  diminished  degree,  for  a  very  long  time.  The 
first  theory  would  have  us  suppose  that  the  temperature  of  the  skin 
is  constantly  rising  during  all  this  time — which  is  hardly  possible. 
The  other  theory  has  simply  to  argue  that  the  process  of  adaptation 
to  temperature  has  limits — i.  e.,  that  the  zero  point,  though  it  can 
be  raised  or  lowered,  cannot  be  raised  or  lowered  very  far.  The 
latter,  or  "adaptation  theory"  has  the  better  of  it  in  this  case;  as 
it  has  also  in  the  case  of  a  sensation  of  warmth  which  lasts  for  a  time 
after  emerging  from  a  hot  bath,  in  spite  of  the  fact  that  the  skin 
at  this  time  is  cooling  off.  However,  such  every-day  observations 
have  not  the  precision  needed  for  a  convincing  test  of  either  theory; 
and  experimental  observations,1  in  which  care  has  been  taken  to 
excite  a  limited  portion  of  the  skin  continuously  and  without  allow- 
ing neighboring  parts  to  be  affected,  have  been  more  favorable  to 
Weber's  theory;  inasmuch  as  prolongation  of  the  stimulus  did  not  pro- 
long the  sensation  beyond  a  minute  or  at  most  three  or  four  minutes. 
§  26.  Other  sensations  from  the  skin  are  apparently  compounds 
or  modifications  of  the  four  classes  already  mentioned.  Hard  and 
soft,  rough  and  smooth,  moist  and  dry,  are  judgments  regarding 
objects,  founded  on  a  combination  of  cutaneous  sensations  and  of 
these  with  muscular  sensations.  Itch  seems  closely  akin  to  pain; 
tickle,  at  least  one  form  of  it,  seems  closely  related  to  touch  proper, 
since  it  can  be  aroused  by  brushing  the  hairs,  which  are  organs  of 
touch.  The  sensation  of  heat  or  of  burning  has  clearly  an  element 
of  pain,  and  probably  arises  from  the  excitation  of  pain-spots  along 
with  warmth-spots.  It  is  also  probable,  from  what  was  said  above 

1  By  Holm,  Skandinavisches  Archiv  /.  Physiol.,  1903,  XIV,  242;  cited  by  Thun- 
berg  in  Nagel's  Handbuch. 


348  THE  QUALITY  OF  SENSATIONS 

regarding  the  paradoxical  sensation  of  cold,  that  a  hot  object  ex- 
cites the  cold-spots  as  well,  so  that  a  burning  sensation  would  be  a 
blend  of  three  elementary  sensations. 

Regarding  pain,  the  view  has  long  been  held  and  is  still  enter- 
tained by  some  authorities,  that  this  sensation  is  the  result  of  ex- 
cessive stimulation  of  any  sensory  nerve;  and  the  painfulness  of 
very  loud  sounds,  or  very  bright  lights,  and  of  high  temperatures, 
was  regarded  as  good  evidence  in  favor  of  this  view.  The  evi- 
dence is  not,  however,  conclusive,  since  it  is  impossible  to  prevent 
excessive  stimulation  from  involving  other  end-organs  besides  those 
of  the  special  sense  supposed  to  be  under  stimulation.  Very  bright 
light  affects  not  only  the  retina,  but  causes  a  strong  reaction  in  the 
iris  and  eyelids.  Loud  sounds  cause  strong  reactions  in  the  mid- 
dle ear;  and  high  temperatures,  as  we  have  just  seen,  affect  the  pain- 
rts  as  well  as  the  warmth-spots.  If  we  distinguish,  as  we  certainly 
uld,  between  mere  unpleasantness  and  pain,  we  can  hardly 
doubt  that  there  is  a  specific  cutaneous  pain,  which  arises  from  the 
excitation  of  certain  definite  points;  and  accordingly  it  has  become 
customary  to  speak  of  a  pain  sense,  as  of  a  warmth  sense,  a  cold 
sense,  and  a  touch  sense.  There  are,  however,  several  different 
qualities  of  sensory  pain  and  ache,  as  from  the  bones,  teeth,  fa- 
tigued muscles,  etc. 

§  27.  Quite  recently  a  new  line  of  study  of  the  cutaneous  senses 
has  been  opened  through  the  work  of  Head  and  his  collaborators.1 
When  a  nerve  supplying  the  sense-organs  in  any  area  of  the  skin 
is  severed  by  accident,  a  certain  region  of  the  skin  is  entirely  de- 
prived of  sensibility;  but,  bordering  this  region  is  a  zone  in  which 
sensibility  is  impaired  in  a  definite  way,  without  being  entirely  de- 
stroyed. This  border  region  may  be  insensitive  to  light  touch  or 
to  minor  degrees  of  warmth  or  cold,  and  be  deprived  of  spatial 
discrimination;  a  prick  may  be  felt,  but  not  be  localizable,  since 
it  gives  rise  to  only  a  diffuse,  tingling  sensation.  In  the  area  in 
which  the  skin  is  totally  insensitive,  a  deep  or  subcutaneous  sensi- 
bility is  still  retained.  Pressure  through  the  skin  is  felt  here;  but, 
if  the  skin  is  lifted  from  the  underlying  tissue  into  a  fold,  no  stimulus 
applied  to  this  fold  of  skin  is  felt.  The  pressure  which  is  appreci- 
ated by  the  subcutaneous  sense  can  itself  be  localized,  and  yet 
spatial  discrimination,  as  between  the  point  and  the  head  of  a  pin, 
or  between  two  points  and  one  point  of  the  compasses,  is  impossible 
with  the  deep  sensibility  alone.  Dull  pain  is  produced  by  strong 
pressure  on  the  subcutaneous  organs. 

1  Head  and  Sherren,  "The  Consequences  of  Injury  to  the  Peripheral  Nerves  of 
Man,"  Brain,  1905,  XXVIII,  241;  Rivers  and  Head,  "  A  Human  Experiment  in 
Nerve  Division,"  Brain,  1908,  XXXI,  323. 


THE  SO-CALLED  MUSCULAR  SENSE  349 

Such  phenomena  show  that  the  severance  of  the  cutaneous  nerve 
has  served  to  isolate  the  subcutaneous  sense,  and  thus  they  lead  to 
a  distinction  between  the  tactile  sense  of  the  skin  and  the  pressure 
sense  of  the  subcutaneous  tissue;  and  also  to  a  distinction  between 
the  pain  sense  of  the  skin  (pricking)  and  the  duller  pain  of  the  sub- 
cutaneous sense.  Besides  this  isolation  of  the  subcutaneous  sense, 
Head  believes  that  the  partial  sensibility  of  the  intermediate  zone 
of  the  skin  represents  the  isolation  of  a  rudimentary  form  of  cu- 
taneous sensibility,  which  is  sensitive  to  pricking  and  to  extremes 
of  heat  and  cold,  from  the  perfected  form  of  the  normal  skin,  which 
is  sensitive  to  warmth  and  coolness  and  to  light  touch,  and  which  has 
the  power  of  spatial  discrimination.  The  more  rudimentary  form 
he  calls  "  protopathic "  sensibility,  and  the  more  perfect  form,  "epi- 
critic"  sensibility.  This  separation  of  two  distinct  forms  of  cu- 
taneous sensibility  has  been  called  in  question;  since  it  has  been 
shown  by  Franz1  and  by  Trotter  and  Davies2  that  the  transition 
from  one  to  the  other  is  really  gradual.  But  the  sufficiently  sharp 
delimitation  of  the  subcutaneous  sense  appears  to  be  correct,  and  a 
certain  region  in  which  the  transition  appears  gradual  is  precisely 
what  we  should  expect.  The  discovery,  therefore,  represents  a  con- 
siderable advance  in  our  .knowledge  of  these  classes  of  sensation. 

§  28.  The  subcutaneous  sense  merges  with  the  so-called  muscular 
sense.  Sensory  end-organs  are  found  in  muscles  and  tendons, 
about  the  joints,  and  in  the  bones  (see  p.  181).  Goldscheider3  and 
others  have  shown  that  cutaneous  anaesthesia  alone  does  not  destroy 
nor  much  impair  the  power  of  perceiving  the  movements  and  posi- 
tions of  the  different  members  of  the  body,  nor  the  power  of  accu- 
rately co-ordinated  movement,  which  latter,  as  well  as,  of  course,  the 
former,  is  destroyed  by  complete  anaesthesia  of  a  member.  At  one 
time,  there  was  much  disposition  to  believe  in  a  definite  joint-sense, 
with  end-organs  in  the  articular  surfaces,  excited  by  the  rubbing 
of  these  surfaces  over  each  other;  but  evidence  is  lacking  of  sense- 
organs  in  these  surfaces.  Yet  the  region  of  the  joints  must  be  the 
location  of  end-organs  important  in  the  perception  of  movement, 
for  Goldscheider  found  that  anaesthetizing  the  joint,  by  passing  a 
current  through  it,  greatly  impaired  the  perception  of  motion.  The 
observations  of  Pillsbury4  indicate  that  the  sensibility  of  the  joint 
region  is  closely  connected  with  the  tendons  which  pass  over  the 
joint.  It  is  apparently  the  movement  of  these  tendons  in  their 

1  Journ.  of  Comp.  Neurol.  and  Psychol,  1909,  XIV,  107,  215. 

2  Journ.  of  PhysioL,  1909,  XXXVIII,  134. 

3  See  Goldscheider's  GesammeUe  Abhandlungen,  Physiologic  des  Muskelsinnes 
(Leipzig,  1909). 

4  American  Journ.  of  Psychol.,  1901,  XII,  346. 


350  THE  QUALITY  OF  SENSATIONS 

grooves,  rather  than  the  movement  of  the  articular  surfaces  over 
each  other,  which  gives  rise  to  sensations  of  movement.  Of  the 
qualities  of  sensation  belonging  to  this  "muscle"  sense  it  is  less  easy 
to  speak  than  of  the  perceptions  of  motion,  position,  and  resistance 
which  are  founded  on  them.  We  may,  however,  distinguish  with 
certainty  between  the  unpleasant  sensations  of  muscular  fatigue 
and  soreness  and  the  more  matter-of-fact  sensations  of  movement 
and  resistance.  Of  these  latter,  several  varieties  can  be  observed;1 
but  nothing  like  a  complete  analysis  or  system  of  such  sensory  quali- 
ties has  yet  been  attempted. 

§  29.  From  the  labyrinth  of  the  inner  ear — the  semicircular 
canals  and  vestibule — arise  a  class  of  sensations  which  may  be 
named  " labyrinthic "  (see  above,  p.  206).  Here  again,  though  it 
is  easy  to  mention  the  perceptions  and  the  reflex  movements  which 
result  from  the  stimulation  of  these  organs,  it  is  not  possible  as  yet 
to  say  much  regarding  the  sensory  qualities  belonging  to  the  laby- 
rinthic sense.  The  swimming  sensations  in  the  head  in  dizziness, 
and  the  milder  degrees  of  the  same  which  can  be  observed  in  rota- 
tion, and  in  going  up  or  down  in  an  elevator,  belong  here  without 
doubt. 

§  30.  Under  the  head  of  visceral  or  organic  sensations  may  be 
grouped  hunger,  thirst,  nausea,  suffocation,  and  probably  a  host 
of  other  sensory  compounds  which  contribute  much  to  our  feeling 
of  well-being  or  illness  and  in  general  to  the  "coensesthesia"  or 
"common  sensation"  of  the  organism.  Analysis  here  is  difficult, 
for  it  is  never  easy  to  analyze  a  sensory  complex  into  its  elements 
unless  we  can  control  and  isolate  the  stimuli.  Little  can  be  done 
at  present  toward  a  physiological  explanation  of  these  sensations, 
except  to  call  attention  to  their  existence.2 

§  31.  In  closing  the  subject  treated  in  the  last  two  chapters, 
attention  is  again  called  to  the  large  amount  and  cumulative  char- 
acter of  the  evidence  afforded  by  the  special  sensations,  considered 
as  respects  their  quality,  for  the  law  of  the  Specific  Energy  of  the 
Nerves.  It  is  impossible  to  account  for  the  above-mentioned  phe- 
nomena without  carrying  this  law  to  a  great  length  in  its  applica- 
tion to  the  special  senses.  We  may  not  be  able  at  present  to  affirm 
that  two  sensations  are  distinguishable  as  respects  quality  only  in 
case  they  are  occasioned  by  two  individually  different  elements  of 
the  nervous  system.  For,  as  we  advance  in  our  investigations  we 
shall  see  even  more  clearly  that  the  quality  of  sensations  depends 
upon  their  quantity,  upon  their  relation  to  preceding  and  con- 

1  See  Woodworth,  Le  Mouvement,  pp.  25  ff.  (Paris,  1903). 
8  For  a  more  extended  treatment  of  the  entire  subject  of  qualities  of  sensation, 
reference  may  be  made  to  Titchener's  Textbook  of  Psychology  (New  York,  1909). 


SPECIFIC  ENERGY  OF  THE  NERVES  351 

temporaneous  sensations,  and  upon  innumerable  considerations 
other  than  merely  the  one  of  what  particular  nerve-fibre  or  element 
of  the  end-apparatus  was  acted  upon  by  the  stimulus.  Moreover, 
there  is  no  warrant  for  saying  that  identically  the  same  nervous 
apparatus  cannot  be  excited  variously  according  to  the  nature  of 
the  stimulus  which  acts  upon  it,  or  according  to  the  combination 
with  other  parts  of  the  system  into  which  it  enters  for  the  time.  It 
is  obvious,  however,  that  the  differentiation  of  function,  and  the  as- 
signment to  specifically  distinct  apparatus  of  particular  nervous  im- 
pressions corresponding  to,  particular  mental  states,  is  carried  to  a 
great  length  in  the  special  senses.  In  this  differentiation  of  func- 
tion it  is  not  wholly  or  chiefly  the  nerve-fibres,  as  such,  which  should 
be  taken  into  account;  it  is  also  the  minute  subdivisions  of  the  end- 
organs  of  sense,  and  the  connections  set  up  within  the  correspond- 
ing regions  of  the  central  organs.  In  accounting  for  those  complex 
sensations  which  appear  in  ordinary  consciousness,  the  law  of  per- 
mutations and  combinations  has,  of  course,  to  be  considered.  A 
vast  variety  of  such  ^sensations  may  be  made  up  by  changing  the 
relations  to  each  other  of  comparatively  few  simple  elements.  But 
in  each  of  the  senses  our  analysis,  when  carried  to  its  utmost  limit, 
leaves  a  number — in  some  of  the  senses  very  large — of  simple  sensa- 
tions, which  apparently  must  have  their  normal  physical  basis  in 
the  excitation  of  specifically  distinct  elements  of  the  nervous  mech- 
anism. 

The  sense  of  smell  apparently  requires  that  the  law  of  the  specific 
energy  of  the  nerves  should  be  carried  to  such  a  length  as  almost 
to  reduce  it  to  an  absurdity.  Histology  has  discovered  only  one 
essential  kind  of  olfactory  end-organ,  and  that  of  comparatively 
simple  structure;  and  yet  experience  gives,  as  the  result  of  its  ex- 
citation, a  bewildering  variety  of  sensations  so  specifically  different 
as  to  baffle  all  our  attempts  to  classify  them.  From  the  case  of 
this  sense  an  argument  may  then  be  derived  which  leads  in  either 
direction.  It  may  be  objected  to  the  law  that  it  is  absurd  to  sup- 
pose a  complexity  of  the  end-organs  of  smell  such  as  to  correspond 
to  each  specific  kind  of  olfactory  stimulus  with  a  specific  sensation 
— for  example,  the  smell  of  musk,  or  of  sulphuretted  hydrogen.  It 
may  be  replied  to  the  objection  that,  in  the  case  of  the  ear,  there 
are  at  least  16,000  or  20,000  distinct  units  of  auditory  end-appa- 
ratus corresponding  to  the  different  musical  tones;  and  it  is  there- 
fore by  no  means  impossible  that  the  entire  regio  olfactoria  may 
contain  enough  specifically  different  forms  of  its  own  peculiar  end- 
apparatus  to  suffice  for  all  the  simple  sensations  of  smell. 

The  sense  of  taste  does  not  occasion  so  many  difficulties  in  rela- 
tion to  the  law  of  the  specific  energy  of  the  nerves.  We  have  seen 


352  THE  QUALITY  OF  SENSATIONS 

that  physiologists  incline  to  reduce  all  the  sensations  of  taste  to  four, 
or  at  most  six,  different  species.  It  is  easy  to  suppose  as  many  spe- 
cifically different  forms  of  the  nervous  apparatus  corresponding 
to  the  different  classes  of  sensations — sweet  and  sour,  salt  and  bit- 
ter, alkaline  and  metallic.  On  combining  these  with  sensations 
of  smell  and  of  touch,  it  is  assumed  to  find  an  explanation  for  all 
the  varieties  of  the  tastes  of  our  daily  experience.  But  even  if  we 
have  greatly  to  increase  the  number  of  primary  sensations  of  the 
gustatory  species,  the  theory  of  specific  functions  for  different  ner- 
vous elements  and  combinations  of  such  elements  would  seem  able 
to  meet  the  demand. 

The  strongest  defence  of  the  most  extreme  form  of  the  theory  of 
the  specific  energy  of  the  nerves  has  hitherto  been  found  in  sensa- 
tions of  musical  sound.  Here  we  undoubtedly  have  a  wide  range 
of  qualitatively  distinct  states  of  consciousness  which  are  appar- 
ently dependent  upon  the  excitation  of  a  correspondingly  large 
number  of  distinct  nervous  elements. 

The  recent  discoveries  as  to  the  existence  of  pressure-spots, 
warmth-spots,  and  cold-spots  in  the  skin  add  important  evidence  to 
that  already  existing  in  favor  of  the  law  under  discussion. 

It  is,  undoubtedly,  still  difficult  to  make  any  thorough-going  appli- 
cation of  the  law  of  specific  energies  to  the  case  of  the  color  sensa- 
tions. The  principal  reasons  for  this  are  twofold:  first,  histology 
has  revealed  no  such  differentiation  of  the  cones  as  would  enable 
us  to  divide  them  into  organs  for  the  several  primary  colors;  and, 
second,  the  peculiar,  antagonistic  relations  of  white  and  black, 
yellow  and  blue,  and  red  and  green,  are  hard  to  explain  by  any 
theory  of  specific  energies.  The  Hering  theory,  as  we  have  seen, 
interprets  these  relations  to  mean  that  the  same  end-organ  is  capa- 
ble of  two  antagonistic  reactions.  Accordingly,  it  would  seem  that 
the  nerve-fibres  connected  with  these  end-organs  must  transmit  two 
kinds  of  impulses  to  the  brain.  The  theory  of  their  specific  energy, 
therefore,  needs  further  testing  before  it  can  be  satisfactorily  fitted 
to  the  facts  of  our  vision  of  color. 

It  will  further  appear,  when  we  consider  the  process  of  localiza- 
tion in  the  so-called  "geometrical  senses"  of  the  eye  and  the  skin, 
that  the  very  possibility  of  such  a  process  demands  a  somewhat 
strict  and  far-reaching  application  of  the  law  of  the  specific  energy 
of  the  nerves.  Precisely  how  we  are  to  state  and  limit  this  law, 
neither  its  opponents  nor  its  advocates  have  as  yet  been  able  satis- 
factorily to  show.  The  exact  expression  of  the  theory  waits  for 
further  evidence  from  experiment,  although  there  can  be  little  doubt 
that  in  its  main  features  it  is  already  secure. 


CHAPTER  III 
THE  QUANTITY  OF  SENSATIONS 

§  1.  By  an  act  of  mental  analysis,  which  all  men  readily  perform, 
changes  in  the  amount  of  sensation  are  distinguished  from  changes 
in  its  quality.  This  distinction,  in  itself  considered,  obviously  re- 
quires for  its  performance  nothing  beyond  what  is  immediately 
given  in  consciousness.  The  simple  fact  of  experience  is,  that  all 
sensations  appear  to  differ  among  themselves,  not  only  with  re- 
spect to  the  nature  of  the  impression  which  serves  to  classify  them 
into  groups  (as  sensations  of  sight,  sound,  etc.),  but  also  with  re- 
spect to  the  degree  in  which  each  particular  impression  possesses 
the  sphere  of  conscious  attention  and  feeling.  The  best  illustra- 
tion of  an  alteration  in  the  intensity  of  sensation,  while  its  charac- 
teristic quality  remains  unaltered,  may  be  derived  from  musical 
tones.  The  dying-out  of  a  single  tone  when  the  bow  is  drawn 
with  decreasing  force  across  the  string  of  a  violin,  or  a  single 
key  of  the  piano  is  struck  and  the  pedal  held,  may  be  considered 
as  a  change  in  the  quantity  of  sensation,  while  its  quality  is  un- 
changed. A  more  complex  case  is  the  experience  we  have  when 
approaching  or  receding  from  a  bell  that  is  sounding  or  a  steam- 
whistle  that  is  blowing.  Noises  of  a  certain  complex  quality — 
such  as  slamming,  hissing,  grating,  etc. — are  continually  described 
as  very  loud,  moderately  loud,  or  of  weak  intensity.  So,  too, 
when  approaching  a  white  or  colored  light,  with  our  attention 
fixed  upon  it,  we  generally  disregard  almost  wholly  the  changes 
in  its  color-tone  which  take  place,  and  consider  chiefly  the  changes 
in  its  intensity  and  apparent  size.  The  pressure  of  different  weights 
upon  different  parts  of  our  skin  is  ordinarily  regarded  as  the  same 
in  quality  and  as  varying  only  in  amount  and  locality.  The  same 
thing  is  true,  in  almost  precisely  the  same  way,  with  sensations 
of  temperature.  The  thing  we  touch  is  called  slightly  cold  or  very 
cold,  somewhat  warm  or  very  hot,  our  attention  being  directed 
chiefly  to  our  judgment  of  the  quantum  of  sense  experience  which 
it  calls  forth.  In  other  words,  it  is  generally  the  same  kind  of 
pressure  and  temperature,  with  a  varying  degree  of  intensity,  of 
which  we  believe  ourselves  to  be  conscious. 

It  is  more  difficult,  however,  even  in  the  most  indefinite  way,  to 
discriminate  between  the  quantities  of  our  sensations  of  smell  and 

353 


354  THE  QUANTITY  OF  SENSATIONS 

taste  and  the  changes  in  specific  quality  of  the  same  sensations.  A 
concentrated  sweet  or  acid  so  strongly  excites  a  variety  of  forms  of 
feeling  which  mingle  indistinguishably  with  the  sensations  of  taste 
that  we  are  compelled  to  attend  to  the  very  decided  qualitative 
changes  which  are  taking  place.  The  increased  intensity  of  the 
sweet  or  sour  we  may  indeed  speak  of  as  "very"  much  of  the  same 
sensation  which  was  excited  in  less  degree  by  the  diluted  form  of 
the  stimulus;  but  we  are'  more  likely  to  regard  it  as  constituting  a 
complete  change  in  the  kind  of  taste.  In  the  same  manner,  at- 
tention is  forcibly  directed  toward  the  kind  of  sensation  which  re- 
sults from  increasing  the  quantity  of  any  specific  sensation  of  smell. 

It  is  further  obvious  that  the  distinction  which  we  make  between 
changes  in  the  quantity  and  changes  in  the  quality  of  our  sensa- 
tions is  to  some  extent  applicable  for  comparing  the  sensations  of 
different  senses.  And  here  the  distinction,  when  applied  to  sub- 
species under  certain  specific  forms  of  sensation,  affords  us  a  means 
of  transition  for  such  comparisons.  Some  yellows  are  bright  and 
others  dull;  and  the  same  thing  is  true  of  the  reds  and  the  blues. 
The  sours,  the  sweets,  the  bitters,  may  be  compared  with  each 
other  as  respects  the  degree  of  intensity  which  they  possess.  We 
may  next,  in  a  very  indefinite  way,  compare  the  quantities  of  the 
sensations  of  the  different  senses  as  they  appear  side  by  side,  or 
successively,  in  consciousness.  We  are  ordinarily  satisfied,  how- 
ever, with  simply  describing  the  varying  degrees  of  intensity  pos- 
sessed by  our  different  sensations  as  "weak"  or  "strong"  (with 
or  without  the  emphatic  "very")  or  as  only  "moderate."  Thus 
we  may  judge  that  both  the  light  which  we  see  and  the  tone  which 
we  hear  (either  simultaneously  or  one  immediately  after  the  other) 
are,  or  are  not,  to  be  classed  together  under  the  same  one  of  these 
three  popular  forms  of  discriminating  degrees  of  intensity. 

§  2.  That  changes  in  the  intensity  of  our  sensations  are  not,  in 
fact,  wholly  independent  of  changes  in  their  specific  nature  has  al- 
ready been  stated.  Only  in  the  case  of  musical  tones  are  we  able 
at  the  same  time  to  attend  carefully  to  both  the  quantity  and  quality 
of  our  sensations,  and  so  discover  with  perfect  confidence  that  the 
former  is  changing  while  the  latter  remains  unchanged.  Even  in 
this  case,  since  the  tones  which  we  ordinarily  hear  are  composite, 
any  considerable  alteration  of  their  intensity  changes  also  their  tone- 
coloring,  through  the  alteration  which  it  produces  in  the  comparative 
intensities  of  the  overtones.  Any  increase  in  the  brightness  of  a  par- 
ticular color  invariably  changes  its  characteristic  color-tone.  A 
white  of  less  intensity  is  not  merely  less  white,  but  it  has  become  a 
gray;  and  by  constantly  diminishing  its  intensity  white  can  be  shaded 
through  the  different  grays  toward  black,  which  is  certainly  not  a 


UNSCIENTIFIC  FORM  OF  ORDINARY  USAGE     355 

feebler  degree  of  the  sensation  of  white.  The  same  dependence  of 
quality  on  quantity  is  true  in  all  sensations  of  smell,  taste,  pressure, 
and  temperature.  It  is  no  less  than  an  inexcusable  psychological 
blunder,  however,  on  this  account  to  consider  "quantity"  of  sen- 
sations as  only  another  name  for  shades  of  quality;  or  to  deny  that 
we  can  apply  terms  of  measurement  to  these  reactions  of  the  mind 
upon  the  excitation  of  the  nervous  apparatus  of  sense.  Scientific 
analysis  confirms  the  distinction  made  by  ordinary  experience  be- 
tween "the  way"  we  feel  and  " how  much"  we  feel  in  any  particular 
way. 

§  3.  All  descriptions  of  the  changing  intensities  of  sensations, 
when  made  on  the  basis  of  ordinary  experience  solely,  leave  the 
subject  in  a  very  indefinite  and  unscientific  form.  That  a  certain 
noise  is  louder  or  weaker  than  another  of  precisely  the  same  kind, 
one  may  be  quite  ready  to  affirm;  one  may  even  be  ready  to  say 
that  one  judges  this  noise  to  be  about  twice  or  three  times  as  loud 
as  the  other.  But  when  more  precise  estimates  are  demanded,  one 
is  obliged  to  hesitate  before  giving  them.  Is  this  musical  tone  ten 
(or  a  hundred)  times  as  loud  as  the  other;  or  is  it  only  nine  and 
nine-tenths  (or  ninety-nine  and  nine-tenths)  as  loud?  Few  would 
venture  so  nice  an  estimate  with  any  confidence.  Yet  the  case  of 
sound  is  much  more  favorable  than  that  of  most  of  the  other  senses 
for  forming  an  exact  judgment  as  to  its  intensity.  It  would  be 
difficult  under  the  most  favorable  circumstances  to  affirm  that  the 
sensation  of  the  light  a  is  twice  or  three  times  as  bright  as  that  of 
the  light  b;  or  that  of  the  shadow  x  one-half  or  one-third  as  bright 
as  y.  The  comparative  intensities  of  different  color-tones  are  yet 
more  difficult  to  fix  subjectively — even  in  the  most  indefinite  way. 
This  particular  yellow  may  seem  about  as  bright  a  color,  of  its  kind, 
as  does  the  red  near  it,  of  its  kind.  But  the  precise  moment  could 
not  readily  be  told  when  the  blue  of  the  sky  appears  exactly  twice 
as  intense  as  the  green  of  the  grass.  Still  further,  all  estimates 
of  the  quantity  of  sensation  approach  the  point  at  which  they  lose 
their  meaning  and  tend  to  become  absurd,  when  we  compare,  for 
example,  sensations  of  smell  or  taste  with  those  of  pressure,  tempera- 
ture, or  sight.  We  never  say:  The  rose  smells  as  sweet  as  it  looks 
red;  or  the  lemon  is  twice  as  sour  as  the  sky  is  blue.  And  yet  each 
qualitatively  different  sensation  is  assumed  to  have  its  place  some- 
where in  that  scale  of  intensities  through  which  the  different  quali- 
ties may  run;  each  may,  therefore,  be  compared  with  every  other, 
with  respect  to  the  general  position  which  it  occupies  in  its  char- 
acteristic scale. 

§  4.  The  variation  of  sensations  in  intensity,  though  it  is  an  ob- 
vious fact  of  the  commonest  experience,  leads,  when  we  attempt  to 


356  THE  QUANTITY  OF  SENSATIONS 

give  it  exact  scientific  form,  to  a  host  of  difficulties,  which  have  been 
the  occasion  of  an  immense  amount  of  discussion  among  psychol- 
ogists. All  the  resources  of  the  higher  mathematics  have  been  em- 
ployed in  order  to  discover  and  demonstrate  some  law  of  universal 
applicability,  which  shall  regulate  satisfactorily  the  quantitative 
relations  existing  between  the  functions  of  the  nervous  mechanism, 
on  the  one  hand,  and  on  the  other,  the  states  of  consciousness  to 
which  psychology  gives  the  name  of  sensations,  or  "sensation-com- 
plexes." In  some  cases,  as  notably  that  of  Fechner,  the  attempt 
has  even  been  made  to  raise  this  alleged  psycho-physical  law  to  the 
dignity  of  a  universal  metaphysical  principle.  Meantime,  the  fact 
has  become  more  and  more  apparent  that  what  we  have,  on  the 
one  side,  is  certain  largely  unknown  but  always  highly  complex 
chemico-physical  and  nervous  reactions,  and  on  the  other  side, 
even  more  complex,  and  often  unanalyzable,  conscious  experiences, 
in  which  the  faculty  of  discrimination  always  plays  the  leading  part, 
and  which  are  quite  uniformly  influenced  by  individual  conditions 
of  mental  habit,  fatigue,  specific  sensitiveness,  either  congenital 
or  acquired,  and  even  individual  idiosyncrasies.  We  do  not  ex- 
pect, then,  to  reach  anything  like  an  exact  and  universal  formula, 
statable  in  terms  of  mathematics,  after  the  fashion  of  the  results 
obtainable  by  a  successful  research  in  physics  or  chemistry.  As 
students  of  psychology  from  the  physiological  point  of  view,  it  is 
our  duty  (so  we  have  always  held)  first  to  discover  the  data,  and  then 
as  far  as  possible  give  them  that  kind  of  interpretation  of  which 
alone  the  science  of  psychology  admits. 

§  5.  The  fundamental  characteristic  of  the  intensity  of  a  given 
sensation,  that  which  fits  it  for  scientific  study  and  measurement,  is 
this:  its  various  degrees  can  be  arranged  in  an  ordered  series,  from 
less  intense  to  more  intense;  and  thus  every  intensity  can  be  judged, 
with  reference  to  any  other,  by  comparing  it  with  a  standard,  or 
some  one  simple  category  of  magnitude.  Such  well-ordered  series 
can  be  found,  not  only  in  reference  to  intensity,  but  also  in  refer- 
ence to  extent  and  duration;  and,  in  some  instances,  in  reference 
to  quality.  Tones,  for  example,  can  be  arranged  in  an  ordered  series 
as  regards  their  pitch;  lights,  as  regards  their  color-tone.  The  scien- 
tific problems  which  arise  in  connection  with  such  series  are  (1)  to 
assign  the  limits  of  the  series;  and  (2)  to  discover,  if  possible,  the 
customary,  or  fixed,  relations  between  the  different  terms  of  the 
series.  As  applied  to  the  pitch  series,  these  problems  lead  to  the 
determination  of  the  lowest  and  highest  pitch,  the  measurement  of 
the  least  noticeable  difference  of  pitch,  and  the  examination  of  the 
"  intervals."  As  applied  to  color,  however,  the  problems  of  measure- 
ment are  reducible  to  the  determination  of  the  limits  of  the  colored 


CHARACTER  OF  QUANTITATIVE  PROBLEMS       357 

spectrum,  the  least  noticeable  difference  in  color,  and  such  relations 
as  that  of  complementary  colors. 

As  applied  to  the  intensity  series,  these  same  problems  call  for 
(1)  the  determination  of  the  weakest  and  strongest  sensations,  of 
any  given  quality,  of  which  we  are  capable;  (2)  the  relation  between 
neighboring  members  of  the  series  which  is  indicated  by  the  least 
noticeable  difference  in  intensity;  and  (3)  the  relations  (if  any  of 
interest  can  be  found)  between  distant  members  of  the  series.  In 
all  this  kind  of  work,  it  is  clearly  impossible  to  deal  with  sensations 
apart  from  the  stimuli  which  give  rise  to  them.  The  stimuli  are 
needed  to  arouse  the  sensations;  control  and  identification  of  the 
stimulus  is  necessary  in  order  to  control  and  (approximately)  iden- 
tify the  sensation;  and  furthermore  measurement  can  be  applied 
directly  to  the  stimulus,  but  not  to  the  sensation.  It  is  clear,  then, 
that  quantitative  statements  regarding  the  intensity  of  sensation 
must  always  be  based  on  quantitatively  determined  stimuli. 

§  6.  The  quantitative  problems  which  arise  in  connection  with 
the  intensity  of  sensation  may  then  be  somewhat  crudely  generalized 
as  follows:  (1)  To  determine  how  little  and  how  much  of  each  kind 
of  stimulus  will  produce  respectively  the  least  and  the  greatest  quan- 
tity of  each  kind  of  sensation  of  which  the  mind  is  capable,  or  to 
find  the  quantitative  limits  within  which  sensations  of  each  sense 
are  possible;  and  (2)  to  determine  the  law  of  the  relation  under  which 
changes  in  the  intensity  of  sensations,  as  estimated  in  conscious- 
ness, are  dependent  upon  changes  in  the  intensity  of  the  stimuli. 

§  7.  Two  methods  of  determining  the  lower  limit,  or  minimum 
of  stimulus  producing  a  sensation,  are  possible.  In  the  use  of  one 
method,  a  weak  stimulus,  but  somewhat  above  the  amount  needed 
to  produce  a  sensation,  is  applied;  its  intensity  is  then  diminished 
by  minute  gradations  until  the  exact  point  is  reached  and  noted  at 
which  it  ceases  to  produce  any  sensation  at  all.  In  the  use  of  the 
other  method  a  stimulus  too  weak  to  produce  any  sensation  is  first 
applied;  its  intensity  is  then  very  gradually  increased  until  it  be- 
gins to  produce  the  smallest  observable  sensation.  Both  ways  may 
be  combined,  and  thus  the  "sensitiveness"  of  each  organ  of  sense, 
and  of  each  part  of  each  organ,  may  be  determined.  Such  sensi- 
tiveness increases,  of  course,  in  inverse  ratio  to  the  amount  of  stim- 
ulus necessary  for  producing  any  sensation  at  all,  or  for  producing 
a  sensation  estimated  as  having  a  definite  degree  of  energy.  The 
effort  to  determine  the  lower  limit  of  sensations  of  sight  and  of 
sound  is  embarrassed  by  the  facts  that  the  retina  is  always  under 
excitation  from  the  chemical  changes  going  on  in  its  pigments,  and 
therefore  has  a  certain  quantum  of  so-called  "light  of  its  own," 
and  that  such  a  thing  as  "absolute  stillness"  cannot  probably  be 


358  THE  QUANTITY  OF  SENSATIONS 

secured  for  the  ear.  Total  absence  of  sensation  in  the  ear,  could 
it  be  secured,  would  not  be  comparable  to  the  black  which  we  see 
with  the  eyes  closed. 

The  upper  limit  of  the  intensity  series  would  be  determined  if 
we  should  find  that  increasing  the  stimulus  beyond  a  certain  limit 
gave  no  further  increase  in  the  sensation.  Practically,  however, 
such  a  determination  is  scarcely  ever  possible,  both  because  of  the 
danger  to  the  sense-organs,  and  also  because  very  intense  stimuli 
affect  other  organs  besides  that  whose  sensations  we  wish  to  examine 
and  so  lead  to  confused  results. 

§  8.  The  invention  of  methods  for  determining  the  least  percepti- 
ble difference  in  intensity,  and  other  relations  between  members 
of  the  intensity  series,  has  been  one  of  the  main  tasks  of  experimental 
psychology,  and  has  engaged  the  attention  of  many  of  the  ablest 
students  of  this  specialty.  The  greatest  contributions,  in  this  mat- 
ter of  method,  were  made  by  Fechner  in  his  Psychophysik  (I860).1 

We  cannot  enter  here  into  the  technical  details  which  must  be 
attended  to  in  any  actual  use  of  these  methods,  but  will  simply 
attempt  to  characterize  them  briefly.  The  following  are  the  prin- 
cipal ones:  (1)  The  "method  of  just  noticeable  differences"  de- 
termines that  difference  in  the  intensity  of  two  stimuli  which  is  just 
large  enough  to  be  recognized  as  such.  Of  two  lights,  for  example, 
which  are  at  first  equally  bright,  one  is  made  brighter  by  slow  de- 
grees, till  it  is  first  judged  brighter;  then  a  second  determination 
is  made,  starting  with  one  light  obviously  the  brighter,  and  diminish- 
ing this  one  till  it  no  longer  seems  brighter  than  the  other.  The 
mean  of  these  two  determinations  gives  a  measure  of  the  least 
noticeable  difference,  or,  it  may  also  be  called,  the  threshold  of 
difference.  It  is  not  true,  however,  as  the  name  might  imply,  that 
this  threshold  is  a  perfectly  fixed  quantity,  below  which  all  dif- 
ferences are  unperceived,  while  above  it  all  are  perceived;  for  per- 
ception is  a  variable  process,  and  a  difference  which  at  one  moment 
may  be  clearly  detected  will  pass  unnoticed  at  another  moment. 
It  is  necessary,  therefore,  to  repeat  the  determinations  a  number 
of  times,  and  to  take  an  average,  according  to  the  well-known  rules 
regulating  such  experimentation. 

(2)  In  the  "method  of  average  error,"  as  in  the  preceding  method, 
there  are  two  stimuli,  one  of  which  is  fixed  in  intensity,  and  the  other 

1  Next  to  Fechner,  the  development  of  these  "  psychophysical  methods  "  owes 
most  to  G.  E.  Mullen  Zur  Grundlegung  der  Psychophysik  (Berlin,  1878);  Die 
Gesichtspunkte  und  die  Tatsachen  der  psychophysischen  Methodik  (Wiesbaden, 
1904).  For  both  an  elementary  account  of  these  methods  and  a  full  history  and 
discussion  of  the  subject,  see  Titchener,  Experimental  Psychology,  Quantitative 
(New  York,  1905). 


THE  LEAST  PERCEPTIBLE  DIFFERENCE  359 

adjustable.  In  the  use  of  this  method,  however,  the  effort  is  made 
to  adjust  the  second  stimulus  so  that  it  shall  appear  equal  to  the 
first.  Some  slight  error  is  sure  to  be  committed;  the  error  will 
vary  from  trial  to  trial,  and  the  average  error  is  determined,  and  also 
any  constant  errors  which  may  appear;  and,  as  well,  the  varia- 
bility of  the  performance.  In  some  respects  the  variability  is  the 
most  important  of  all  of  these  determinations,  since  it  gives  a  measure 
of  the  variation  which  a  stimulus  can  undergo  without  ceasing  to 
appear  the  same. 

(3)  In  the  "method  of  right  and  wrong  cases,"  neither  of  the  two 
stimuli  is  adjustable,  but  they  are  fixed  at  a  difference  too  small  to 
be  perceived  with  certainty  and  regularity,  but  not  too  small  to  be 
detected  most  of  the  time.     With  this  fixed  difference,  a  long  series 
of  judgments  is  made,  and  the  per  cent,  of  right  judgments  gives  a 
measure  of  the  perceptibility  of  the  chosen  difference  between  the 
stimuli.     If,  for  example,  a  weight  of  105  grams  is  judged  heavier 
than  a  weight  of  100  grams  70  per  cent,  of  the  time,  while  205  is 
judged  heavier  than  200  only  60  per  cent,  of  the  time,  it  is  clear  that 
the  difference  200-205  is  less  perceptible  than  the  difference  100- 
105;  and  if  200—210  is,  like  100-105,  judged  correctly  70  per  cent, 
of  the  time,   then  these  two  differences  are  equally  perceptible. 
Mathematical  analysis,  based  on  the  theory  of  probability,  enables 
us  to  go  further  than  this,  and  compute,  from  the  observed  per  cent, 
of  right  cases  with  a  given  difference,  the  difference  which  should 
give  any  desired  per  cent,  of  right  cases.1 

While  the  three  preceding  methods  deal  with  the  perception  of 
small  differences — differences  at  or  near  the  threshold — the  two 
which  remain  to  be  noticed  deal  with  larger,  supraliminal  differences. 

(4)  The  " method  of  mean  gradations"  uses  three  stimuli,  two 
of  which  are  fixed,  while  the  third  is  to  be  so  adjusted  as  to  seem 
midway  between  the  other  two.     The  point  is  to  see  whether  the 
stimulus  which  gives  this  "middle  sensation"  is  the  arithmetical 
mean  between  the  extreme  stimuli,  or  the  geometrical  mean,  or 
some  other  proportion.     Various  similar  problems,  involving  the 
relations  of  distant  members  of  the  intensity  series,  can  be  ap- 
proached in  the  same  way. 

1  It  would  seem  that  the  "  method  of  right  and  wrong  cases "  can  scarcely  be 
freed  from  certain  inherent  difficulties;  the  chief  of  which  concern,  first,  the  proper 
treatment  of  those  cases  in  which  the  judgment  is  undecided;  and  second,  the 
relation  of  the  values  found  by  this  method  to  those  found  by  the  method  of 
the  least  noticeable  difference.  Neither  of  the  solutions  hitherto  proposed  for 
these  difficulties  seerns  altogether  satisfactory.  For  a  discussion  of  the  relations 
of  the  two  methods  of  measurement  from  the  point  of  view  of  their  comparative 
value,  the  student  of  the  subject  may  refer  to  Fullerton  and  Cattell,  On  the 
Perception  of  Small  Differences,  pp.  10,  35  (Philadelphia,  1892). 


360  THE  QUANTITY  OF  SENSATIONS 

(5)  What  may  be  called  the  "discrimination  time  method"  is 
of  recent  introduction.1  It  consists  in  determining  the  time  re- 
quired to  judge  of  a  difference  between  two  stimuli.  If,  starting 
at  the  threshold,  we  gradually  increase  the  difference  between  the 
stimuli,  requiring  the  observer  to  make  his  judgments  as  promptly 
as  possible,  we  find  the  judgment  rather  slow  near  the  threshold, 
and  more  and  more  prompt  as  the  difference  is  increased.  The 
impulse  to  judge,  or  the  "steering  force"  exerted  by  the  pair  of 
stimuli,  goes  on  increasing  far  beyond  the  threshold;  and  this  is  a 
fact  of  some  theoretic  moment,  as  showing  the  artificiality  of  the 
threshold.  Now  differences  that  are  judged  rightly  and  in  equal 
times  may  be  called  subjectively  equal;  they  are  equal  in  their  ef- 
fect on  perception,  or  equally  perceptible — this  is  the  basis  of  the 
method. 

"""§  9.  The  problem  to  which  these  methods  have  chiefly  been  ap- 
'  plied  is  that  of  determining  equally  perceptible,  or  equal-appearing 
differences,  at  all  parts  of  the  scale  of  intensities.  The  commence- 
ment of  this  study,  which  soon  took  on  the  proportions  of  one  of 
the  most  important  topics  of  experimental  psychology,  dates  from 
about  1830,  and  is  due  to  the  physiologist  E.  H.  Weber.2  Desiring 
to  determine  the  power  of  the  skin  and  of  the  muscle-sense  to  dis- 
criminate weights,  Weber  devised  the  method  of  least  noticeable 
differences,  and  found  that,  when  poised  in  the  hand,  a  weight  of 
30  ounces  could  just  be  distinguished  from  one  of  29  ounces;  but 
that  if  he  started  with  30  drams  instead  of  ounces,  the  least  noticea- 
ble difference  was  one  dram.  It  seemed  that  the  least  noticeable 
difference  was  not  itself  a  fixed  quantity,  but  varied  with  the  stimu- 
lus, being  always  a  certain  fixed  fraction  of  the  stimulus.  Weber 
found,  however,  that  the  fraction  which  gave  the  least  noticeable 
difference  was  not  the  same  for  all  the  senses :  it  was  larger,  for  ex- 
ample, in  case  of  pressure  on  the  skin  than  in  case  of  the  lifting 
of  weights;  but  in  the  case  of  judging  the  length  of  lines  it  was  much 
smaller,  or  only  about  ifo — i.  e.,  a  line  of  100  units  could  barely 
be  distinguished  from  a  line  of  101  units.  Weber  therefore  con- 
cluded, on  generalizing  from  his  results,  that  the  least  noticeable, 
difference,  in  each  kind  of  sense-perception,  is  a  constant  fraction 
of  the  stimulus. 

Some  years  later,  Fechner  became  acquainted  with  this  general- 

1  Henmon's  The  Time  of  Perception  as  a  Measure  of  Differences  in  Sensations 
(New  York,  1906)  is  the  most  important  study  made  by  the  use  of  this  method, 
and  contains  a  history  of  the  method. 

2  An  account  of  Weber's  experiments  can  be  found  reprinted  in  his  collected 
works  (1851)  and  also  in  his  article  in  Wagner's  Handworterbuch  der  Physiologic, 
1846,  III,  ii,  481. 


WEBER'S  LAW  AND  FECHNER'S  FORMULAS        361 

ization,  and  being  greatly  impressed  with  its  importance,  he  named 
it  Weber's  law,  and  made  it  the  corner-stone  of  his  Psychophysics^f 
Fechner  tested  this  law  by  extensive  series  of  experiments,  whicTThe 
conducted  according  to  several  of  the  above-mentioned  methods. 
And  he  further  sought  to  utilize  it  as  a  means  for  attaining  to  a  scien- 
tific measure  of  all  our  sensory  experience.  To  accomplish  this, 
he  had  to  make  one  or  two  assumptions.  He  must,  of  course,  as- 
sume that  every  sensation  is  a  measurable  quantity,  and  this  im- 
plies that  sensations  can  be  subdivided,  added  and  subtracted;  and 
he  further  assumed  that  the  least  noticeable  difference  corresponds 
to  a  genuine  unit  of  sensation,  and  that  all  least  noticeable  differ- 
ences, within  the  same  kind  of  sense-perception,  represent  equal 
steps  or  units  of  sensation.  With  these  assumptions,  Fechner  could 
express  Weber's  law  in  either  of  the  two  following  ways:  (1)  The 
addition  of  equal  units  of  sensation  is  accomplished  by  the  successive 
multiplication  of  the  stimulus  by  a  constant  fraction;  or  (2)  to  make  I 
sensation  increase  in  arithmetical  progression,  the  stimulus  must  \ 
increase  in  geometrical  progression.  Now  when  two  quantities  are 
related,  in  this  way,  the  one  increasing  in  arithmetical  progression 
while  the  other  increases  in  geometrical  progression,  the  former  is 
proportional  to  the  logarithm  of  the  latter;  and  thus  Fechner  reached 
the  most  compact  form  of  his  modification  of  Weber's  law,  which 
reads  that  sensation  is  proportional  to  the  logarithm  of  the  stimulus.1  \ 
This  modification  is  called  Fechner's  law;  and  it  is  to  be  noted  that, 
as  it  involves  certain  assumptions  from  which  Weber's  law  is  free, 
the  latter  is  the  more  empirical  expression  of  the  facts,  so  far  as, 
indeed,  the  facts  are  found  to  agree  with  it. 

§  10.  That  Weber's  law  is  at  least  an  approximation  to  the  truth 
is  borne  out  by  our  most  common  experiences.     These  evince  the^ 
simple  fact  that  an  amount  of  difference  which  is  easily  percepti-  '• 

1  For  the  detailed  mathematical  discussion  and  expression  of  Weber's  law  the 
reader  is  referred  to  the  technical  works,  especially  of  Fechner  and  G.  E.  Miiller. 
A  simple  statement  of  Fechner 's  principle  may  be  given  as  follows:  Let  £T=the 
^intensity  of  the  light  of  one-half  of  a  white  field;  jf^the  smallest  fraction  of 
stimulus  added  to  H  that  will  produce  an  observable  increase  in  this  intensity; 
and  H  +  ff¥  =  the  intensity  of  the  other  half  of  the  same  field.  Then  let  S  = 
the  sensation  produced  by  H;  £  +  s  =  the  sensation  produced  by  #  +  T|^;  and  s 
will,  of  course,  represent  the  so-called  least  observable  difference  at  this  point  in 
the  scale.  We  have,  then,  H  produces  S;  #  +  Tf¥,  or  Hi  H,  produces  S+s; 
Hi  ff +  ijf  J?,  or  Hi-Hi  H,  produces  £  +  s  +  s;  and  so  on.  That  is  to  say,  if 
s  is  to  be  kept  of  the  same  magnitude,  then  H  must  be  multiplied  by  the  same 
magnitude  (Hi). 

The  three  fundamental  formulas  which  Fechner  has  employed  to  state  and 
demonstrate  the  law  are  the  following:  Let  S  be  the  magnitude  of  the  sensa- 
tion caused  by  the  stimulus  S,  and  AS  a  just  observable  increase  in  this  sen- 


362  THE  QUANTITY  OF  SENSATIONS 

'ble  when  the  stimuli  are  weak  becomes  imperceptible  when  the 
stimuli  are  strong.  If,  for  example,  when  a  single  electric  bulb 
is  burning,  one  more  is  turned  on,  the  increase  in  illumination  is 
very  marked;  but  when  a  hundred  are  burning,  the  addition  of  one 
more  is  barely  perceptible;  and  when  a  thousand  are  burning  the 
addition  of  one  more  is  quite  unnoticed.  To  hear  the  pin  drop, 
the  room  must  be  very  quiet.  But  such  e  very-day  observations  are 
not  a  sufficient  test  of  the  law,  which  states  something  definite  and 
exact — namely,  that  the  least  perceptible  difference  is  a  constant 
fraction  of  the  stimulus;  or,  when  somewhat  generalized,  that  equally 
perceptible  differences  require  the  stimuli  to  be  in  a  constant  ratio. 
Moreover,  while  the  empirical  data  upon  which  the  advocates  of 
Weber's  law  rely  are  very  numerous,  their  value  and  trustworthi- 
ness are  often  much  diminished  by  the  fact  that  many  of  these  ex- 
perimenters have  failed  to  isolate  sufficiently  the  exact  problem  which 
it  was  desired  to  solve.  Nevertheless,  the  data  show  that  the  law 
summarizes  many  facts  reasonably  well  within  a  certain  range  of 
sensations  lying  near  the  middle  of  the  scale  of  quantity.  The  same 
general  fact  of  experience,  which  this  law  attempts  to  summarize, 
holds,  roughly,  of  our  perception  of  magnitudes  of  space  and  time, 
as  well  as  of  our  estimate  of  intensities  of  sensation.  Near  both  the 
upper  and  the  lower  limits  the  law  fails  to  prove  applicable;  even  in 
the  regions  and  under  the  circumstances  which  are  most  favorable 
it  is  only  approximately  true.  Many  fluctuations  of  unknown  sig- 
nificance and  origin  occur  in  all  the  senses. 

§  11.  In  the  following  brief  summary  of  the  empirical  results 
of  a  study  of  the  intensity  of  sensation,  we  shall  group  together 
under  each  sense  the  data  on  the  two  chief  problems: — that  of  the 
least  observable  stimulus;  and  that  of  the  least  observable  difference 
between  two  stimuli.  Weber's  law  is  concerned  only  with  the  latter; 

sation  which  is  caused  by  an  increase  of  the  stimulus  =  AS.     Let  C  be  a  con- 

CA2 

stant  dependent  on  the  units  chosen  for  S  and  S.  Then  AS=—^-.  Let  it  be 
further  assumed  that  A5  remains  constant  whatever  values  for  S  and  AS  are  as- 
sumed; then  dS  =  C^,  and  by  integration  S  =  C  log.  S,  which  is  Fechner's  "funda- 
mental formula."  But  if  the  stimulus  is  just  below  the  least  observable  amount, 
and  be  =S°,  then  substituting  in  the  above  formula  we  have  0  =  C  log  S°; 
from  which  Fechner  derives  formula  No.  2  (the  formula  of  measurement) — 

namely,  S  =  C  log.  J,  which  means  that  the  magnitude  of  the  sensation  is 
"  negative, "  in  case  the  stimulus  sinks  below  the  least  observable.  If  two  sensations 
(S  and  S')  are  observably  different,  then  S—S'  =C  (log.  S— log.  S')  =  (7  log  §/; 
this  is  called  the  "formula  of  difference,"  and  means  that  the  difference  in  the 
intensity  of  two  sensations  is  proportional  to  the  logarithm  of  the  quotient  of  the 
magnitudes  of  their  stimuli. 


THE  PERCEPTION  OF  WEIGHT 


363 


and  wherever  this  law  is  empirically  true,  it  is  possible  to  express  the 
result,  in  regard  to  the  least  observable  difference,  or  the  discrimi- 
native sensibility  of  the  sense  in  question,  in  the  form  of  a  single 
fraction.  It  should  be  noted,  however,  that  a  single  sense  may 
furnish  more  than  one  intensity  series,  and  that  Weber's  law  must 
accordingly  be  tested  in  each  such  series  separately. 

§  12.  We  begin  where  Weber  began,  with  the  perception  of  weight. 
Weight  is  appreciated  partly  by  pressure  on  the  skin  and  subcu- 
taneous pressure  organs,  and  partly  by  the  muscular  sense,  as  in 
lifting  and  poising  the  weight.  One  of  Weber's  results  was  to  the 
effect  that  a  smaller  difference  between  weights  can  be  detected 
when  they  are  lifted  than  when  they  are  simply  allowed  to  press  on 
the  skin,  and  this  result  is  now  generally  accepted.  Weber  also 
decided  that  the  discriminative  sensitivity,  for  lifted  weights,  was 
expressible  as  a  constant  fraction,  about  -fa,  of  the  total  weight. 
His  experiments  were,  however,  confined  to  a  narrow  range  of  weight. 

Biedermann  and  Lowit,  by  the  method  of  just  observable  differ- 
ences, obtained  results  departing  widely  from  Weber's  law.1  By 
experimenting  with  weights  varying  from  10  to  500  grams  they 
found  that  the  sensitiveness  to  pressure  rose  with  the  increase  of 
the  weights  from  10  to  400  grams,  and  then  fell  off  rapidly,  as  the 
following  table  will  show: 


Absolute  weight 
Grams 

Least  observable  difference 
Grams 

Quotient  of  sensitiveness 

10 

0.7 

iV 

50 

1.7 

*v 

100 

2.4 

& 

200 

3.6 

A 

300 

4.6 

* 

400 

5.2 

lv 

450 

6.5 

A 

500 

25.5 

& 

The  trustworthiness  of  these  results  is  impaired,  however,  by  the 
fact  that  no  method,  except  the  doubtful  one  of  directing  "atten- 
tion" exclusively  to  the  sensations  of  pressure,  was  employed  to 
exclude  the  disturbing  effect  of  the  muscular  sensations.  The 
same  observers  concluded,  also,  that  the  fineness  of  the  muscular 
sense,  when  isolated,  does  not  vary  according  to  Weber's  law.  They 
fixed  it  at  -fa  for  weights  of  250  grams,  YTT  f°r  weights  of  2,500 
grams,  -gV  for  weights  of  2,750  grams. 

1  See  Hering,  Sitzgsber.  d.  Wiener  Acad.,  LXXII,  iii,  342  f. 


364  THE  QUANTITY  OF  SENSATIONS 

The  absolute  sensitiveness  of  the  complex  sensations  dealt  with 
in  the  above  experiments  differs  greatly  for  different  localities  on  the 
surface  of  the  body.  This  kind  of  sensitiveness  was  at  one  time 
thought  to  be  chiefly  dependent  upon  the  number  of  the  nervous 
elements  present  in  the  skin,  its  thickness,  the  character  of  its 
tension  over  the  underlying  parts,  etc.;  but  its  variations  are  by 
no  means  parallel  with  those  of  the  sharpness  of  the  sense  of  lo- 
cality. The  foregoing  and  similar  conclusions  all  need  to  be  re- 
vised in  the  light  of  Goldscheider's  determinations  of  the  pressure- 
spots. 

Several  other  authors  have  tested  Weber's  law  in  the  perception 
of  lifted  weights,  and  have  regularly  found  the  least  perceptible 
fraction  of  the  stimulus  to  decrease  as  the  total  weight  increased. 
The  least  perceptible  difference  of  weight  does,  indeed,  increase  in 
absolute  magnitude  as  the  total  weight  increases,  but  it  increases 
more  slowly  than  the  weight,  contrary  to  Weber's  law.  The  same 
is  true  of  other  forms  of  perception  depending  on  the  muscle-sense, 
such  as  the  extent  or  force  of  a  movement.  In  all  these  varieties 
of  muscle-sense  perception,  the  least  noticeable  difference  increases 
more  slowly  than  the  total  magnitude;  the  fraction  which  should 
remain  constant  grows  smaller  as  the  magnitude  increases.  It  is 
rather  disappointing  to  find  that  the  sphere  of  perception  which 
first  suggested  the  formulation  of  Weber's  law  should  prove  to  be 
the  sphere,  of  all  others,  to  which  it  is  least  applicable. 

§  13.  Let  us  now  turn  to  the  determinations  of  the  least  observa- 
ble stimulus  in  the  realm  of  the  muscle-sense.  Such  determina- 
tions have  been  chiefly  carried  out  by  Goldscheider,1  who  has  meas- 
ured the  least  perceptible  extent  of  movement  at  various  joints. 
This  varies  greatly  for  different  joints.  The  most  sensitive  joints, 
i.  e.,  those  at  which  the  least  angular  rotation  is  perceived,  are  the 
wrist,  shoulder,  and  the  joint  between  the  hand  and  the  finger;  at 
these  joints  a  rotation  of  .30  to  .40°  can  be  perceived.  Less  sensi- 
tive are  the  elbow,  hip,  and  knee,  with  thresholds  of  .40  to  .80°;  still 
less  sensitive  are  the  ankle  and  the  two  joints  within  each  finger 
(.75-1.50°);  and  least  sensitive  of  all  are  the  joints  within  the  foot 
(2-4°).  The  measure  of  sensitivity  varies  with  the  speed  of  the 
impressed  movement,  the  joints  being  more  sensitive  for  rapid  than 
for  slow  movements.  For  active  movements  (that  is,  movements 
made  voluntarily  by  the  observer's  own  muscular  action),  the  sen- 
sitivity is  perhaps  slightly  greater  than  for  passive  or  impressed 
movements;  but  the  difference,  if  any,  is  very  small. 

§  14.  If  we  next  turn  to  the  skin  and  subcutaneous  pressure 
organs,  we  easily  observe  that  the  absolute  sensitiveness  of  different 
1  Gesammelte  Abhandlungen,  II. 


SENSITIVENESS  TO  LIGHT  PRESSURE  365 

parts  differs  greatly.  Aubert  and  Kammler  found  the  lightest 
weight  which  produced  a  sensation  of  touch  to  be  0.002  gram  on 
the  forehead,  temples,  and  dorsal  side  of  the  forearm  and  hands; 
0.003  gram  for  the  volar  side  of  the  forearm;  0.005  gram  for  the 
nose,  lips,  chin,  eyelids,  and  skin  of  abdomen;  0.005-0.015  gram 
for  the  volar  side  of  the  fingers;  and  1  gram  for  the  finger-nails  and 
skin  of  the  heel.  Later  observations  have  to  some  extent  varied 
these  results;  and  it  has  come  to  be  recognized  that  the  most  sig- 
nificant experiments  are  those  which  determine  the  least  percepti- 
ble pressure  on  single  touch-spots.  Von  Frey1  concludes  that  in- 
dividual touch-spots  differ  comparatively  little  from  each  other  in 
sensitiveness,  the  threshold  ranging  from  .5  to  4  grams  pressure 
per  square  millimetre  of  the  surface  pressed  on.  Kiesow2  has  de- 
termined the  sensitivity  of  touch-spots  in  various  regions  of  the  trunk 
and  limbs,  and  finds  it  to  vary  little  within  the  limbs,  but  to  be  com- 
paratively blunt  on  the  chest  and  back.  The  presence  of  hairs 
or  hairlets  greatly  increases  this  kind  of  sensitivity. 

The  sensitivity  to  differences  of  pressure  also  varies  from  one 
area  of  skin  to  another,  being  greatest  on  the  face,  where  the  least 
noticeable  difference  may  be  as  low  as  ^  of  the  total  pressure, 
whereas  on  the  leg  the  fraction  rises  above  yV3  Probably  the 
most  thorough  test  of  Weber's  law  in  this  field  has  been  made  by 
Stratton,4  who  found  that  the  fraction  expressing  the  least  observa- 
ble increase  in  an  existing  pressure  was  large  for  such  small  press- 
ures as  10  grams,  and  decreased  considerably  up  to  75  grams,  but 
from  here  on  remained  fairly  constant  up  to  200  grams ;  accordingly, 
Weber's  law  held  good  for  a  medium  range  of  stimuli  but  not  for 
weak  stimuli.  The  skin,  as  well  as  the  joint  sense,  is  more  sensi- 
tive to  rapid  than  to  slow  changes. 

§  15.  Extraordinary  difficulties  accompany  the  attempt  to  apply 
Weber's  law  to  sensations  of  temperature.  As  has  already  been  seen 
(compare  p.  346),  we  do  not  know  exactly  what  to  measure — whether 
the  rising  and  falling  of  the  thermic  apparatus,  or  its  actual  tempera- 
ture in  relation  to  its  own  zero-point — as  constituting  the  quanti- 
tative changes  in  the  stimuli.  Even  Fechner  admits  that  Weber's 
law  does  not  apply  to  the  sensitiveness  of  the  hand  to  changes  in 
temperature  when  it  is  itself  cooling  off;  but  he  thinks  the  law 
holds  good  approximately  for  degrees  of  warmth  varying  between 
25°  and  37.5°  C.  (77°-99.5°  Fahr.),  if  18.71°  C.  (65.66°  Fahr.)  be 

1  Pfliiger's  Archiv  f.  d.  gesammte  PhysioL,  1900,  LXXXII,  399. 

2  Wundt's  Philosophised  Studien,  1902,  XIX,  307. 

3  Eulenberg,  Zeitschrift  /.  rationelle  Medecin,  1861,  X,  cited  after  Sherrington 
in  Schafer's  Textbook  of  Physiology,  1900,  II,  929. 

4  Wimdt's  Philosophische  Studien,  1896,  XII,  525, 


366  THE  QUANTITY  OF  SENSATIONS 

taken  as  -the  zero-point.  The  assumption  of  this  zero-point  is, 
however,  arbitrary.  No  general  rule  for  the  quantity  of  sensations 
of  temperature  can  well  be  given  except  this:  the  skin  is  most  sensi- 
tive to  changes  which  lie  near  its  own  zero-point.  In  comparing 
two  temperatures  it  is  most  favorable  to  nice  discrimination  that 
one  should  lie  slightly  above,  the  other  slightly  below,  this  point. 
The  degrees  of  the  thermometer  between  which  the  maximum  of 
sensitiveness  is  attainable  are  given  differently  by  different  ob- 
servers: By  Nothnagel,  27°-33°  C.  (80.6°-91.4°  Fahr.);  by  Linde- 
mann,  26°-39°  C.,  by  Alsberg,  35°-39°  C.;  by  Fechner,  12°-25° 
C. — where  it  is  so  great  as  not  to  be  easily  measurable  by  a  good 
quicksilver  thermometer  (about  -fa0  Fahr.).  Cold  and  heat  alike, 
when  applied  for  some  time,  reduce  greatly  the  sensitiveness  of 
the  skin  to  minute  changes  of  temperature;  by  heat  it  can  be  so 
dulled  as  not  to  distinguish  alterations  of  less  than  4°  or  J°  Fahr.; 
by  cold  it  can  be  rendered  insensible  to  changes  measuring  from 
2°  to  5J°. 

Another  complicating  circumstance  is  that  the  sense  of  tempera- 
ture depends  for  its  fineness  upon  the  extent  and  locality  of  the  sur- 
face excited.  Weber  found  that  water  at  29^°  R.,  in  which  the 
whole  hand  was  immersed,  seemed  warmer  than  that  at  32°  R.,  to  a 
single  finger.  Nothnagel  placed  the  following  values  upon  the  fine- 
ness of  discrimination,  for  minute  variations  in  temperature,  of  dif- 
ferent parts  of  the  body:  Middle  breast,  0.6°  C.;  sides  of  the  same, 
0.4°;  middle  of  the  back,  1.2°;  sides  of  the  same,  0.9°;  hollow  of  the 
hand,  0.5°-0.4°;  back  of  the  same,  0.3°;  parts  of  upper  and  lower 
arm,  0.2°;  cheeks,  0.4°-0.2°;  temples,  0.4°-0.3°. 

More  recent  investigations  have  shown  that  the  table  of  sensi- 
tiveness for  the  different  parts  of  the  body  must  take  account  of 
the  division  of  the  temperature-sense  into  two  species,  and  of  the 
locality  of  the  warmth-spots  and  cold-spots  in  all  such  different  parts. 
On  the  basis  of  experiment  with  areas  of  the  skin  whose  topography, 
with  respect  to  the  temperature-sense  had  previously  been  investi- 
gated, Goldscheider  has  given  a  lengthy  statement1  of  the  sensi- 
tiveness of  different  parts  of  the  body.  Thus  he  finds  that  the  skin 
of  the  head  is,  in  general,  little  developed  for  the  sense  of  cold,  and 
only  in  a  few  places  for  the  sense  of  heat.  The  sensitiveness  of  the 
forehead  to  cold  is  intense,  but  to  heat  only  moderate;  that  of  the 
breast  to  cold  moderate  along  the  sternum,  and  elsewhere  very  in- 
tense, while  to  heat  it  is  only  moderate  except  near  the  nipples; 
that  of  the  back  everywhere  very  intense  to  cold,  and  only  moderate 
to  heat;  while  in  all  parts  of  the  hand  the  intensity  of  sensitiveness 

1  See  the  Archiv  /.  Anat.  u.  PhysioL,  Pysiolog.  Abth.,  1885,  Supplement-Band, 
pp.  60  ff. 


SENSITIVENESS  OF  ACOUSTIC  PERCEPTION       367 

to  both  cold  and  heat  is  alike.  In  general,  the  skin  in  the  median 
line  of  the  body  seems  much  less  sensitive  to  changes  in  temperature 
than  at  its  sides;  and  the  number  of  thermic  elements  (according 
to  Goldscheider,  the  distributory  fibrils  of  the  temperature-nerves), 
the  thickness  of  the  skin,  etc.,  are  determining  factors. 

§  16.  The  possibility  of  executing  or  appreciating  a  musical 
passage  in  which  the  intensity  of  the  successive  tones  is  brought 
to  a  certain  standard  of  memory,  or  in  which  these  tones  are  nicely 
shaded  so  as  to  constitute  a  crescendo  or  a  diminuendo,  appears  to 
depend  upon  applying  to  sensations  of  sound  some  law  resembling 
that  of  Weber.  It  is  partly  by  comparing  such  sensations  with 
their  images  in  memory  that  the  singer  or  player  reproduces  certain 
notes  previously  executed,  with  about  the  same  stress  of  tone. 
Moreover,  in  order  to  shade  the  relative  intensities  of  successive 
tones,  our  appreciation  of  their  differences  needs  to  be  much  greater 
for  those  that  have  a  low  degree  of  intensity.  Many  obstacles, 
however,  stand  in  the  way  of  determining  either  the  lower  limit 
or  the  least  observable  difference  for  sensations  of  sound.  The 
greatest  difficulty  is  of  a  physical  nature — namely,  to  obtain  a  source 
of  sound  which  shall  be  free  from  disturbing  variations  in  pitch 
and  clang,  both  of  which  greatly  affect  the  apparent  intensity  of 
sound;  and  especially  to  secure  a  physical  measure  of  the  intensity 
of  the  stimulus.  If  sound  is  generated,  as  has  frequently  been  the 
case  in  tests  of  Weber's  law,  by  dropping  a  metal  or  hard  rubber  ball 
upon  a  block  of  wood  or  slate,  it  is  easy  to  calculate  the  energy  of 
impact  from  the  weight  of  the  ball  and  the  height  of  its  fall.  But 
the  real  stimulus  is  not  this  impact,  but  the  vibrations  of  air  which 
strike  the  tympanic  membrane;  for  a  full  determination  of  the  stimu- 
lus, we  should  therefore  need  to  know  how  much  of  the  energy  of 
impact  went  into  the  production  of  vibrations  of  the  air,  and  how  the 
amplitude  of  these  vibrations  decreased  in  its  passage  to  the  ear. 
In  regard  to  the  latter  point,  the  law  of  inverse  squares  would  prob- 
ably hold  in  a  space  that  was  perfectly  open,  and  so  free  from  all 
reflection;  but  in  a  closed  room  reflection  and  resonance  make  it 
impossible  to  deduce  the  intensity  at  the  ear  from  the  distance  be- 
tween the  ear  and  the  sounding  body.  Comparative  tests  may,  how- 
ever, be  made  by  keeping  both  the  apparatus  and  the  observer  at 
the  same  places  in  the  same  room  throughout.  In  regard  to  the 
question  as  to  how  the  intensity  of  the  sound  varies  with  the  height 
of  the  fall,  there  has  been  much  difference  of  opinion,  but  perhaps  it 
is  safe  to  assume  that,  for  a  given  piece  of  apparatus,  the  intensity 
varies  as  the  height  of  the  fall  (and  therefore  as  the  energy  of  im- 
pact).1 If  this  is  granted,  the  least  perceptible  difference  in  inten- 
1  See  Kampfe,  Philosophische  Studien,  1893,  VIII,  526. 


368  THE  QUANTITY  OF  SENSATIONS 

sity  of  sound  (noise)  as  produced  by  this  apparatus,  is  about  £; 
i.  e.,  the  two  noises  can  just  be  distinguished  when  the  heights  of 
fall  are  as  3  to  4.     This  ratio  remains  fairly  constant  within  a  mod-  / 
erate  range  of  intensity,  in  confirmation  of  Weber's  law. 

§  17.  Such  an  experiment  as  the  above,  though  it  does  not  meas- 
ure the  real  stimulus  at  the  tympanic  membrane,  is  satisfactory  for 
the  testing  of  Weber's  law;  but  not  for  determining  the  least  stimu- 
lus which  can  be  perceived.  Through  the  work  of  several  authors, 
among  whom  may  be  mentioned  Lord  Rayleigh  and  Max  Wien, 
an  approximation  to  the  desired  measure  can  now  be  given.  The 
sound  given  out  by  a  telephone  receiver  can  be  calculated  if  the  ex- 
cursions of  the  telephone  plate  are  known ;  and  these  excursions  can 
be  measured  by  direct  microscopic  examination.  If  now  a  tele- 
phone receiver  be  held  to  the  ear,  and  actuated  by  an  electric  current 
which  causes  the  plate  to  vibrate  with  a  known  amplitude,  as  well 
as  a  known  frequency,  the  amplitude  of  the  vibrations  entering  the 
ear  is  known,  and  it  can  be  assumed  that  this  does  not  change  much 
in  the  passage  to  the  tympanic  membrane.  By  such  means,  Wien1 
has  found  that  a  tone  is  barely  audible  when  the  energy  of  the  at- 
mospheric vibrations  is  from  .00032  to  .000,000,000,002,5  ergs  (per 
second,  over  an  area  of  one  square  centimetre).  Now  the  erg  is 
itself  a  small  unit;  and  since  the  air  vibrations  need  not  act  for  a  whole 
second,  a  small  fraction  of  a  second  being  long  enough,  the  above 
figures  would  need  to  be  considerably  reduced  in  order  to  express 
the  minimum  stimulus  which  will  produce  the  sensation  of  sound. 
Other  authorities,  it  should  be  said,  have  not  assigned  values  as 
small  as  the  smallest  of  Wien,  but  they  all  lie  within  the  range  of 
those  cited  from  this  author. 

Individuals  differ  greatly  in  absolute  sensitivity;  Bruner2  found 
the  energy  of  the  least  noticeable  stimulus  to  vary  in  the  ratio  of 
1  to  400,  among  normally  hearing  individuals  of  the  white  race. 
The  sensitivity  to  sound  varies  also,  and  greatly,  with  the  pitch. 
The  largest  of  the  numbers,  quoted  above  from  Wien,  is  the  energy 
required  for  a  low  tone  (50  vibrations  per  second);  the  smallest  is 
for  tones  of  1,600  and  3,200  vibrations.  The  ear  is,  accordingly, 
more  than  100,000,000  times  as  sensitive  to  the  high  tones  as  to 
the  low  ones;  but  the  threshold  is  nearly  constant  in  the  interval 
from  800  to  6,400  vibrations.  It  increases  rapidly  above  and  below 
this  range.  It  will  be  seen  that  the  ear  is  most  sensitive  to  decidedly 
high  tones  (from  "high  G,"  or  g2,  up  for  about  three  octaves).  This 
may  appear  strange  in  view  of  the  fact  that  these  high  notes  lie 
beyond  the  ordinary  use  of  the  human  voice;  but  the  importance  of 

1  Pfliiger's  Archiv  f.  d.  gesammte  Physiol,  1903,  XCVII,  1. 
a  The  Hearing  of  Primitive  Peoples,  p.  95  (New  York,  1908). 


QUANTITATIVE  DISCRIMINATIONS  OF  SIGHT      369 

overtones  in  the  appreciation  of  the  clang,  or  complex  character,  of 
a  sound,  should  be  borne  in  mind  in  this  connection.  The  ability 
to  distinguish  different  noises,  voices,  vocal  sounds,  etc.,  is  as  im- 
portant as  anything  in  hearing,  and  requires  that  the  ear  shall  be 
sensitive  to  high  and  faint  overtones. 

§  18.  Wien  also  employed  the  telephone  method  for  determining 
the  least  noticeable  difference  of  intensity  of  tones.  He  found  the 
fraction  expressing  the  least  noticeable  difference  to  vary  with  the 
pitch  of  the  tone  (-J-  to  -J-  from  a  to  a1);  but  for  each  tone  it  pre- 
serves a  fair  equality  over  a  considerable  middle  range  of  intensity. 
There  is,  therefore,  good  ground  for  concluding  that  Weber's  law 
is  fairly  applicable  to  intensity  of  sound. 

§  19.  Attention  was  early  called  to  the  law  of  judgment  in  esti- 
mating the  quantitative  relations  of  sensations  of  sight,  on  account 
of  its  connection  with  astronomical  observation.  In  the  eighteenth 
century  French  physicists  had  already  begun  to  investigate  the 
sensitiveness  of  the  eye  to  varying  intensities  of  light.  Bouguer,  in 
answer  to  the  question,  What  force  must  a  light  have  in  order  to 
make  a  more  feeble  one  disappear?  placed  the  fraction  of  least 
observable  difference  in  the  intensities  of  two  shadows  at  ^j-.  That 
the  magnitudes  of  the  stars  are  not  to  be  classified  according  to 
their  absolute  brightness  as  determined  by  photometric  observa- 
tions, was,  of  course,  assumed  by  Sir  John  Herschel  when  he  made 
the  latter  vary  in  the  series  1  :  J  :  i  :  tV,  while  the  former  vary  in 
the  series  1:2:3:4.  That  the  least  observable  difference  in  the 
intensity  of  two  sensations  of  sight  is  absolutely  much  smaller  for 
those  of  the  lowest  grade  of  intensity  is  a  truth  needed  to  explain 
many  every-day  experiences.  For  example,  the  finer  gradations  of 
shade  in  a  lithograph  or  photograph  are  not  lost  when  we  take  it 
from  the  open  sunlight  into  a  rather  dimly  lighted  room;  we  can 
also  observe  them  through  smoked  glass,  if  it  be  not  too  black. 
Through  the  same  media  we  can  measure  rather  delicate  shades 
of  brightness  on  the  clouds.  We  observe,  however,  that  in  all 
such  cases  either  too  great  or  too  weak  intensity  of  the  light 
destroys  our  power  to  distinguish  the  finest  gradations  of  its 
intensity. 

§  20.  It  has  already  been  shown  (p.  325)  that  the  retina  is  never 
free  from  light  of  its  own  which  has  a  varying  intensity;  this  fact 
greatly  increases  the  difficulty  of  fixing  accurately  either  the  lower 
limit  or  the  least  observable  difference  of  visual  sensations.  In  the 
effort  to  apply  Weber's  law  to  sensations  of  color,  the  laws  of  change 
in  the  quality  operate  to  obscure  the  laws  of  change  in  the  quantity 
of  the  sensations.  Experiments  with  shadows  for  the  sake  of  test- 
ing Weber's  law  were  first  conducted  by  A.  W.  Volkmann  and  others, 


370  THE  QUANTITY  OF  SENSATIONS 

under  the  direction  of  Fechner.1  By  measuring  the  distance  to 
which  a  candle  must  be  removed  from  an  object  in  order  that  the 
shadow  produced  by  its  light  might  disappear  in  that  of  another 
candle  of  like  intensity  situated  at  a  fixed  near  distance  from  the  ob- 
ject, the  quotient  for  the  least  observable  difference  was  found  to  be 
rJ-fr.  This  quotient  was  also  found  to  remain  nearly  constant  for 
absolute  intensities  varying  from  1  to  38.79.  If,  however,  the  light 
of  the  background  diminished  to  0.36  in  intensity,  marked  varia- 
tions in  the  law  occurred;  the  difference  in  the  brightness  of  the  two 
shadows  had  then  to  be  greater  than  Y^  to  be  observable.  Later 
experiments  of  the  same  observer  yielded  results  less  favorable  to 
Weber's  law.2  The  quotient  was  found  to  vary  from  -g-J.^  for  weak 
intensities  of  light  to  T^  for  stronger  intensities. 

By  using  rotating  disks  and  comparing  the  grayish  circles  made 
upon  them  when  revolving  rapidly,  through  the  admixture  of  small 
black  stripes  with  the  white  of  their  surfaces  ("Masson's  Disks"), 
Helmholtz3  found  the  medium  value  of  the  quotient  of  least  observ- 
able difference  to  be  TJ-j;  this  quotient  is  not  constant,  however,  and 
increases,  especially  for  sensations  near  the  upper  or  the  lower  limit. 
By  changing  the  method  somewhat,  Aubert  obtained  a  variation 
°f  TST  to  T-Jhr  in  the  degree  of  sensitiveness  to  differences  in  the 
brightness  of  lights,  even  when  not  going  above  the  middle  of  the 
scale  of  intensity.  Experiments  with  such  intensities  as  lie  nearest 
the  limits  showed  much  greater  departures  from  Weber's  law.  Just 
above  the  lower  limit,  an  addition  of  even  J  to  J  to  the  stimulus 
might  be  necessary  in  order  to  produce  an  observable  difference 
in  the  resulting  sensation.  Similar  results  have  been  obtained  by 
Delboeuf,  but,  on  the  whole,  more  favorable  to  Weber's  law  than 
the  results  of  Aubert. 

§  21.  Of  the  many  experiments  which  have  been  undertaken 
with  the  object  of  testing  Weber's  law  in  the  comparison  of  intensi- 
ties of  light,  those  of  Konig  and  Brudhun4  apparently  cover  the 
greatest  range  of  intensities  and  so  afford  the  most  complete  test. 
These  experimenters  used  intensities  varying  in  the  proportion  of 
1  to  50,000,000,  and  found  the  least  perceptible  difference  to  be  a 
nearly  constant  fraction  (.017  to  .018  of  the  total  intensity)  over  a 
considerable  range  of  medium  intensities.  But  the  fraction  became 
considerably  larger  for  very  feeble,  or  for  very  intense,  light  (.036 
for  the  most  intense  employed,  and  .695  for  the  weakest).  We 
may  accordingly  regard  Weber's  law  as  verified  for  the  middle  range 

1  See  Elemente  d.  Psychophysik,  pp.  148  f. 

2  A.  W.  Volkmann,  Physiolog.  Untersuchungen,  I,  pp.  56  f . 

3  Physiologische  Optik,  pp.  315  f. 

4  Sitzungsberichte  der  Berliner  Akad.  d.  Wissensch.,  1888  and  1889. 


MINIMUM  SENSATIONS  OF  LIGHT  371 

of  intensity  of  light.  The  fraction  which  expresses  the  least  per- 
ceptible difference  varies  greatly,  however,  with  the  special  condi- 
tions of  observation ;  it  is  smallest  when  the  two  lights  to  be  compared 
appear  side  by  side  with  no  intervening  space,  so  that  the  effect 
of  contrast  helps  discrimination.  When  the  lights  are  separated, 
comparison  is  much  less  precise,  and  if  one  light  is  seen  a  second 
after  the  other  has  been  removed 1  the  fraction  may  rise  as  high  as  J. 

Older  experiments  seemed  to  show  that  the  sensitivity  of  the 
eye  to  differences  in  intensity  varies  according  to  the  color  of  the 
light;  but  the  authors  just  quoted  found  that  this  was  not  the  case, 
within  the  same  range  of  brightness.  The  comparison  of  the  bright- 
ness of  one  color  with  that  of  a  different  color  is,  however,  difficult 
and  uncertain. 

§  22.  The  minimum  of  the  intensity  of  light  appreciable  by  the 
eye  is  dependent  on  many  circumstances,  of  which  the  most  im- 
portant is  the  condition  of  adaptation  to  light  or  dark  (see  p.  195). 
The  color  or  wave-length  also  makes  a  great  difference,  as  also  the 
part  of  the  retina  stimulated,  and  the  extent  and  duration  of  the 
stimulus.  The  eye  is  much  more  sensitive  to  rays  from  the  middle 
of  the  spectrum,  especially  yellow  and  green,  than  to  rays  from  near 
the  red  or  the  violet  end.  The  central  area  of  clear  vision  of  the 
retina  is  less  sensitive  to  very  dim  light  than  is  the  region  lying  just 
about  it;  this  difference  appears  during  dark  adaptation.  Lights 
of  very  small  area  or  duration  will  make  no  impression,  although 
the  same  intensity,  with  greater  extent  or  duration,  would  suffice 
to  give  a  sensation.  S.  P.  Langley2  sought  to  determine  the  energy 
of  the  least  quantity  of  light  which  could,  under  the  most  favorable 
conditions,  produce  a  perceptible  impression,  and  reached  a  figure 
of  .00000003  ergs.  Though  this  is  a  very  small  quantity  of  energy, 
it  is  considerably  greater  than  the  minimum  determined  by  Wien  as 
required  to  excite  an  auditory  sensation. 

§  23.  Weber  applied  his  own  law  to  so-called  extensive  sensa- 
tions of  light.  He  showed  that  in  judging  of  the  comparative 
length  of  lines  the  least  observable  difference  is,  for  each  person,  a 
tolerably  constant  fraction  of  the  absolute  length  of  the  line  with 
which  the  comparison  is  made.  This  fraction  is  different  for  differ- 
ent persons;  and  has  a  range  from  ^  to  T^TF*  Fechner3  defends 
the  validity  of  the  law  for  lines  of  lengths  varying  between  10  and 
240  mm.  (f  to  9-fc  in.),  with  the  eye  removed  from  300  to  800  mm. 
(12-32  in.).  The  lower  limit  for  such  cases  has  been  fixed  by  A.  W. 
Volkmann  at  lines  of  length  from  0.2  to  3.6  mm.  It  is  obvious, 

1  See  Fullerton  and  Cattell,  The  Perception  of  Small  Differences,  1892,  p.  140. 

2  Philosophical  Magazine,  1889,  XXVII,  1. 
1  Elements  d.  Psychophysik,  I,  211  f. 


372  THE  QUANTITY  OF  SENSATIONS 

however,  that  we  are  here  not  dealing  with  pure  quantity  of  visual 
sensations,  but  with  judgments  of  local  relation  which,  in  case  the 
eyes  are  moved,  have  their  basis,  at  least  partly,  in  our  power  to  dis- 
criminate minute  differences  in  the  sensations  of  the  muscular  sense 
connected  with  such  movements. 

§  24.  The  law  of  Weber  can,  of  course,  derive  little  or  no  sup- 
port from  sensations  of  taste  and  smell.  In  the  case  of  these  two 
senses  our  knowledge  of  both  series  of  quantities — of  the  intensity 
of  the  stimulus  and  of  the  amount  of  specific  sensation  which  re- 
sults from  its  application — is  altogether  too  inadequate  to  admit 
of  trustworthy  comparison.  We  cannot  measure  forms  of  energy 
like  those  by  which  smellable  particles  and  tas table  solutions  act 
on  the  end-organs  of  sense,  until  we  have  a  unit  of  measurement  and 
some  information  as  to  what  the  object  is  to  which  the  standard 
should  be  applied.  Nor  can  we  compare  amounts  of  sensations 
that  are  so  largely  matters  of  individual  origin  and  capricious 
change,  and  that  are  so  overlaid  with  other  forms  of  feeling,  as  are 
the  sensations  of  these  senses.  Moreover,  the  element  of  time — both 
as  respects  the  interval  elapsing  between  the  two  sensations  com- 
pared and  also  the  order  in  which  the  sensations  follow  each  other 
— is  here  a  very  important  influence. 

The  intensity  of  taste  depends  upon  a  variety  of  circumstances 
besides  the  objective  quantity  of  the  stimulus.  Among  these  cir- 
cumstances is  the  extent  of  surface  excited.  Camerer1  found  by 
experimenting  with  common  salt  in  solutions  of  different  degrees 
of  concentration  that  the  number  of  correct  guesses  increased  almost 
in  exact  proportion  to  the  number  of  gustatory  papilla?  upon  which 
the  solutions  were  placed.  Certain  mechanical  and  thermic  con- 
ditions also  have  a  great  influence.  Substances  even  in  fluid  form, 
when  quickly  swallowed,  have  little  taste;  pressing  and  rubbing 
against  the  gustatory  organs,  movement  of  the  tastable  matter  in 
the  mouth,  increase  the  excitatory  effect  of  the  stimuli.  It  is 
doubtful  whether  this  effect  is  due  solely  to  the  mechanical  result 
of  spreading  the  stimulus  over  the  surface  and  urging  it  into  the 
pores  against  the  end-organs  of  the  sense,  or  in  part  also  to  some 
direct  physiological  cause.  The  influence  of  temperature  on  the 
intensity  of  sensations  of  taste  is  well  known.  Weber  showed  that 
if  the  tongue  is  held  for  J  to  1  minute  in  very  cold  water,  or  in  wa- 
ter of  about  125°  Fahr.,  the  sweet  taste  of  sugar  can  no  longer  be 
perceived.  Cold  also  destroys  for  a  time  the  susceptibility  to  bit- 
ter tastes.  Keppler2  endeavored  to  test  Weber's  law  by  determining 
the  sensitiveness  to  minute  changes  in  the  four  principal  kinds  of 

1  See  Zeitschr.  /.  Biologie,  1870,  VI,  pp.  440  f. 

2  Pfliiger's  Archiv,  1869,  II,  449  f. 


INTENSITY  OF   GUSTATORY  SENSATIONS          373 

taste;  and  arrived  at  a  negative  result.  Fechner,  however,  con- 
siders that  Keppler's  experiments  with  common  salt  confirm  We- 
ber's law,  and  that  his  other  experiments  were  not  adapted  to  yield 
any  assured  result.  We  can  only  repeat  the  statement  that  other 
causes  than  mere  increase  in  the  quantity  of  the  stimulus  so  largely 
determine  the  intensity  of  the  resulting  sensations  as  to  discredit 
any  arguments  from  the  experiments  either  for  or  against  applying 
Weber's  law  to  sensations  of  taste. 

§  25.  The  experiments  of  Valentin  and  others,  to  determine  how 
weak  solutions  of  various  substances  will  excite  the  end-organs  of 
taste,  are  chiefly  valuable  as  gratifying  our  curiosity.  The  figures 
are  not  to  be  accepted  as  exact,  but  as  showing  in  general  the  ex- 
treme fineness  of  this  sense,  and  the  great  difference  of  different 
substances  in  their  power  to  excite  it.  Valentin  found,  for  exam- 
ple, that  0.24  gram  of  a  solution  containing  1.2  per  cent,  of  cane- 
sugar  excited  the  sensation  of  sweet;  a  solution  containing  -%fa 
part  of  common  salt  was  scarcely  detectable;  of  sulphuric  acid 
TToWfr  could  be  discerned,  loooTnnr  not;  extract  of  aloes  contain- 
ing woVoir  could  be  distinguished  from  distilled  water;  ssooir 
of  sulphate  of  quinine  was  plainly  observable,  and  the  observer 
thought  he  could  detect  a  slight  trace  of  bitter  when  the  solution 
was  diluted  to  1000000  of  this  substance.  In  general,  a  smaller 
absolute  quantity  of  stimulus,  when  in  a  relatively  concentrated 
solution,  will  suffice  to  excite  the  end-organs  of  taste.  It  will 
readily  be  seen  that  the  minimum  of  some  of  these  substances 
which  will  give  rise  to  a  sensation  under  the  most  favorable  cir- 
cumstances is  exceedingly  small. 

§  26.  The  intensity  of  sensations  of  smell  is  also  largely  depend- 
ent on  other  causes  than  changes  in  the  quantity  of  the  stimuli. 
The  amount  of  sensation  appears  to  be  largely  governed  by  the 
extent  of  surface  excited;  since  it  is  greater  when  we  smell  with 
both  nostrils,  and  with  the  current  of  inspiration  which  carries  the 
exciting  particles  over  more  of  the  sensitive  membrane.  No  as- 
sured results  on  this  point,  however,  have  yet  been  reached.  Val- 
entin supposes  that  a  smaller  number  of  odorous  particles  will 
excite  sensation  if  presented  in  a  concentrated  rather  than  a  dilute 
form.  When  the  intensity  of  the  stimulus  increases  beyond  a  cer- 
tain point,  the  character  of  the  resulting  sensation  changes — often- 
times from  a  pleasant  to  an  unpleasant  tone  of  feeling.  All  are 
familiar  with  the  fact  that  a  large  increase  of  some  smells — for 
example,  musk — does  not  give  the  same  kind  of  sensation. 

This  sense  has  a  great  degree  of  "sharpness,"  or  power  to  be  ex- 
cited by  small  quantities  of  stimulus,  as  distinguished  from  "fine- 
ness," or  power  to  distinguish  minute  variations  in  the  sensations. 


374  THE  QUANTITY  OF  SENSATIONS 

It  is  undoubtedly  different  in  different  species  of  animals,  as  de- 
pendent upon  unknown  differences  in  their  psycho-physical  con- 
stitution; but  it  is  tolerably  uniform  among  men  where  there  is  the 
same  cultivation  of  it,  and  the  same  concentration  of  attention.  It 
is  well  known  that  certain  animals  have  an  astonishing  fineness  of 
smell,  and  are  able  by  it  even  to  detect  the  individual  variations 
that  are  quite  imperceptible  to  man.  Little  value  can  be  attached 
to  the  results  reached  by  experiments  to  fix  the  least  quantity  of 
smellable  substances  which  can  excite  the  human  end-organs  of 
this  sense.  In  general,  we  can  say  that  incredibly  small  quanti- 
ties of  some  substances  will  suffice.  Valentin  found  that  a  current 
of  air  containing  -3-0  0*000  of  vapor  of  bromine  excited  a  strong  un- 
pleasant sensation.  Atmosphere  polluted  with  even  1700000  of 
sulphuretted  hydrogen  could  be  detected.  It  was  calculated  by 
this  observer  that  -a  o  o  o  o  o  o  °f  a  milligram  of  alcoholic  extract  of 
musk  is  about  as  little  as  can  be  perceived;  of  mercaptan,  a  still 
smaller  quantity  is  perceptible,  since  the  odor  can  be  detected  when 
a  litre  of  air  contains  only  T8iroVo"oTr  of  a  milligram  of  this  substance ; 
and  it  is  probable  that  only  a  small  part  of  a  litre  would  be  inhaled 
in  snuffing  the  air. 

Zwaardemaker1  has  devised  an  olfactometer,  which,  though  not 
measuring  the  stimulus  in  absolute  units,  permits  of  relative  de- 
terminations, and  has  applied  his  instrument  to  the  testing  of 
Weber's  law.  The  results  were  necessarily  inexact,  but  the  law  was 
believed  to  be  approximately  verified. 

§  27.  A  review  of  the  facts  bearing  on  the  validity  of  Weber's  law 
shows,  first,  that  the  law  is  regularly  departed  from  at  both  the  lower 
and  the  upper  extremes  of  the  scale  of  intensities;  next,  that  it  holds 
very  well  for  an  extensive  middle  range  of  intensities  of  light;  further, 
that  it  seems  to  hold  approximately  for  the  middle  range  of  inten- 
sities of  sound,  smell,  and  pressure  on  the  skin,  though  in  none  of 
these  cases  has  anything  like  the  full  range  of  intensities  been 
brought  under  examination.  In  the  perception  of  lifted  weights, 
and  generally  in  perceptions  dependent  on  the  muscular  sense, 
the  departures  from  the  law,  even  within  the  middle  range,  are  great 
and  all  in  the  same  direction,  so  that  it  is  impossible  to  regard  the 
law  as  holding  for  these  kinds  of  perception. 

§  28.  The  value  of  Weber's  law  is  so  restricted,  even  as  stating 
a  general  fact  of  experience,  that  it  would  seem  scarcely  necessary 
to  discuss  at  length  its  higher  significance.  Three  possible  modes 
of  explanation  have  all  had  their  defenders;  these  are  the  physio- 
logical, the  psycho-physical,  and  the  psychological.  The  first  of 
the  three  assumes  that  the  physical  construction  of  the  nervous 
1  Physiologic  des  Geruchs,  1895. 


EXPLANATIONS   OF  WEBER'S  LAW  375 

system,  including  chiefly  the  end-organs  of  sense  and  their  central 
representatives  and  connections,  is  such  as  to  supply  the  reason 
for  this  relation  between  the  intensity  of  sensations  and  that  of 
their  stimuli.  And  certainly,  if  we  were  to  adopt  off-hand  the  sim- 
plest assumption,  it  would  be  that  the  quantitative  relation  between 
the  last  antecedent  molecular  changes  in  the  brain  and  the  mental 
changes  to  which  they  give  rise,  is  one  of  simple  proportion;  the  more 
work  done  by  means  of  the  excitation  in  the  appropriate  cerebral 
centres,  the  more  of  physical  basis  laid,  as  it  were,  for  a  resulting 
quantity  of  psychical  movement. 

If,  then,  the  sensations  vary  in  quantity  in  an  arithmetical  pro- 
portion, while  their  external  stimuli  vary  in  a  geometrical  propor- 
tion, the  explanation  of  the  fact  must  be  found  somewhere  in  the 
chain  of  events  between  the  external  stimuli  and  the  nerve-commo- 
tions set  up  as  a  result  in  the  appropriate  centres  of  the  brain.  And 
without  doubt,  in  all  of  the  senses,  the  end-organs  profoundly  mod- 
ify the  intensity  of  the  stimulus  they  receive.  In  the  so-called  chem- 
ical senses  (smell,  taste,  sight)  a  profound  quantitative  modifica- 
tion takes  place,  even  before  the  stimulus  reaches  the  fibrils  of  the 
sensory  nerve.  In  the  case  of  the  mechanical  sense  of  hearing  we 
cannot  say  how  much  of  the  effect  stated  in  Weber's  law  may  not 
have  been  gained  even  before  the  acoustic  waves  set  agoing  the 
nervous  elements  of  the  organ  of  Corti.  As  to  profounder  modifi- 
cations in  the  same  direction  by  reason  of  the  interaction  of  different 
nerve-elements  in  the  brain  we  are  yet  more  ignorant. 

The  psycho-physical  explanation  of  Weber's  law  is  that  adopted 
by  Fechner.  This  explanation  insists  upon  making  the  law  one  of 
the  utmost  generality  and  of  the  highest  import  as  stating  the  re- 
lations between  organic  and  spiritual  activities.  Although  Fech- 
ner's  view  confessedly  grew  out  of  his  speculation  that  body  and 
mind  are  only  two  phenomenal  aspects,  as  it  were,  of  one  and  the 
same  underlying  reality,  it  has  been  defended  by  him  with  a  great 
amount  of  mathematical  science  and  experimental  research.  No 
other  form  of  explanation,  however,  takes  us  so  much  into  the 
regions  of  utter  obscurity.  Why  the  quantitative  relations  of  body 
and  mind  should  be  such,  and  such  only,  that  a  geometrical  series 
of  changes  in  the  one  should  invariably  be  represented  by  an  arith- 
metical series  of  changes  in  the  other,  must  indeed  remain  an  ulti- 
mate mystery.  And  the  experimental  proof  of  Weber's  law  is  as 
yet  much  too  incomplete  to  make  us  ready  to  accept  it  as  an  ulti- 
mate psycho-physical  principle. 

The  psychological  explanation  of  Weber's  law  resolves  it  into  a 
special  case  under  the  greater  law  of  the  relativity  of  our  inner  \ 
states.     It  is  not  so  much,  then,  a  law  of  the  absolute  quantity  of 


376  THE  QUANTITY  OF  SENSATIONS 

sensations  as  dependent  on  stimuli,  but  rather  a  law  of  our  ap- 
prehension in  consciousness  of  the  relation  of  our  own  feelings.  In 
general,  it  may  be  said  that  every  mental  state  has  its  value  de- 
termined, both  as  respects  its  quality  and  its  so-called  quantity,  by 
its  relation  to  other  states.  It  is  the  amount  of  change  rather  than 
the  absolute  amount  of  feeling  which  the  mental  apperception  esti- 
mates. That  some  psychological  explanation  is  needed  to  account 
for  the  facts  there  can  be  no  doubt  when  we  consider  how  impor- 
tant are  the  elements  of  attention,  mental  habit,  power  of  acute 
discrimination,  etc.,  in  determining  our  estimates  of  the  quanti- 
tative relations  of  our  sensations.  Estimates — that  is,  acts  of  the 
/  comparing  judgment — are  involved  in  the  experience  upon  which 
reliance  is  placed  for  a  demonstration  of  Weber's  law. 

§  29.  It  does  not  seem  possible,  however,  to  make  a  strict  de- 
duction of  Weber's  law  from  the  principle  of  relativity  alone ;  for  the 
proportionality  required  by  Weber's  law  is  not  the  only  form  of 
relation  that  can  hold  between  quantities.  Wundt  has  laid  some 
stress  on  the  "method  of  mean  gradations"  as  being  specially 
suited  to  test  the  view  that,  in  comparing  different  intensities,  we 
judge  by  ratios  rather  than  by  additions  and  subtractions.  If  we 
judge  by  ratios,  the  subjective  mean  between  two  intensities  should 
be  given  by  the  geometrical  mean  between  the  stimuli.  The  re- 
sults, on  application  of  the  method,  have  been  uneven :  in  some  cases 
the  subjective  mean  has  corresponded  rather  closely  to  the  geo- 
metrical mean  of  the  stimuli,  and  in  others  to  the  arithmetical  mean. 
This  has  led  Wundt  to  offer  the  suggestion  that  two  kinds  of  com- 
parison are  possible — by  ratios  and  by  absolute  differences — ac- 
cording to  the  psychological  attitude  induced  in  the  observer  by 
the  conditions  of  the  experiment.  This  importance  of  the  psycho- 
logical attitude  he  regards  as  good  evidence  in  favor  of  the  psycho- 
logical interpretation  of  Weber's  law.1  The  prevailing  opinion  at 
the  present  day  probably  favors  some  form  of  physiological  inter- 
pretation for  Weber's  law.  In  this  connection  a  suggestion  of  Fuller- 
ton  and  Cattell 2  is  of  interest.  They  would  interpret  the  least  notice- 
able difference,  like  the  average  error,  as  belonging  to  the  class  of 
"errors  of  observation,"  and  as  due,  fundamentally,  to  the  varia- 
bility which  the  process  of  discrimination  shares  with  all  other  or- 
ganic processes.  In  general,  the  variability  of  a  large  phenomenon  is 
greater  than  that  of  a  small;  and  the  perception  of  a  strong  stimu- 
lus may  probably  be  regarded  as  involving  a  larger  organic  response 
than  the  perception  of  a  weak  stimulus,  and  thus  as  involving  more 
sources  of  variability.  Accordingly,  the  error  of  observation,  and 

1  Physiologische  Psychologic,  1902,  I,  544;  6th  ed.,  1908,  I,  635. 

2  Op.  cit. 


FECHNER'S  INTERPRETATION  377 

the  least  noticeable  difference,  would  increase  with  the  stimulus. 
It  need  not,  however,  increase  as  fast  as  the  stimulus;  and  indeed 
these  authors  suggest  that  it  should  typically  increase  according  to 
the  general  law  of  composition  of  independent  variations — namely, 
according  as  the  square  root  of  the  stimulus.  This  formula  fits 
the  facts  better  than  Weber's  law,  in  the  case  of  perception  of  weight 
and  bodily  movement,  and  the  deviations  from  Weber's  law  in 
the  other  departments  of  sense-perception  are  usually  toward  the 
law  of  the  square  root. 

This  ingenious  hypothesis  may  be  helped  out  by  ascribing  the 
departures  from  Weber's  law  to  the  peculiarities  of  structure  in  the 
end-organs  and  to  the  complexity  of  the  physiological  process  in- 
volved in  different  sense-experiences.  It  is  to  be  noted,  however, 
that  it  does  not  do  away  with  the  required  psychological  explana- 
tion, but,  the  rather,  involves  it  as  its  necessary  correlate.  For 
conscious  discrimination  is  involved  in  all  the  experiments,  of  every 
kind  and  by  whatever  method  tested,  that  are  cited  in  the  interests 
of  Weber 's  law.  And  conscious  discrimination,  as  viewed  from  the 
introspective  point  of  view,  is  an  exceedingly  complex  and  variable 
process. 

§  30.  As  an  experimental  problem,  our  confidence  in  the  scientific 
accuracy  of  Weber's,  or  indeed  of  any  other  law  looked  upon  as 
an  invariable  rule  of  our  sense-experience,  quantitatively  considered, 
would  be  much  increased,  if  there  could  be  any  agreement  as  to  ex- 
act standards  of  measurement,  and  as  to  the  values  of  the  different 
ways  of  treating  the  experimental  data.  But,  as  yet  there  is  no 
such  agreement. 

Fechner  interpreted  Weber's  law  as  affording  a  measure  of 
sensations,  and  so  modified  the  more  empirical  expression  of 
Weber  as  to  express  the  quantity  of  sensation  in  terms  of  the  loga- 
rithm of  the  stimulus.  This  conception  implies  the  possibility  of 
adding  one  sensation  to  another,  and  of  treating  the  least  notice- 
able difference  as  a  certain  quantum  of  sensation.  Against  this 
it  has  been  argued,  with  much  force,  that  the  least  noticeable  differ- 
ence is  not  a  sensation,  but  simply  a  fact  about  perception;  that 
every  sensation,  be  it  of  great  or  small  intensity,  is,  for  conscious- 
ness, a  unit  and  not  made  up  of  smaller  sensations;  and  that,  in 
short,  a  sensation  is  not  a  measurable  quantity,  but  that  what  we 
call  a  difference  in  intensity  is  merely  a  difference  in  one  of  the  many 
directions  along  which  sensations  differ — a  certain  kind  of  quali- 
tative difference.  Probably  this  general  point  of  view  has  been 
rather  generally  accepted,  in  opposition  to  Fechner.  But  it  has 
been  found  possible  to  retain  Fechner's  logarithmic  formula,  while 


378  THE  QUANTITY  OF  SENSATIONS 

revising  the  interpretation  of  it.1  Granted  that  it  is  no  longer  con- 
sidered as  giving  a  measure  of  sensation;  it  may  be  retained  as  indi- 
cating the  position  of  a  sensation  in  the  scale  of  intensities.  Any 
convenient  intensity  can  be  taken  as  the  zero  or  centre  of  reference, 
and  any  convenient  unit  can  be  chosen,  and  then  the  location  of 
any  intensity  can  be  indicated  in  multiples  of  the  unit  above  or 
below  the  chosen  zero.  This  view  does  not  require  us,  as  Fech- 
ner's  did,  to  conceive  of  a  sensation  as  made  up  of  smaller  sensa- 
tions, but  only  to  conceive  the  scale  of  intensities  of  any  kind  of 
sensation  as  marked  off  in  equal  units.  Fechner's  formula  then 
shows  the  relation  of  points  in  the  intensity  series  to  the  corres- 
ponding points  in  the  series  of  stimuli  which  produce  the  sen- 
sations. 

It  is  doubtful,  however,  if  even  this  improved  interpretation  of 
Fechner's  law  can  be  regarded  as  a  justifiable  way  of  stating  the 
facts  discovered  in  measuring  least  noticeable  differences.  One 
difficulty  may  be  expressed  as  follows:  If  a  least  noticeable  differ- 
ence is  taken  as  a  unit,  the  difference  in  intensity  between  any  two 
sensations  must  be  equal  to  a  certain  number  of  these  units.  The 
units  must  therefore  be  additive;  they  must  fit  together,  one  after 
another,  without  loss.  But  this,  in  general,  they  will  not  do,  be- 
cause of  the  adaptation  of  the  sense-organ  to  different  intensities 
of  the  stimulus.  In  the  case  of  sensations  of  sight,  for  example, 
on  account  of  the  adaptation  of  the  eye,  it  may  even  happen  that 
the  same  intensity  of  sensation  is  aroused  by  stimuli  of  widely  dif- 
ferent strength,  in  spite  of  the  fact  that,  in  passing  gradually  from 
one  to  the  other,  we  have  perceived  many  successive  increases  of 
intensity.  Our  units  have  telescoped;  we  have  been  like  a  man 
on  a  ship,  carefully  pacing  off  the  distance  between  two  points 
on  the  neighboring  shore,  unmindful  of  the  fact  that  the  ship  is 
moving  in  the  opposite  direction.  The  same  difficulty  would 
be  met  in  other  senses  besides  sight,  since  all  have  some  power  of 
adaptation. 

§  31.  It  seems  better,  then,  to  drop  Fechner's  logarithmic  law, 
and  abide  by  the  more  empirical  expression  of  Weber.  The  facts 
which  are  discovered  in  experiments  on  the  least  noticeable  differ- 
ence are  not,  directly  at  least,  facts  regarding  the  intensity  of  sensa- 
tion, but  facts  regarding  the  discrimination  of  intensities  of  the  stim- 

1  Those  who  have  contributed  to  this  revision  are  principally  Delboeuf,  Ele- 
ments de  psychophysique,  1883;  Stumpf,  Tonpsychologie,  1883;  and  Ebbinghaus, 
Sitzungsberichte  d.  Berl.  Akad.  d.  Wissensch.,  1887.  Convenient  references  are 
Ebbinghaus,  Grundzuge  der  Psychologie,  1905,  p.  529,  and  Titchener,  Experimental 
Psychology,  Quantitative,  Instructor's  Manual,  1905,  p.  cxvi. 


SUMMARY  OF  RESULTS  379 

ulus  j1  and  Weber's  law,  where  it  holds  good,  expresses  an  important 
truth  regarding  the  perception  of  intensities,  the  interpretation  of 
which,  as  already  explained,  probably  must  involve  both  physi- 
ological and  psychological  factors.  Indeed,  we  shall  have  to  return 
to  the  discussion  of  the  whole  subject  of  our  sense-experience,  from 
higher  points  of  view. 

1  That  all  the  studies  which  have  been  considered  in  this  chapter  are,  properly 
speaking,  studies  in  the  process  and  limitations  of  sense-perception,  and  not  sim- 
ply in  the  intensity  of  sensation,  is  a  growing  conviction  among  psychologists, 
and  has  been  presented  with  special  force  by  Miiller,  in  Die  Gesichtspunkte  und 
Tatsachen  der  psychophysichen  Methodik,  p.  234  (Wiesbaden,  1904),  and  by 
Aliotta,  in  La  Misura  in  Psicologia  Sperimentale,  p.  103  (Firenze,  1905). 


CHAPTER  IV 
PRESENTATIONS  OF  SENSE,  OR  SENSE-PERCEPTIONS 

§  1.  It  has  already  been  made  clear  that  those  hypothetical  ele- 
ments which  we  have  called  "simple  sensations,"  and  whose  vari- 
ations in  quality  and  quantity  have  been  found  to  be  dependent 
on  the  locality,  structural  peculiarities,  and  characteristic  physical 
and  chemical  processes,  of  the  nervous  system,  have  themselves  no 
real  existence  in  our  sensuous  experience.  We  are  never  con- 
sciously aware  of  simple,  unlocalized  sensations,  as  such.  Even  our 
most  carefully  prepared  means  of  scientific  analysis  do  not  succeed 
in  disentangling  them  from  the  intricate  complex  of  experience  in 
which  we  find  them  to  be  actually  engaged.  For  this  experience 
of  feeling,  smelling,  tasting,  hearing,  and  seeing  things,  is  of  some- 
thing infinitely  more  complex.  And  to  it,  when  viewed  from  the 
psychological  point  of  view,  such  titles  as  the  following  have  been 
given:  "Sense-perceptions,"  or  "apperceptions,"  "presentations 
of  sense,"  "sense  experience,"  or  when  designated  in  a  manner 
seeming  to  imply  less  of  introspective  theory,  "objects  of  sense." 

It  is  the  ambition  of  physiological  psychology,  as  it  is  of  course 
the  aim  of  psychological  science  from  whatever  point  of  view  its 
problems  may  be  approached,  to  explain  experience.  And  expla- 
nation here,  as  elsewhere,  would  involve  an  analysis  of  these  com- 
pounds (if  we  may  be  pardoned  a  somewhat  inappropriate  figure 
of  speech)  into  their  simpler  elements;  or  better,  a  descriptive  his- 
tory of  the  genesis  and  growth  of  the  more  complex  from  the  simpler 
and  more  nearly  primitive:  a  study  of  the  causes  which  have  given 
rise  to  the  process  of  development,  and  of  the  relations  which  have 
been  called  into  existence  between  its  different  products;  and,  finally, 
as  far  as  possible,  the  establishment  of  those  most  general  facts  of 
relationship,  or  so-called  laws,  which  have  presided  over  the  whole 
course  of  this  development. 

§  2.  But  while  our  study  shares  to  the  full  in  the  desire  to  accom- 
plish this  aim  of  all  psychological  science,  it  has  its  own  somewhat 
special  stand-point  to  maintain,  its  special  obligations  to  fulfil, 
and  its  somewhat  special  problems  to  investigate.  All  these  have 
been  sufficiently  many  times  over  defined.  We  are  studying  the 
whole  subject  from  the  point  of  view  of  one  who  lays  emphasis 

380 


NATURE  AND  STAGES  OF  SENSE-PERCEPTION     381 

on  the  relations  existing  between  the  development  of  mental  life 
and  the  growth  and  increasingly  complicated  functionings  of  the 
nervous  mechanism.  Our  special  ambition,  accordingly,  is  to  dis- 
cover, or  to  conjecture,  the  most  plausible  explanation  of  the  devel- 
opment of  the  mental  life  of  the  human  individual  by  correlating 
it  with  the  genesis  and  growth  of  the  human  nervous  mechanism. 

It  is  plain,  however,  that  from  this  time  onward  we  must  be  more 
than  ever  dependent  for  our  problems,  for  suggestions  as  to  the  best 
manner  of  approaching  the  study  of  them,  and  even  for  suggestions 
as  to  the  directions  in  which  to  look  for  their  answer,  upon  intro- 
spective psychology.  It  is  conscious  experience  which  we  wish  to 
explain.  And  since  the  particular  kind  of  experience,  which  we  ob- 
tain through  the  use  of  our  senses,  by  no  means  consists  in  the 
mere  having  of  simple,  or  of  indefinitely  complex,  sensations,  we 
must  constantly  bear  in  mind  the  fact  that — as  everybody  knows — 
attention,  association,  memory,  discrimination,  judgment,  are  all 
involved  in  gaining  a  knowledge  of  things.  All  these  so-called 
faculties,  however,  have  a  genesis  and  development  which,  while 
it  involves  a  constant  and  ever  more  complicated  system  of  actions 
and  reactions,  may  in  many  respects  be  made  the  subjects,  each 
one,  of  a  special  investigation  from  the  physiological  point  of  view. 

§  3.  We  shall  now  briefly  summarize  some  of  those  conclu- 
sions, or  suggestions  from  introspective  psychology,  which  have 
most  bearing  on  every  attempt  to  build  up  a  theory  of  the  presenta- 
tions of  sense,  or  so-called  sense-perception. 

(1)  Sensations  are,  when  considered  from  the  introspective  point 
of  view,  modes  of  our  being  affected.     There  is  a  wide  interval 
between  our  consciousness  of  being  ourselves  affected  and  the  per- 
ception of  "things"  as  objective  and  having  qualities  of  their  own. 
This  interval  is  filled,  in  nature,  by  the  development  of  mental 
life  as  conditioned  upon  its  environment  of  sense-stimuli;  it  must  be 
filled  in  psychological  theory,  by  a  description  of  the  process  of  de- 
velopment.    But  how  shall  such  a  description  be  obtained?    The 
psychologist  does  not  remember  by  what  stages  he  first  learned  to 
see  or  feel  the  extended  and  external  objects  of  sense.     The  child 
cannot  describe   the   process    to   the   psychologist.     The   infant's 
first  sense-experience  is  already  exceedingly  complex;  and  in  all 
stages  of  human  growth  the  analyzable  contents  of  consciousness 
represent  only  very  imperfectly  the  nature  of  the  basis  upon  which 
they  rest. 

(2)  The  forms  of  being  and  happening  in  the  world,  outside  of  the 
body,  furnish  in  themselves  no  adequate  explanation  whatever  of 
the  presentations  of  sense.     This  is  as  true  of  the  colored  or  smooth 
extension  of  an  object  as  it  is  of  its  sweet  taste  or  disagreeable  smell. 


382  PRESENTATIONS  OF  SENSE 

Whatever  exists  eotfra-mentally,  so  far  as  its  pure  existence  goes,  is 
of  no  account  to  the  mind.  It  is  only  as  so-called  " things"  act  upon 
us,  or — in  other  words — get  themselves  expressed  by  causing  changes 
in  our  mental  states,  that  any  theory  of  knowledge  by  the  senses  can 
make  use  of  them.  What  is  true  of  all  that  exists  and  happens  out- 
side of  the  body  is  just  as  true  of  all  the  bodily  conditions  and  proc- 
esses. Strictly  speaking,  they  can  in  themselves  furnish  no  ade- 
quate explanation  for  the  rise  and  development  of  the  presentations 
of  sense.  Only  psychical  factors  can  be  built  into  mental  products. 
The  simple  sensations  have  no  theoretical  value  except  as  they  may 
be  considered  as  psychical  elements.  The  image  on  the  retina,  for 
example,  is  a  necessary  physical  condition  of  the  clear  vision  of 
outside  objects;  it  may  also  become  an  object  for  the  inspection 
of  another  observer.  But  the  retinal  image  never  becomes  a  kind 
of  inner  object  for  one's  own  brain  or  mind.  The  mind  is  never  to 
be  conceived  of  as  contemplating  a  spatial  picture  of  its  object 
formed  somewhere  within  the  cerebral  substance. 

Even  more  obvious  is  the  worthlessness,  for  purposes  of  strictly 
psychological  analysis,  of  all  theory  as  to  the  precise  spatial  ar- 
rangement of  the  fibrils  of  sensory  nerves  within  the  skin  or  muscular 
fibre.  We  have  nothing  approaching  an  immediate  cognition  of 
the  extended  network  of  sensory  fibrils  in  the  skin  or  muscles;  much 
less  of  the  extended  muscle  or  area  of  the  skin.  No  copy  in  space- 
form  of  the  various  simultaneous  or  successive  rubbings  and  stretch- 
ings of  these  peripheral  fibrils  is  propagated  to  the  brain;  and  if  it 
were,  the  mind  could  not  be  regarded  as  taking  account  of  any  of 
these  neural  processes. 

(3)  A  further  negative  statement  may  be  made  with  entire  confi- 
dence. The  place  at  which  each  organ  of  sense  is  found  in  the  pe- 
riphery of  the  body,  or  the  place  at  which  any  such  organ  is  acted 
on  by  the  stimulus,  cannot  of  itself  furnish  a  reason  for  the  spatial 
perception  of  such  place  or  for  distinguishing  it  from  other  places 
near  or  remote.  The  locality  where  a  stimulus  is  applied,  except 
as  this  locality  affects  the  mental  coloring  or  qualitative  shading 
of  the  sensations  which  result,  is  a  matter  of  complete  indifference 
to  the  mind. 

§  4.  In  contrast  to  all  theories  like  those  just  rejected,  the  fol- 
lowing positive  affirmations  are  to  be  held  firmly.  (1)  Sensations, 
as  the  elements  of  so-called  "presentations  of  sense,"  are  psychical 
states  whose  place — so  far  as  they  can  be  said  to  have  one  and  to 
speak  figuratively — is  in  consciousness.  The  transference  of  these 
sensations  from  mere  mental  states  to  an  interpretation  of  physical 
processes  located  in  the  periphery  of  the  body,  or  of  qualities  of 
things  projected  in  space  external  to  the  body,  is  a  mental  act.  It 


NECESSITY  OF  MENTAL  SYNTHESIS  383 

may  rather  be  said  to  be  a  mental  achievement;  for  it  is  an  act  which 
in  its  perfection  results  from  a  long  and  intricate  process  of  psychical 
as  well  as  physical  development. 

(2)  The  presentation  of  sense,  or  rather,  the  thing  perceived, 
has  "space-form";  it  is  extended,  and  consists  of  an  indefinite  num- 
ber of  visible  or  tangible  parts  that  are  systematically  arranged  be- 
side each  other  into  a  continuous  whole;  it  is  related  with  respect  to 
position,  magnitude,  etc.,  to  other  similar  objects  of  sense.  And, 
indeed,  the  one  most  important  characteristic  which  the  presenta- 
tions of  sense  possess  is  space-form.  "Space-form"  (whatever 
metaphysics  may  decide  to  be  the  nature,  origin,  and  validity  of 
our  idea  of  space)  must  be  regarded  by  psychology  simply  as  the 
mental  form  of  the  presentations  of  sense.  The  problem  which 
physiological  psychology  has  to  solve  in  this  direction  may  then  be 
stated  as  follows:  On  the  basis  of  what  combinations  of  physical  and 
nervous  processes  do  the  different  resulting  sensations  come  to  be  com- 
bined into  presentations  of  sense  under  the  characteristic  of  space- 
form  f 

§  5.  The  most  complete  answer  possible  to  the  question  just 
raised  is  obliged  to  recognize  the  following  particular  truths: 

(1)  A  combination  (or  "fusion,"  or  "synthesis,"  or  "association") 
of  two  or  more  qualitatively  different  series  of  sensations  is  ordinarily 
— if  not  absolutely — necessary  in  order  that  presentations  of  sense  in 
space-form  may  be  constructed.     A  series  of  sensations  of  one  kind 
only,  like  the  pure  differences  in  pitch  of  musical  tone,  or  of  degrees 
of  brightness  and  saturation  of  color-tone,  or  of  pressure,  tempera- 
ture, or  muscular  innervation,  is  not  adapted  to  form  the  material 
for  constructing  extended  objects  of  sense. 

(2)  The  characteristic  differences  in  quality  of  the  sensations  of 
some  of  the  senses,  and  so  their  adaptability  to  form  graded  series, 
are  such  as  to  fit  these  sensations  for  combination  with  other  simi- 
lar sensations  into  the  presentations  of  sense  under  space-form;   the 
sensations  of  other  senses  have  not, — at  least,  to  the  same  extent, — 
these  characteristic  differences  and  this  adaptability.     We  may  then 
speak  of  peculiarly  " spatial  series"  of  sensations,  and  of  other  series 
of  sensations  as — at  least,  relatively — non-spatial.     The  sensations 
of  smell,  for  example,  are  manifestly  not  fitted  to  form  a  so-called 
spatial  series;  indeed,  they  are  incapable  of  being  arranged  in  any 
series  at  all.     On  the  contrary,  the  various  series  of  complex  sensa- 
tions that  come  through  the  eye  and  skin  (including  those  of  the 
muscular  sense)  are  qualitatively  adapted  to  enter  into  such  rela- 
tions to  each  other  as  shall  give  a  ground  in  their  combined  ex- 
istence for  a  perception  of  things.     Accordingly  the  eye  and  skin 
are  the  so-called  "geometrical  senses." 


384  PRESENTATIONS  OF  SENSE 

(3)  The  locally  different  parts  of  the  organ  of  sense — if  this 
organ  is  itself  to  become  known  (as  in  the  case  of  the  skin),  or  if 
through  its  being  stimulated  an  extended  object  outside  of  the  body 
is  to  be  perceived  (as  in  the  case  of  both  skin  and  eye) — must  have 
some  mental  representative  in  the  sensations  which  stimulation 
of  each  calls  forth.     It  is  therefore  assumed  that  every  complex 
sensuous  experience,  besides  its  general  characteristic  quality  as 
belonging  to  this  or  that  particular  sense,  must  have  a  peculiar 
"local  stamp,"  or  shade,  or  mixture  of  quality,  dependent  upon  the 
place  of  the  organism  at  which  the  stimuli  are  applied;   otherwise 
such  experience  cannot  serve  as  a  factor  in  the  construction  of  an 
extended  object  of  sense.     This  peculiar  local  stamp,  or  shade,  or 
mixture  of  quality  has  been  called  a  "local  sign."     It  is  to  Lotze 
that  we  owe  the  first  elaborate  theory  of  "local  signs,"  and  of  their 
relation  to  the  formation  of  the  presentations  of  sense. 

(4)  Various  stages  in  the  process  of  elaborating  the  objects  of 
perception  must  be  recognized.     Thus  the  knowledge  of  the  things 
we  handle — the  fork,  the  tool,  the  pen — stands  at  a  farther  remove 
from  the  simplest  perceptions  of  touch  than  does  the  discrimina- 
tion of  one  area  at  the  surface  of  the  body  as  warmer  or  under  more 
pressure  than  the  surrounding  spots.     Two  noteworthy  stages,  or 
"epoch-making"  achievements,  in  the  process  of  elaborating  the 
presentations  of  sense  would  seem  to  require  a  special  consideration. 
These  have  been  spoken  of  as  "localization"  or  the  assigning  of 
the  sensuous  experience  to  more  or  less  definitely  fixed  points  or 
areas  of  the  body;  and  "eccentric  projection"   (sometimes  called 
"eccentric  perception"),  or  the  recognition  by  the  senses  of  the 
qualities  of  objects  as  situated  within  a  field  of  space  and  either  in 
contact  with,  or  more  or  less  remotely  distant  from,  the  body. 

(5)  The  entire  process  of  elaborating  the  presentations  of  sense, 
or  objects  of  perception  by  the  senses,  presupposes  for  its  explana- 
tion a  constant  activity  of  the  mind  in  reacting  upon  the  stimuli 
which  produce  various  forms  of  molecular  disturbance  in  the  ner- 
vous system;  and,  furthermore,  its  activity  in  combining  the  sensa- 
tions  into  ever  more   complex  forms.     This   combining   activity 
is  best  called  "synthetic"  or  constructive.1     It  may,  indeed,  always 
have  a  physical  basis  in  some  central  organic  combination  of  the 

'The  word  "synthesis"  for  this  mental  activity  is  employed  and  defended 
by  Wundt  (Physiolog.  Psychologic,  2d  ed.,  ii,  pp.  28  f.,  164  f.,  177),  who  justly 
objects  to  the  word  "association"  and  the  theories  which  have  used  the  word, 
because  of  their  concealment  of  the  truth  that  the  process  imparts  new  proper- 
ties to  its  product.  He  also  calls  attention  (p.  175)  to  the  fact  that  John  Stuart 
Mill,  a  chief  defender  of  the  "association  hypothesis,"  virtually  admits  the 
theory  of  a  mental  synthesis  by  using  the  term  "psychical  chemistry." 


NATIVISTIC  AND  EMPIRISTIC  SCHOOLS  385 

neural  processes  which  result  from  stimulating,  simultaneously  or 
in  the  right  succession,  the  different  end-organs  and  areas  of  the 
end-organs  of  sense.  And,  indeed,  our  science  assumes  that  this  is 
so;  although  about  this,  as  a  matter  of  accurate  knowledge,  we  are 
almost  wholly  in  the  dark. 

§  6.  It  follows,  then,  that  an  analysis  of  the  presentations  of  sense 
leads  us  to  find  our  explanation  of  certain  primary  facts  and  results 
in  the  nature  of  the  Mind  itself.  It  is  in  vain  to  object  that  to  do 
this  leaves  the  subject,  ultimately,  still  shrouded  in  mystery.  As 
a  matter  of  fact,  the  analysis  of  psycho-physical  science  does  end 
in  the  recognition  of  ultimate  mystery.  This  is  no  reproach  to  it; 
nor  is  it  a  failure  or  fault  peculiar  to  it  alone.  All  physical  science, 
even,  is  obliged  to  accept  the  same  result  from  its  keenest  analyses 
when  most  vigorously  pushed. 

The  foregoing  remarks  indicate  what  is  the  correct  attitude  of 
the  science  of  physiological  psychology — so  far  as  it  is  necessary  for 
it  to  take  any  position  whatever — toward  the  two  rival  theories 
as  to  the  nature  and  origin  of  presentations  of  sense.  These  theo- 
ries have  been  named  the  "nativistic"  (or  intuitional)  and  the  "em- 
piristic."  Properly  speaking,  they  are  not  two  fundamentally 
different  theories,  but  rather  two  tendencies  which  appear  in  the  at- 
titude assumed  by  two  classes  of  observers  toward  the  admission 
of  certain  alleged  facts,  or  in  the  manner  of  explaining  such  facts 
as  are  admitted  by  all.  These  different  tendencies  are  largely  due 
to  differences  of  position  on  certain  fundamental  philosophical  ques- 
tions. Thus  influenced,  the  advocates  of  the  so-called  "Nativistic 
School"  prefer  to  emphasize  the  intuitional  and  underived  activ- 
ities of  the  mind. 

The  so-called  "Empiristic  School,"  on  the  other  hand,  is  inclined 
to  give  little  or  no  place  to  the  mind's  native  intuition;  it  prefers 
to  fill  the  gaps  in  the  explanation  as  based  on  experiment,  with 
probable  conjecture  and  hypothesis.  It  often  aims  to  show  how 
what  we  call  "mind"  is  itself  rather  the  result  of  a  genesis  induced 
by  the  activity  of  things  through  the  nervous  system.  The  one 
school  is  inclined  to  look  upon  the  space-form,  which  presentations 
of  sense  possess,  as  the  mind's  form,  in  some  large  sense  native  to 
it  and  not  to  be  explained  as  the  result  of  a  development.  The 
other  is  inclined  to  look  upon  space-form  as  wholly  a  form  which 
"things"  have  come  to  acquire,  and  which  will  be  fully  explained 
when  science  has  described  the  empirical  process  by  which  solely 
this  acquisition  is  gained  for  them. 

§  7.  Certain  principles  adopted  both  by  the  empiristic  and  by  the 
nativistic  school  have  their  undoubted  rights;  and  no  satisfactory 
theory  of  sense-perception  can  be  framed  without  admitting  them. 


386  PRESENTATIONS  OF  SENSE 

There  can  be  no  doubt  that  the  presentations  of  sense  which  so 
largely  constitute  our  every-day  adult  experience  are  not  direct  re- 
sults of  untrained  organic  and  mental  activities;  they  are  not  sim- 
ple intuitions  dependent  solely  on  the  native  and  inherent  powers 
of  the  mind.  With  whatever  speed  and  certainty  they  are  formed, 
and  however  the  impression  they  make  is  characterized  by  a  per- 
fect "immediateness,"  they  are  really  extremely  complex  products, 
involving  not  only  the  organic  habit  of  the  species  and  individual 
peculiarities  of  mind  and  body,  but  also  the  acquisitions  of  experi- 
ence through  memory,  attention,  association,  and  so-called  "instinc- 
tive inference."  All  this  is  as  true  of  the  unhesitating  localization 
of  a  burning  or  cutting  pain  in  some  area  of  the  skin  as  it  is  of  the 
most  deliberate  judgment  about  the  distance  of  a  mountain. 

On  the  other  hand,  however  far  the  "empiricist"  may  succeed 
in  resolving  these  "intuitions"  of  sense  into  more  nearly  prim- 
itive elements,  and  however  minutely  he  may  describe  the  processes 
and  laws  of  their  development,  he  will  never  succeed  in  withholding 
from  the  mind  itself  the  ascription  of  all  its  so-called  native  powers. 
The  elements  reached  by  his  most  complete  analysis  must  always 
be  considered  as  reactions  of  the  mind  upon  the  stimulation  of  the 
nervous  centres  through  the  end-organs  of  sense;  they  all  imply  a 
native  disposition  and  ability  of  the  subject  of  the  sensations.  And 
both  theories  must  alike  admit  that  the  nature  of  the  elements  and 
of  the  synthetic  process  is  conditioned  at  every  step  upon  the  action 
of  the  central  nervous  mechanism  as  sensitive  and  excited  through 
stimulation  of  the  end-organs  of  sense.  Nevertheless,  the  triumphs 
of  scientific  research  all  lie  along  the  line  of  the  construction  of  a 
more  and  more  perfect  genetic  theory. 

§  8.  Before  proceeding  to  illustrate  and  confirm  in  detail  the 
principles  already  laid  down,  several  questions  raised  by  the  mere 
statement  of  these  principles  require  an  answer.  And  first:  What 
are  those  characteristic  differences  in  quality  which  the  sensations  be- 
longing to  some  of  the  senses  possess,  and  which  adapt  them  to  com- 
bine into  presentations  of  sense  under  space-form  ?  In  other  words, 
what  kinds  of  sensations  are  fitted  to  constitute  a  so-called  "spatial 
series"  ?  Plainly,  it  is  not  necessary  that  those  elements  of  the  com- 
plex objects  of  sense,  which  make  the  objects  appear  to  be  composed 
of  parts  set  together  side  by  side,  should  themselves  be  immediately 
known  as  side  by  side.  What  is  really  necessary  is  that  both  series 
of  sensations,  if  they  are  to  be  combined  into  one  presentation  of 
sense,  shall  be  capable  of  clearly  and  reciprocally  determining  each 
other  as  series  of  sensations.  They  must  both  have,  that  is  to  say, 
the  common  qualities  and  mutual  relations  of  a  so-called  "spatial 


NATURE  OF  A  SPATIAL  SERIES  387 

(1)  Of  the  qualities  which  characterize  spatial  series  the  following 
are  the  most  important:  Series  of  sensations  of  like  quality,  which 
are  adapted  to  combine  into  extended  objects  of  sense,  must  admit  of 
easy,  rapid,  and  frequent  repetition  in  varying  order  of  arrangement. 
If  a  portion  of  the  body  be  moved,  as,  for  example,  a  finger,  an  arm, 
a  leg,  or  the  bending  of  the  back — a  graded  series  of  sensations,  due 
to  the  varying  quality  and  quantity  of  strain  upon  the  different 
muscles,  joints,  etc.,  is  the  result.  This  series  is  composed  of  indi- 
vidual compound  sensations  that  shade  into  each  other  with  no 
apparent  interruption,  each  of  them  having  a  certain  value  and 
temporal  position  in  consciousness.  In  adult  experience  the  series 
is  rapidly  concluded,  and  instantaneously  interpreted  as  a  whole. 
But  they  may  be  reproduced  in  a  measure  by  slowly  moving  a  limb 
in  any  direction,  and  endeavoring  to  pay  strict  and  exclusive  atten- 
tion to  the  succession  of  feelings  which  results.  Every  motion  of 
each  limb,  from  about  the  same  position  a  to  about  the  same  posi- 
tion m,  relative  to  the  whole  body,  with  similar  energy,  speed,  and 
other  concomitant  circumstances,  yields  a  nearly  identical  series 
of  sensations  (a,  /3,  7,  .  .  .  /*).  Other  motions  of  different  limbs, 
or  differing  otherwise  (in  energy,  speed,  point  of  starting  or  of  con- 
clusion, etc.),  yield  series  differing  in  the  value  and  ordering  of 
their  individual  members.  What  is  true  of  the  muscular  sensations 
that  result  from  the  movement  of  the  limbs  is  also  true  of  the  ac- 
companying sensations  of  the  skin,  such  as  arise  from  changes  in 
its  tension,  etc.  These  sensations,  however,  largely  blend  with  the 
series  of  muscular  sensations  so  as  to  be  nearly  or  quite  inseparable 
in  consciousness.  The  same  thing  also  holds  good  of  the  series 
of  tactual  sensations  (sensations  of  light  pressure  or  touch  proper) 
developed  by  moving  an  object  over  the  skin,  or  by  moving  a  tactile 
organ  (especially  the  hand)  over  an  object  at  rest.  The  muscular 
and  tactual  sensations  which  result  from  motion  of  the  eye  also 
have  the  qualities  of  a  graded  spatial  series. 

Accordingly,  senses  like  those  of  the  eye  and  hand,  which  have 
organs  capable  of  rapid  and  precise  motion,  are  equipped  with  a 
peripheral  mechanism  adapted  to  the  production  of  so-called  spa- 
tial series  of  sensations.  The  succession  of  sensations  of  light  and 
color  which  accompany  the  movement  of  an  object  in  the  field  of 
vision,  or  of  the  glance  from  one  object  to  another,  are  of  the  kind 
favorable  to  forming  a  spatial  series.  In  all  these  cases  the  rate  of 
the  sensations  is  important.  Either  too  slow  or  too  rapid  movement 
of  the  organ  will  not  yield  a  spatial  series  of  sensations.  Moreover, 
such  series  are  capable  of  repetition,  not  only  forward,  as  a,  ft,  7,  8, 
...  /*,  or  in  inverse  order,  as  p,  \,  K,  .  .  .  ft,  a,  but  also  in  an 
endless  variety  as  an  intersecting  network  of  sensations. 


388  PRESENTATIONS  OF  SENSE 

(2)  The  second  class  of  qualifications  which  must  be  possessed 
by  a  spatial  series  of  sensations  secures  their  habitual  combination 
with  other  series,  also  of  a  spatial  kind.     They  must  be  in  nature 
comparable  and  associable  with  each  other,  and,  in  fact,  simultane- 
ously experienced  by  the  mind.     In  singing  a  musical  scale  a  series 
of  sounds  is  accompanied  by  another  series  of  muscular  and  tactual 
sensations  occasioned  by  the  use  of  the  vocal  organs;  both  series 
may  be  produced  in  inverse  order  by  singing  the  same  scale  back- 
ward.    Thus  we  know  not  only  that  we  are  singing  the  scale  with 
the  vocal  organs,  but  also  that  we  are  at  the  same  time  hearing  it 
with  the  ear.     We  know  both  these  facts,  however,  through  sensa- 
tions of  muscle  and  skin  that  have  already  become  inseparably 
associated  and  localized  in  our  own  body. 

On  the  contrary,  from  the  dawn  of  consciousness  onward  through 
all  the  development  of  experience,  series  of  sensations  of  light  and 
color  are  constantly  accompanied  by,  and  combined  with,  other  se- 
ries of  tactual  and  muscular  sensations  of  the  eye.  So,  too,  the 
different  series  of  sensations  that  arise  from  the  irritation  of  the 
nerves  in  muscle  and  skin  are,  of  necessity,  habitually  combined. 
In  forming  the  field  of  touch,  the  fact  that  certain  parts  of  the  pe- 
riphery of  the  body  so  frequently  come  into  contact  with  other  parts 
is  of  the  highest  significance.  Two  series  of  complex  sensations, 
corresponding  to  the  terms  " touching"  and  "being  touched/'  are 
thus  brought  into  juxtaposition,  as  it  were,  in  consciousness.  This 
"juxtaposition"  in  consciousness  is  not  itself,  of  course,  a  spatial 
juxtaposition;  the  former  is,  however,  the  necessary  precondition 
of  the  latter. 

(3)  The  third  characteristic  of  the  spatial  series  of  sensations 
is  the  possession  of  a  system  of  local  signs.     It  may  safely  be  as- 
sumed that  on  neither  side — that  of  an  active  nervous  mechanism, 
or  that  of  a  conscious,  sensuous  experience — are  the  means  that 
make  an  interpretation  of  the  locality  of  our  body  as  affected  by  the 
stimuli,  of  the  spatial  relation  t>ne  to  another  of  its  parts,  of  the  whole 
body  to  its  environment,  and  of  the  different  objects  in  this  environ- 
ment one  to  another,  a  simple  affair.     On  the  contrary,  those  feel-j 
ings,  on  the  interpretation  of  which,  as  "local  signs,"  our  sense-per-j 
ceptions  all  depend,  are  always  exceedingly  complex.     And  cor- 
respondingly complex  are   the   quickly  changing  and  combining 
functions  of  the  nervous  mechanism.     For  this  reason,  if  no  other, 
it  is  uniformly  difficult  to  submit  them  to  a  complete  analysis. 
Several  views,  for  example,  are  possible  as  to  the  nature  of  the  local 
signs  of  the  skin.     It  has  been  held  that  they  are  not  qualitative 
differences  at  all,  but  differences  in  the  intensity  and  time  course 
of  the  tactual  sensations.     Again,  it  has  been  held  that  the  local 


NATURE  OF  THE  LOCAL  SIGNS  389 

signs  of  touch  are  qualitativedifferences_of  sensation  dependent 
upon  the  modifications  whicTTTEe  stimulus  undergoes  on  account 
of  the  changing  character  of  the  skin  witn  respect  to  tension,  nature 
of  the  substance  of  muscle,  tendon,  and  bone  over  which  it  is 
stretched,  etc.  Finally,  it  may  also  be  held  that  the  local  signs  of 
the  skin  are  qualitative  differences  of  sensation  peculiar  to  the  differ- 
ent nervous  ^laments  yiating  in  different  parts  of  this  organ 
sense.  They  are  the  direct  result,  that  is,  of  the  mind's  reaction 
upon  the  specific  energies  of  the  nervous  elements  as  called  out  by 
the  stimulus.  This  is,  of  course,  to  fall  back  upon  the  ultimate 
mystery  involved  in  the  original  nature  of  that  reaction  which  the 
mind  makes  as  dependent  upon  the  locally  individual  nervous  ele- 
ments being  stimulated. 

What  is  certain  of  the  feelings,  or  "sensation-complexes,"  in 
dependence  on  which  this  class  of  objects  of  sense  becomes  known 
to  us,  is  yet  more  certain  of  those  on  the  interpretation  of  which  all 
our  knowledge  of  visual  objects  is  dependent.  Here,  not  only  is 
it  true  that  no  one  theory  among  the  several  proposed  seems  ade- 
quate to  account  for  all  our  experience,  but  something  may  possibly 
be  taken  from  them  all  which  will  prove  of  assistance.  And  more 
refinements  borrowed  from  the  psychology  of  association,  memory, 
habit,  and  judgment,  must  be  added  in  order  to  approximate,  even 
somewhat  remotely,  a  satisfactory  theory  of  the  local  signs  of  the 
eye. 

§  9.  In  view  of  all  the  evidence,  it  would  seem  that  the  gene 
theory  of  local  signs  must  be  constructed  in  somewhat  the  follow- 
ing way:  Within  certain  limits,  which  it  is  impossible  for  science 
as  yet  definitely  to  fix,  the  irritation  of  the  different  nervous  ele- 
ments of  certain  organs  of  sense  gives  rise  to  sensations  which  dif- 
fer in  the  shading  of  their  quality  according  to  the  locality  in  the 

^jgE^a*****"*1"*"  i  "••^'••••'^••••••P"*1     "••'"*  t/  O  9P^MMMK9Vi> 

organ  at  which  the  elements  are  situated.  This  is  probably  true  of 
both  peripheral  and  central  areas  of  the  total  organ.  It  is  true  of 
the  latter  areas  as  dependent  on  the  excitation  of  the  former.  The 
simultaneous  irritation  of  several  locally  related  elements  of  the 
organ  (and  the  irritation  is  seldom  or  never  confined  to  a  single 
element)  results,  then,  in  a  certain  mixture  of  feeling  dependent 
upon  the  number  and  local  relation  of  all  the  elements  thus  simul- 
taneously irritated.  For  example,  the  color-tone  of  the  complex 
sensations  aroused  by  irritating  together  the  retinal  elements  a,  /3,  y, 
5,  etc.,  differs  from  that  aroused  by  irritating  the  elements  7,  S,  e,  £, 
etc.  The  same  thing  holds  true  of  locally  related  nervous  elements 
of  the  skin.  Just  how  much  in  every  case  of  the  local  coloring  is 
due,  on  the  physiological  side,  to  differences  in  structure  and  how 
much  to  differences  in  processes,  how  much  to  peripheral  elements 


390  PRESENTATIONS  OF  SENSE 

and  how  much  to  central  nervous  connections,  it  may  be  impossible 
to  say.  Each  of  the  spatial  series  of  sensations  is  characterized  by 
this  shading  of  its  elements.  We  must,  therefore,  hold  that  every 
sense  which  is  the  medium  of  space-perceptions  has  a  system  of 
local  signs  of  its  own. 

Further:  not  only  each  "geometrical  sense,"  but  also  each  of  the 
"spatial  series"  of  sensations  arising  through  the  total  operation 
of  that  sense,  consists  of  members  that  have  a  local  coloring  pe- 
culiar to  the  series.  Thus  the  spatial  series  of  tactual  impressions 
produced  by  moving  an  object  from  a  to  d  on  the  hand  differs  from 
that  produced  by  moving  it  from  a  to  n;  the  series  of  muscular  sen- 
sations developed  by  raising  one  pound  differs,  with  respect  to  the 
color-tone  of  its  members,  from  that  developed  by  raising  two 
pounds,  with  the  hand. 

But  another  important  consideration  remains.  The  local  signs 
of  the  different  spatial  series  which  frequently  combine  in  the  opera- 
tion of  the  same  organ  must  necessarily  modify  each  other.  Hence 
there  arise  ever  more  complex  admixtures  of  feeling  dependent 
upon  the  combined  specific  energies  of  the  nervous  elements  simul- 
taneously excited,  with  a  given  amount  of  energy  and  with  given 
relations  to  preceding  conditions.  -We  define  the  local  sign,  then, 
as  that  mixture  of  feeling  which  gives  to  the  sense-experience  its 
peculiar  coloring,  and  is  dependent  upon  the  combined  result  of  excit- 
ing the  nerves  of  a  given  locality  of  the  organ. 

§  10.  The  most  noteworthy  stages,  or  "epoch-making"  achieve- 
ments, in  the  process  of  elaborating  the  presentations  of  sense,  have 
been  declared  to  be  " localization"  and  " eccentric  projection."  The 
first,  primarily,  gives  us  the  knowledge  of  our  own  body,  mainly  by 
passive  sensations  of  touch;  the  knowledge  of  our  own  body  which 
comes  through  sight  is  by  eccentric  projection.  We  immediately 
feel  the  peripheral  parts  of  the  body  as  the  places  where  the  sensa- 
tions are  localized;  we  see  some  of  the  same  parts  as  projected  in 
space  before  our  eyes.  Objects  that  are  not  a  part  of  ourselves  are 
given  to  us  as  projected  eccentrically,  either  by  touch  through  their 
being  in  contact  with  the  skin  and  occasioning  sensations  of  mus- 
cular exertion,  or  by  sight  as  having  distance  in  its  field  of  vision. 

Localization  and  projection  are  not,  however,  to  be  regarded  as 
two  phases  of  one  and  the  same  process;  we  do  not  first  have  the 
presentations  of  sense  as  parts  of  the  periphery  of  our  bodies,  and 
then,  on  further  experience,  push  them  beyond  this  periphery,  either 
to  an  infinitesimal  distance  or  to  one  remote.  Localization  and  ec- 
centric projection  are  rather  two  processes,  largely  unlike,  which 
go  on  contemporaneously  and  are  set  up  chiefly  on  the  basis  of  dif- 
ferent classes  of  sensations.  Where  two  parts  of  the  sensitive  skin 


ACTIVITY  OF  HIGHER  FACULTIES  391 

of  our  own  bodies  come  together,  the  conditions  for  both  of  the  above- 
mentioned  processes  are  fulfilled.  Accordingly,  one  part  has  local- 
ized in  it  those  complex  sensations  which  make  us  aware  that  this 
part  of  our  body  is  touching  something;  the  other  has  localized  in 
it  those  sensations  which  make  us  aware  that  this  part  is  being 
touched  by  something.  Which  of  the  two  parts  shall  be  regarded 
as  touching,  and  which  as  being  touched,  depends  on  various  con- 
siderations. Those  members  of  the  body  which  are  most  used  in 
active  touch  are  generally  known  as  touching,  and  the  less  active 
parts  as  being  touched. 

§  11.  Two  things  more  must  constantly  be  borne  in  mind  in  any 
attempt  to  construct  even  a  fairly  plausible  theory  of  sense-percep- 
tion in  terms  of  physiological  psychology.  First:  so  far  as  we  can 
penetrate  the  mysteries  of  beginning  mental  life,  there  is  never  at 
any  stage  an  experience  corresponding  to  "pure  sensations,"  or 
"simple  sensations."  Such  terms,  if  employed  at  all,  must  always 
be  understood  as  applying  to  hypothetical  elements  of  already  com- 
plex psychoses,  which  are,  however,  of  value,  especially  to  the 
student  of  physiological  psychology  as  enabling  him  to  correlate  the 
mental  life,  in  its  development,  with  the  increasing  complexity  of 
the  secondary  activities  of  the  brain  which  result  from  the  combina- 
tion of  its  more  primary  reactions  to  the  elementary  sensory  impulses. 
That  there  is  growth  in  knowledge  as  to  the  spatial  and  temporal 
relations  of  the  bodily  members,  and  of  external  objects  and  the 
sequence  of  events  among  things,  there  can  be  no  manner  of  doubt. 
Our  problem  is  to  account  for  this  growth. 

But  second:  the  so-called  "higher  faculties"  of  the  mind  are  as 
truly  implied  in  the  very  beginnings  of  sensuous  experience  as  in 
its  latest  developments.  The  infant  attends,  discriminates,  judges, 
and  so  learns,  as  truly — and  in  all  probability,  in  essentially  the  same 
way — as  the  adult  man  of  science.  Indeed,  the  most  astonishing 
and  antecedently  incredible  thing  about  the  whole  of  these  earlier 
stages  is  the  intensity,  and  the  fine  quality,  of  those  mental  activ- 
ities which  initiate,  conduct,  and  control  all  the  more  primitive 
processes  of  learning  to  know,  by  the  senses,  the  bodily  organism 
and  external  things.  For  these  secondary  and  "synthetic"  reactions, 
the  highly  developed,  but  as  yet  unused,  cerebral  hemispheres  of  the 
human  infant  seem  especially  adapted. 

The  foregoing  principles  must  now  be  illustrated  and  confirmed 
by  a  brief  statement  of  facts  which  relate  to  the  formation  and  de- 
velopment of  presentations  of  sense  by  a  synthesis  of  simple  sen- 
sations. Attention  will,  for  obvious  reasons,  be  directed  almost 
exclusively  to  those  presentations  of  sense  which  come  through  the 
eye  and  skin,  including  in  both  the  influence  of  muscular  sensations. 


392  PRESENTATIONS  OF  SENSE 

§  12.  Perceptions  of  Smell  differ  only  in  fineness,  duration,  and  ac- 
companying tone  of  feeling;  they  have  no  size  or  shape,  no  spatial 
properties  of  any  kind.  Considered  apart  from  their  accompani- 
ment of  muscular  and  tactual  sensations,  they  cannot  even  be  said 
to  be  localized.  Fineness  of  smell,  or  power  to  make  minute  dis- 
tinctions in  quality,  and  so  infer  the  presence  or  direction  of  an  ob- 
ject previously  known  to  excite  such  quality  of  sensations,  differs 
greatly  in  different  species  of  animals  and  in  different  individuals 
of  the  same  species.  The  exploits  of  some  animals  give  ground  for 
the  conjecture  that  every  species,  and  even  every  individual,  has  an 
odor  of  its  own.  The  direction  and  nature  of  the  object  which 
causes  the  sensations  are  judged  by  variations  of  intensity  on  turn- 
ing the  head,  or  on  approaching  or  receding  from  the  object.  Sen- 
sations of  smell  are  known  to  come  through  the  nose,  by  localizing 
there  the  accompanying  muscular  and  tactual  sensations  with  their 
strong  tone  of  feeling.  This  is  readily  done,  since  we  draw  the  air 
through  the  nostrils  and  feel  its  double  effects  in  producing  the  two 
classes  of  sensations.  As  to  the  simultaneous  influence  of  two  smells, 
little  is  known  beyond  the  fact  that  the  stronger  overwhelms  the 
weaker.  The  power  of  discrimination  may,  of  course,  be  culti- 
vated in  this  sense  as  in  every  other.1 

§  13.  Most  of  the  remarks  just  made  as  to  perceptions  of  smell 
apply  also  to  Perceptions  of  Taste.  Sensations  of  taste,  however,  are 
much  more  closely  connected  with  those  of  touch;  since  the  tongue 
is  a  chief  organ  of  active  touch.  It  is  the  tactual  and  muscular 
sensations,  and  not  the  purely  qualitative  affections  of  taste,  which 
are  localized  in  the  mouth.  Concerning  contrast  and  compensation 
of  tastes,  little  is  known  which  does  not  belong  to  ordinary  experi- 
ence. Valentin2  alleges  that  when  a  sour  mass  is  laid  on  one  half, 
and  a  bitter  mass  on  the  other  half,  of  the  root  of  the  tongue,  the 
predominating  taste  may  sometimes  be  determined  by  our  choice. 
It  is  well  known  that  certain  tastes  compensate  each  other,  as  it 
were,  in  experience,  without  any  chemical  equivalence  of  their  prop- 
erties. The  sugar  neutralizes  the  acid  of  the  lemonade,  not  in  the 
vessel  that  contains  the  mixture,  but  in  the  nervous  system  of  him 
who  drinks  it.  Briicke  holds3  that  the  neutralizing  of  one  sensa- 
tion of  taste  by  the  other  takes  place  in  the  brain.  The  sensation 
of  bitter  is  especially  difficult  to  cover  or  neutralize. 

§  14.  Perceptions  of  Hearing  next  demand  consideration.  More 
difficulty  accompanies  the  effort  to  establish  the  proposition  that 

1  On  the  whole  subject  see  von  Vintschgau's  monograph  in  Hermann,  Handb. 
d.  Physiol,  III,  pp.  225  ff. 

2  Lehrbuch  der  Physiol.  d.  Menschen,  etc.,  Abth.  ii,  p.  308  (2d  ed.). 

3  Vorlesungen  uber  Physiol.  (ed.  1884),  ii,  p.  262. 


LOCALIZATION  OF  SOUNDS  393 

sensations  of  sound  are  not  directly  localized,  but  are  projected 
in  a  space  constituted  chiefly  by  the  eye  and  the  hand,  through 
complicated  indirect  inferences. 

The  chief  facts  which  must  be  accounted  for  by  a  theory  for  the 
localization  of  sounds  are  the  following:  A  sound  can  be  recog- 
nized with  certainty  as  coming  from  the  right  or  the  left  side,  or  as 
coming  from  a  point  either  to  the  right  or  left  of  the  median  plane 
of  the  head.  Moreover,  the  angle  at  which  the  sound  approaches 
the  ear,  as  measured  from  the  median  plane,  can  be  recognized  with 
considerable  accuracy.  Or,  if  a  sound  is  produced  within  this 
plane  (extended  into  space),  this  fact  can  be  well  recognized.  But 
as  to  the  direction  within  this  plane  from  which  the  sound  comes — 
whether  from  above,  before,  or  behind — judgment  is  uncertain  and 
subject  to  large  errors.  And  the  same  difficulty  is  experienced  in 
judging  the  exact  direction  of  sounds  which  come  from  the  side. 
How  much  their  direction  differs  from  the  median  plane  can,  as 
was  stated,  be  told;  but  whether  from  before,  behind,  up  or  down, 
is  only  poorly  distinguished.  In  other  words,  if  a  sphere  be  con- 
ceived as  surrounding  the  head,  with  the  north  and  south  poles 
located  opposite  the  ears  and  the  equator  coinciding  with  the  median 
plane  of  the  head,  then  the  latitude  of  a  sound  can  be  detected  with 
considerable  accuracy,  but  the  longitude  is  subject  to  much  error. 
Judgment  of  the  latitude  is,  however,  most  accurate  near  the  equa- 
tor, and  least  accurate  near  the  poles.1 

§  15.  Another  set  of  facts  comes  to  light  when  different  sounds 
are  made  to  affect  the  two  ears.  If  the  two  sounds  are  of  different 
pitch  or  timbre,  each  is  apt  to  be  heard  and  localized  separately; 
but  if  the  two  are  alike,  except  that  one  is  stronger  than  the  other, 
they  usually  appear  as  one,  which  is  localized  on  the  side  of  the 
stronger  stimulus;  and  if  they  are  alike  in  intensity  as  well  as  in 
pitch  and  timbre,  usually  one  sound  is  heard,  which  seems  to  come 
from  the  median  plane.  This  last  result,  which  is  as  instructive 
as  it  is  curious,  can  be  obtained  by  sounding  two  tuning  forks  of  the 
same  pitch,  with  equal  loudness,  one  opposite  each  ear;  or  by  con- 
veying a  sound  through  a  branched  tube,  one  branch  being  inserted 
in  each  meatus;  or,  finally,  by  employing  a  branched  telephone  cir- 
cuit, with  a  receiver  held  to  each  ear.  In  the  latter  two  cases,  the 
sound  is  subjectively  localized  in  the  interior  of  the  head.  Similar 
results  are  obtained  by  applying  the  shank  of  a  vibrating  tuning 
fork  to  the  skull  at  various  points;  the  tone  is  localized  in  that  ear 
which  is  more  strongly  excited,  but  if  the  fork  is  applied  at  a  point 

1  See  D.  Starch,  University  of  Iowa  Studies  in  Psychology,  1905,  IV,  1;  and  Psy- 
chological Review,  Monograph  Supplement,  No.  XXXVIII,  1908. 


394  PRESENTATIONS  OF  SENSE 

in  the  median  plane,  the  sound  is  localized  in  the  median  plane,  and 
often  in  the  interior  of  the  head.  When  sound  is  thus  conveyed 
to  the  ear  by  bone  conduction,  closing  the  meatus  of  one  ear  by 
holding  the  palm  against  it  increases  the  effect  on  that  ear,  and 
causes  the  sound  to  appear  louder,  and,  therefore,  to  be  localized 
in  the  ear  that  is  closed  (Weber's  experiment).  As  such  conduction 
is,  in  large  measure,  the  means  by  which  the  sound  of  our  own  voices 
reaches  our  ears,  the  same  experiment  can  be  tried  by  humming  a 
low  note  with  closed  lips,  and  observing  the  effect  of  closing  the  ex- 
ternal meatus  with  the  palm.  If  one  ear  is  thus  closed,  the  sound 
is  localized  in  that  ear;  if  both  ears  are  closed  at  once,  the  sound 
appears  to  come  from  the  interior  of  the  head. 

§  16.  The  first  step  toward  an  explanation  of  the  power  of  local- 
izing sounds  is  thus  made  clear:  evidently,  as  the  last-mentioned  ex- 
periment shows,  each  ear  has  a  "local  sign"  of  its  own,  by  which  a 
stimulus  affecting  chiefly  one  ear  is  distinguished  from  a  stimulus 
affecting  the  other  ear.  It  should  be  noted  that  sound  never  affects 
one  ear  alone,  unless  the  other  ear  is  totally  deaf;  for  conduction  by 
the  bones  occurs  between  the  two  ears  to  a  surprising  degree.  Nei- 
ther closing  the  external  meatus  of  one  ear,  nor  bringing  the  source 
of  sound  close  to  the  other  ear,  suffices  to  produce  strictly  monaural 
hearing.  These  devices  do,  however,  ensure  a  stronger  excitation 
of  one  ear  than  of  the  other.  There  can  be  no  doubt — as  a  second 
indication  for  a  correct  theory  of  the  localization  of  sounds — that  a 
sound  which  excites  one  ear  more  strongly  than  the  other  is  local- 
ized on  the  more  excited  side;  nor  that  when  both  ears  are  excited 
equally,  the  sound  is  localized  in  the  median  plane.  It  would  seem, 
accordingly,  that  the  decisive  factor  in  localizing  sounds  to  the  right 
or  left  must  be  the  relative  intensities  of  the  stimulation  of  the  two 
ears.  This  factor  would  account  for  the  more  accurate  element  in 
localization.  The  much  less  accurate  sense  of  locality  for  front 
and  back  may  be  dependent  on  the  position  of  the  auricle,  which, 
in  the  human  being,  would  seem  to  favor  the  entrance  of  sounds 
from  the  front;  just  as,  in  animals  possessing  movable  ears,  the 
localization  of  sounds  certainly  appears  to  depend  on  adjusting 
the  position  of  the  pinna  most  favorably  for  the  direction  of  the 
sound. 

§  17.  The  principle  of  relative  intensities  can  therefore  be  ap- 
plied satisfactorily  to  the  case  of  sounds  originating  near  the  ear; 
but  difficulty  arises  in  the  case  of  sounds  from  distant  sources,  for 
here  the  difference  in  distance  of  the  ears  from  the  source  of  sound 
is  too  small,  in  comparison  with  the  total  distance,  to  account 
for  a  different  intensity  of  excitation  of  the  two  ears.  One  ear  is 
practically  as  far  from  the  source  as  the  other;  both  ears  should 


LOCALIZATION  OF  SOUNDS  395 

therefore  be  equally  excited,  and  the  sound  be  localized  in  the  median 
plane,  instead  of  being,  as  it  often  is,  very  clearly  and  correctly  as- 
signed to  one  side  or  the  other.  The  ear  which  is  toward  the  source 
of  sound  is  indeed  exposed  to  the  direct  impact  of  the  waves,  whereas 
the  other  would  seem  to  be  shielded  from  them  by  the  head.  But 
here  we  meet  a  physical  difficulty,  for,  as  Lord  Rayleigh  has  shown,1 
the  head  is  too  small  a  shield  to  cast  a  "sound  shadow,"  at  least 
when  the  sound  is  of  low  pitch  and,  therefore,  of  great  wave-length 
with  respect  to  the  diameter  of  the  head.  Sound-waves  bend  read- 
ily around  the  head,  and  enter  the  further  ear  with,  it  would  seem, 
little  possible  diminution  of  their  energy.  The  waves  entering  the 
two  ears  would,  however,  be  at  any  moment  in  different  phases, 
and  Lord  Rayleigh  believed  that  this  difference  of  phase  afforded 
a  basis  for  distinguishing  the  direction  of  the  sound — a  view  which 
appears  rather  improbable.  Myers  and  Wilson,2  taking  account  of 
bone  conduction,  are,  however,  able  to  show  that  the  difference  of 
phase  must  result  in  a  difference  in  intensity  at  the  two  ears.  It 
should  also  be  noted  that,  as  the  orifice  of  the  ear  lies  nearer  to  the 
back  than  to  the  front  or  top  of  the  head,  the  path  of  sound-waves 
around  the  head  is  shorter  in  some  directions  than  in  others,  and 
that,  accordingly,  the  waves  which  reach  the  further  ear  will  them- 
selves vary  in  phase  and  therefore  interfere  with  each  other  and  have 
less  effective  intensity  than  the  sounds  which,  approaching  the 
nearer  ear  across  clear  space,  act  on  it  without  this  mutual  inter- 
ference. High  tones,  with  short  wave-length,  would  evidently  be 
more  subject  to  this  interference  than  low  tones.  At  any  rate,  there 
is  no  doubt  that  the  nearer  ear  is  more  intensely  excited  than  the 
further  ear,  for  this  can  be  proved  by  direct  measurements  of  the 
least  audible  sound,  coming  from  various  directions.3  A  weaker 
sound  can  be  heard  when  the  source  is  directly  opposite  the 
right  or  left  ear,  than  when  it  is  situated  in  or  near  the  median 
plane. 

The  quality  or  timbre  of  a  sound  is  also  affected  by  the  angle 
from  which  it  approaches  the  ear,  and  is  different  for  the  nearer 
and  for  the  farther  ear. 

The  shorter  the  waves,  the  more  effectively  does  the  head  screen 
the  farther  ear;  it  thus  cuts  out  the  higher  partial  tones  of  a  clang 
or  noise  more  than  the  lower  components,  and  changes  the  quality 
of  the  sound.  This  difference  in  the  quality  of  sound  to  the  two 
ears,  according  to  the  direction  from  which  the  sound  comes,  may  be 

1  Nature,  1876,  XIV,  32;  Philosophical  Magazine,  1907,  XIII,  214. 

2  Proceedings  of  the  Royal  Society,  1908,  A,  LXXX,  260;  British  Journal  of  Psy- 
chology, 1908,  II,  363. 

3  See  Starch,  op.  cit. 


396  PRESENTATIONS  OF  SENSE 

of  importance  not  only  in  the  binaural  localization  of  sounds  to  the 
right  or  left,  but  also  in  the  (often  considerable)  power  of  localiza- 
tion as  possessed  by  a  one-eared  person;1  and,  further,  in  the  as- 
signment of  sounds  to  front  and  rear,  above  and  below. 

§  18.  What  has  thus  far  been  said  of  localization  of  sound  has  re- 
ferred to  the  perception  of  its  direction.  The  judgment  of  its  dis- 
tance must  be  dismissed  with  a  few  words.  If  the  sound  is  familiar, 
the  distance  of  its  source  can  be  judged  from  its  apparent  intensity. 
But  even  when  the  intensity  of  the  sound  at  its  source  is  not  known, 
it  may  still  be  possible  to  recognize  the  distance  from  which  it  comes. 
It  is  probable  that  the  change  in  the  quality  of  a  sound  with  dis- 
tance, due  to  the  dropping  out  of  the  weaker  overtones,  has  much  to 
do  with  the  recognition  of  distance.  It  is  significant  that  simple 
tones  are  more  poorly  localized,  both  as  to  distance  and  as  to  direc- 
tion, than  clangs  and  noises  containing  an  abundance  of  high  par- 
tial tones.2  In  one-eared  individuals  pure  tones  are  entirely  un- 
localizable. 

The  proposition  would  seem  then  to  be  established  that  the  so- 
called  perceptions  of  hearing  are  localized  by  means  of  the  varying 
intensities  and  complex  qualities  (or  "local  signs")  of  the  sensations 
of  sound,  in  a  field  of  space  which  has  been  "constructed" — so  to 
say — out  of  other  forms  of  sense-experience. 

§  19.  An  account  of  the  process  by  which  a  Field  of  Touch  is  con- 
structed, and  extended  objects  are  known  as  in  contact  with  the 
skin  at  definite  points  or  areas  of  it,  must  begin  by  enumerating  the 
data  which  the  mind  has  for  such  activity.  The  most  important 
of  these  data  are  indicated  by  certain  facts  as  to  the  fineness  of  the 
so-called  "sense  of  locality"  belonging  to  the  skin.  E.  H.  Weber 
first  attempted  a  rule  for  measuring  the  degree  of  this  fineness  ac- 
curately; he  also  mapped  out  the  entire  field  of  the  surface  of  the 
body  into  areas  differing  greatly  in  their  fineness.3  For  a  measur- 
ing instrument  he  used  the  two  points  of  a  pair  of  dividers,  blunted 
so  as  to  prevent  the  sensation  of  being  pricked;  the  principle  of 
measurement  was  that  the  minimum  distance  apart  at  which  the 
two  points,  when  touching  the  skin  of  any  region,  are  felt  as  two 
localized  sensations,  is  the  measure  of  the  sensitiveness  to  local  dis- 
tinction of  that  region.  The  following  table  gives  some  of  the  re- 
sults of  Weber's  experiments;  the  figures  indicate  the  number  of 

1  See  Angell  and  Fite,  Psychological  Review,  1901,  VIII,  225,  449. 

2  On  this  point,  see  J.  R.  Angell,  Psychological  Review,  1903,  X,  1.     For  general 
discussion  and  literature,  see  K.  L.  Schaefer,  in  Nagel's  Handbuch  der  Physiologic, 
1905,  III,  573,  and  C.  S.  Myers,  Textbook  of  Experimental  Psychology,  1909,  286. 

3  Annot.  Anatom.,  vii,  p.  4  f.;  Wagner's  Handworterb.  d.  PhysioL,  III,  Abth. 
ii,  p.  529  f. 


CONSTRUCTION  OF  THE  FIELD  OF  TOUCH       397 

millimetres1  apart  which  the  points  of  the  dividers  were  when  the 
given  area  of  the  organ  was  just  able  to  distinguish  them : 

Tip  of  the  tongue 1 

Volar  side  of  the  last  phalanx  of  the  finger 2 

Red  part  of  the  lips " 5 

Volar  side  of  the  second  and  dorsal  side  of  the  third  phalanx  of  the  finger    .  7 

White  of  the  lips,  and  metacarpus  of  the  thumb 9 

Cheek,  and  plantar  side  of  the  last  phalanx  of  the  great-toe 11 

Dorsal  side  of  the  first  phalanx  of  the  finger 16 

Skin  on  the  back  part  of  cheek-bone,  and  forehead 23 

Back  of  the  hand 31 

Knee-pan,  and  surrounding  region .36 

Forearm,  lower  leg,  back  of  the  foot  near  the  toes 40 

Skin  of  the  nape,  and  of  the  back  in  the  five  upper  cervical  vertebrae  ...  54 

Skin  of  the  middle  of  the  back,  and  of  the  upper  arm  and  leg 68 

Weber  also  found  that  the  fineness  of  the  sense  of  locality  is 
greater  in  a  transverse  than  in  a  longitudinal  direction,  on  both  arms 
and  legs.  On  these  surfaces  of  the  skin  the  "sensation-circles," 
or  areas  within  which  the  minimum  distances  of  the  dividers'  points 
are  felt  as  two  points,  have  an  elliptical  shape,  with  their  long  axes 
up  and  down.  That  the  size  of  the  sensation-circles,  or  the  fineness 
of  the  sense  of  locality,  largely  forms  the  basis  for  our  judgments 
of  the  position,  number,  and  magnitude  of  the  localized  sensations 
in  the  field  of  touch  may  be  shown  by  a  simple  experiment.  If 
the  points  of  the  dividers  be  separated  somewhat  less  than  is  neces- 
sary in  order  to  distinguish  them  as  two  on  the  cheek  just  in  front 
of  the  ear,  and  then  (the  distance  apart  of  the  points  remaining  un- 
changed) be  slowly  moved  until  one  point  rests  upon  the  upper  and 
the  other  upon  the  lower  lip,  to  a  person  blindfold,  and  unpreju- 
diced by  knowing  what  is  to  take  place,  the  point  first  felt  as  one 
will  appear  to  become  two,  and  then  the  two  recede  from  each 
other  continually  as  the  parts  with  a  finer  sense  of  locality  are  trav- 
ersed. The  same  experiment  may  be  tried  upon  any  other  part 
of  the  body.  It  appears,  therefore,  that  the  mental  representation 
of  the  magnitude  of  the  distance  between  two  impressions  varies  in 
inverse  proportion  to  the  real  magnitude  of  the  smallest  perceiv- 
able distance,  on  any  given  area  of  the  skin.  The  same  principle 
appears  to  hold  good  when  all  the  space  between  the  impressions 
is  filled  up,  so  as  to  make  a  continuum  of  localized  sensations. 

§  20.  The  explanation  of  Weber's  "sensation-circles"  of  the  skin 
has  been  the  subject  of  much  debate.  It  is  natural  at  first  to  as- 

1  The  numbers  were  given  by  Weber  in  Parisian  lines;  in  the  table  they  are 
taken  from  WTundt,  Physiolog.  Psychologic  (2d  ed.),  ii,  p.  7,  who  has  reduced  them 
to  even  millimetres. 


398  PRESENTATIONS  OF  SENSE 

sume  that  each  entire  circle  is  provided  with  one  and  only  one 
nerve-fibre,  whose  terminal  expansion  covers  the  circle,  and  whose 
excitation  is  represented  in  consciousness  by  a  sensation  of  a  spe- 
cific value.  Doubtless  certain  anatomical  differences  in  the  nerve- 
fibres  of  the  skin,  and  certain  corresponding  physiological  differ- 
ences in  their  function,  must  be  assumed  as  the  basis  of  every  the- 
ory to  account  for  the  skin's  sense  of  locality.  But  Goldscheider's 
experiments  show  that  a  number  of  pressure-spots  must  be  recog- 
nized within  each  sensation-circle,  and  each  pressure-spot  at  least 
should  have  a  sensory  fibre.  Moreover,  every  point  within  each 
sensation-circle  is  itself  sensitive  (however  large  the  circle  may  be), 
and  the  limits  of  none  of  the  circles  are  fixed  as  would  be  the  ex- 
panse of  a  single  nerve-fibre  distributed  over  them.  Still  further, 
different  individuals  differ  greatly  in  the  size  of  these  circles  (and 
we  cannot  well  suppose  a  corresponding  difference  in  the  number 
of  sensory  nerves  of  the  skin),  and  practice  suddenly  and  greatly 
diminishes  the  area  covered  by  a  single  circle.  It  must  at  least  be 
admitted  that  "the  smallest  perceivable  distance  is  not  a  direct 
measure  for  the  diameter  of  the  sensation-circle."  Weber  himself 
assumed  that  sensation-circles  always  contain  a  number  of  isolated 
nerve-fibres;  and  that,  in  order  to  have  the  impression  of  two  local- 
ized sensations,  several  unexcited  fibres  must  exist  between  the  two 
excited.  The  number  of  these  unexcited  fibres  serves  the  mind  as 
a  kind  of  means  for  the  approximate  measurement  of  distances  on 
the  skin.  The  highly  conjectural,  and  in  general  the  unpsycho- 
logical,  nature  of  all  these  explanations  renders  them  unsatisfactory. 
§  21.  When  we  attempt  to  apply  the  theory  of  local  signs  to  this 
subject,  difficulty  arises  in  assigning  a  conclusive  reason  why  the 
different  areas  of  the  skin  should  differ  so  greatly  in  the  fineness  of 
their  capacity  for  making  local  distinctions.  In  the  view  of  Lotze,1 
this  difference  is  chiefly  due  to  the  varying  character  of  the  areas 
of  the  skin  with  respect  to  richness  in  nerve-fibres,  thickness  and 
so  sensitiveness,  support  and  tension  according  as  the  skin  is 
stretched  over  underlying  soft  or  hard  parts — fat,  muscle,  tendon, 
bone,  etc.  Doubtless  all  such  influences  enter  into  the  determina- 
tion of  that  mixture  of  feeling  which  characterizes  the  local  signs 
of  the  skin.  The  theory  suggested  by  Vierordt,2  on  the  basis  of 
experiments  made  by  himself  and  his  pupils,  should  also  be  men- 
tioned. This  investigator  concluded  that  the  fineness  of  the  sense 
of  locality  belonging  to  any  area  of  the  skin  increases  in  direct  pro- 
portion with  the  distance  of  that  area  from  the  axes  about  which  it 

1  See  Medicin.  Psychologic,  p.  405  f. 

2  Pfluger's  Archiv,  1869,  II,  pp.  297  ff.;  and  Zeitschr.  f.  Biologic  VI,  VII,  IX, 
X,  XI. 


EXPLANATION  OF  WEBER'S  "SENSATION-CIRCLES"  399 

is  rotated.  The  relative  fineness  of  the  organ's  local  sense  is  a 
function  of  its  mobility.  Thus  an  uninterrupted  increase  of  the 
power  of  localization  exists  in  the  arm  from  the  acromion  to  the 
tips  of  the  fingers;  an  increase  of  its  movableness,  on  the  whole,  also 
exists.  If  a  value  of  100  be  assigned  to  the  power  of  discrimination 
exercised  at  the  acromion,  151  will  represent  that  of  the  upper  arm, 
272  that  of  the  lower  arm,  659  of  the  hand,  2,417  of  the  thumb,  and 
2,582  of  the  tips  of  the  fingers.  In  estimating  the  relative  movable- 
ness  of  these  different  parts,  it  should  be  remembered  that  they 
not  only  all  move  in  an  enlarging  circuit  from  the  shoulder-joint 
downward,  but  that  each  of  them  from  the  elbow-joint  downward 
has  its  special  increased  circuit  and  more  numerous  forms  of  mo- 
tion. 

But  even  if  Vierordt's  law  could  be  strictly  demonstrated  for 
every  portion  of  the  body,  its  meaning  would  have  to  be  translated 
into  other  terms  in  order  to  be  of  any  real  service  to  psychology. 
It  is  therefore  suggested  by  Funke  that  the  increased  power  of 
discrimination  which  belongs  to  the  more  movable  areas  of  the  skin 
is  really  due  to  the  superior  facility  which  they  thus  have  for  exer- 
cise; it  therefore  falls  under  the  law  of  habit.  Furthermore — as 
we  have  occasion  to  remark  concerning  many  similar  functions  of 
the  mind  in  correlation  with  the  nervous  mechanism — the  effect 
of  acquired  habit  is  not  limited  to  the  experience  of  the  individual; 
it  belongs  also  to  the  race.  The  superior  fineness  of  local  sense  in 
some  parts  of  the  body  may  therefore  be  regarded  as  largely  native 
to  the  individual. 

§  22.  The  view  which  must  be  taken  of  Weber's  "sensation-cir- 
cles," and  of  the  entire  subject  of  the  localization  of  areas  of  press- 
ure on  the  skin,  has  been  largely  changed  by  the  more  recent  experi- 
ments of  Goldscheider1  and  others.  We  have  already  seen  (p.  344) 
that  the  finest  point,  when  it  touches  a  "pressure-spot,"  produces 
a  sensation  of  pressure,  and  not  one  of  being  pricked;  but  touching 
other  spots  does  not  produce  a  sensation  of  pressure  at  all.  It  musL 
be  held,  then,  that  the  sensations  produced  by  laying  a  single 
blunted  dividers'  point  upon  the  skin,  as  in  Weber's  classical  ex- 
periment, are  really  very  complex,  and  are  composed  of  the  sensa- 
tions from  several  pressure-spots  blended  with  other  sensations 
from  the  rest  of  the  same  area  not  covered  by  the  pressure-spots. 
The  fineness  of  discrimination  possible  in  any  area  of  the  skin  de- 
pends, then,  upon  how  all  the  points  irritated  stand  related  to  the 
specific  pressure-spots.  Goldscheider  found  that  only  when  two 
irritating  points  touch  two  pressure-spots  are  they  felt  as  two.  But 

1  Archiv  f.  Anat.  u.  Physiol,  Physiolog.  Abth.,  1885,  Supplement-Band,  pp. 
1-104;  especially,  p.  84  f. 


400 


PRESENTATIONS  OF  SENSE 


when  one  of  the  points  touches  a  pressure-spot,  and  the  other  touches 
some  place  in  the  contiguous  area  of  skin  which  is  free  from  such 
spots,  the  two  points  are  not  both  felt;  in  this  case  only  the  one  rest- 
ing on  the  pressure-spot  is  felt. 

The  table  of  minimum  distances  at  which  two  points  can  be  felt 
as  two,  when  the  exact  nature  of  the  area  of  the  skin  on  which  we 
are  experimenting  is  known,  and  everything  made  as  favorable  as 
possible,  consists  of  numbers  very  much  reduced  from  those  of 
Weber.  Following  are  some  citations  from  Goldscheider's  table: 


Part  of  the  body  mm. 

Back 4-6 

Breast 0.8 

Forehead 0.5-1.0 

Cheek 0.4-0.6 

Nose  and  chin 0.3 

Upper  and  lower  arm .     .     .  0 . 5-1 . 0 


Part  of  the  body  mm. 

Back  of  hand 0.3-0.6 

I.  and  II.  phalanges  (volar)  .  0 . 2-0 . 4 

I.  and  II.  phalanges  (dorsal)  0.4-0.8 

Upper  leg 3.0 

Lower  leg 0.8-2.0 

Back,  and  sole  of  foot     .     .0.8-1.0 


§  23.  Yet  more  recent  experiments  by  Von  Frey1  and  his  col- 
laborators, Bruckner  and  Metzner,  differ  considerably  from  those 
of  Goldscheider  in  the  absolute  values  assigned  to  the  touch-spots. 
. —  When  two  points  are  applied  to  neighboring  spots,  the  threshold 
is  indeed  very  small  provided  the  points  are  applied  successively, 
but  not  when  both  are  applied  simultaneously,  as  in  the  experi- 
ments of  Weber.  It  had  previously  been  known  that  a  difference 
of  location  could  be  perceived  with  much  less  distance  between  the 
points  stimulated  when  one  was  touched  after  the  other  than  when 
both  were  touched  at  once.2  Von  Frey  and  Metzner,  applying 
stimuli  to  previously  identified  touch-spots,  concluded  that  a  dif- 
ference of  location  could  be  appreciated  between  any  two  touch- 
spots,  no  matter  how  near  together  they  might  be,  when  one  was  ex- 
cited shortly  (say  J  to  1  second)  after  the  other,  but  not  when  both 
were  excited  simultaneously.  In  the  latter  case,  the  "two-point 
threshold"  was  not  much  less  when  touch-spots  were  specifically 
touched  than  when  the  compasses  were  applied  in  the  usual  indiscrimi- 
nate manner.  The  perception  of  simultaneous  double  touch,  there- 
fore, is  evidently  a  perception  made  under  difficulties,  and  cannot 
give  an  ultimate  measure  of  the  fineness  of  the  system  of  local  signs. 
Other  facts  tend  in  the  same  direction.  If  two  points  are  simul- 
taneously applied  to  the  skin  at  a  distance  too  small  to  permit  of 
a  clear  recognition  of  twoness,  still  the  sensation  aroused  may  dif- 
fer from  that  of  a  single  point,  in  possessing  a  certain  breadth.  In 
pathological  cases,  notably  after  injury  to  the  nerves  supplying 
some  region  of  the  skin,  the  power  of  discriminating  two  points 

1  Zeitschrift  fur  Psychologie,  1901,  XXVI,  33,  and  1902,  XXIX,  161. 

2  See  Judd,  Wundt's  Philosophische  Studien,  1896,  XII,  409. 


THE   "TWO-POINT  THRESHOLD"  401 

from  one  may  be  practically  abolished,  while  nevertheless  the  power 
of  localizing  single  touches  may  remain  excellent,  being  provided 
for  by  the  subcutaneous  sense,  which  possesses  good  powers  of 
localization  but  none  of  spatial  discrimination.1 

§  24.  We  conclude,  then,  that  the  discrimination  of  two  points 
applied  to  the  skin  is  not  simply  related  to  the  system  of  local  signs, 
but  is,  as  we  should  expect,  closely  bound  up  with  other  mental 
factors.  Of  these  may  be  mentioned  attention,  practice,  sugges- 
tion, and  association.  Under  the  head  of  attention  we  note  that 
persons  unfamiliar  with  the  compass  test  suffer  much  more  than 
trained  observers  from  the  strain  to  which  they  are  subjected; 
previous  mental  or  bodily  exertion  seems  to  make  it  difficult  to 
adapt  the  attention  to  the  rather  unusual  demands  of  the  test.  For 
this  reason  this  method  has  been  regarded  by  some  as  a  suitable 
device  for  testing  mental  and  muscular  fatigue.2  The  direction 
of  attentiorfTs  also  an  important  factor  in  determining  the  threshold; 
for  if  the  observer  seeks  to  interpret  the  impression  of  breadth  that 
arises  from  two  points  which  are  applied  close  together,  he  may 
translate  this  into  terms  of  double  touch,  whereas  if  he  insists 
a  clear  impression  of  doubleness,  he  needs  to  have  the  points  much 
farther  separated.3  Such  differences  in  the  direction  of  attention 
probably  account  in  part  for  the  enormous  individual  differences 
which  seem  to  exist  in  the  fineness  of  this  kind  of  perception.  A.  W. 
Volkmann4  showed  the  remarkable  effect  of  exercise  upon  the  culti- 
vation of  the  sense  of  locality.  After  fixing  the  value  of  the  least 
perceivable  differences  of  locality  for  a  number  of  small  areas  in 
the  field  of  touch,  Volkmann  found  that  each  successive  series  of 
experiments  with  each  area  increased  its  fineness  of  perception, 
until  within  a  few  hours  twice  the  original  degree  of  fineness  could 
be  reached.  The  growth  in  perceptive  skill  of  the  skin  was  slower 
at  first  for  areas  not  ordinarily  used  for  touch;  quicker  for  those 
accustomed  to  daily  use.  The  improvement  ceased  at  a  certain 
limit,  and  was  soon  lost  by  disuse,  so  that  a  few  months  out  of 

1  Compare  Head  and  Sherren,  Brain,  1905,  XXIX,  109;  Spearman,  British 
Journal  of  Psychology,  1905,  I,  286. 

2  Griesbach,  Archiv  fur  Hygeine,  1895,  XXIV,  and  Internationales  Archiv  fur 
Schulhygeine,  1905,  I,  317.     Griesbach  introduced  the  two-point  threshold  as  an 
index  of  mental  fatigue,  especially  in  school  pupils.     He  asserts  that  the  two 
points  must  be  more  widely  separated  after  mental  work,  in  order  that  they 
may  be  felt  as  two.     Of  others  who  have  tried  the  method,  some  regard  it  as 
valid,  but  many  have  got  only  negative  results.     See  a  summary  of  this  and  other 
points  in  the  recent  literature  by  Spearman  in  Archiv  fur  die  gesammte  Psychol- 
ogic, 1906. 

3  Binet,  Annee  Psychologique,  1903,  IX,  199. 

*  Berichte  d.  sdchsischen  Gesellschaft  d.  Wissenschaften,  1858,  pp.  38  f. 


402  PRESENTATIONS  OF  SENSE 

practice  served  to  reduce  the  acquired  tact  of  any  area  to  its  origi- 
nal condition.  A  most  surprising  discovery  of  this  experimenter 
was,  that  the  practice  exclusively  of  a  member  of  the  body  on  one 
side  resulted  in  improving  the  fineness  of  touch  of  the  correspond- 
ing member  of  the  other  side.  Thus,  if  the  smallest  perceivable 
distance  for  the  tip  of  a  left  finger  was,  to  begin  with,  0.75  line, 
and  that  of  the  corresponding  place  on  the  right  finger,  0.85,  prac- 
tice with  the  left  finger  exclusively  reduced  the  distance  for  both 
fingers — for  the  left  to  0.45  line,  and  for  the  right  to  0.4. 

It  is  well  known  that  the  blind,  who  have  no  spatial  series  of 
sensations  or  presentations  of  extended  objects  by  the  eye,  attain  by 
exercise  a  high  degree  of  fineness  for  certain  space-perceptions  of 
the  skin.1  In  the  case  of  those  who  have  sight,  the  most  movable 
and  discriminating  organs  of  the  skin — such  as  the  tips  of  the  fin- 
gers— are  capable  of  being  cultivated  to  great  delicacy  of  touch; 
but  Funke2  did  not  succeed,  even  by  an  education  lasting  an  en- 
tire month,  in  reducing  the  obtuseness  of  the  skin  of  the  back  be- 
tween the  shoulder-blades  and  in  the  lumbar  region  more  than  by 
about  one-fourth. 

§  25.  These  earlier  results  are  subject  to  some  modification  in 
consequence  of  later  work.  The  rapid  improvement  with  train- 
ing is  ascribed  by  Tawney3  to  the  influence  of  suggestion,  while 
Judd  and  Von  Frey  and  Metzner  found  that  the  improvement  did 
not  occur  in  the  form  of  the  experiment  in  which  the  two  points 
are  excited  successively.  Probably  the  improvement  which  occurs 
in  the  case  of  simultaneous  application  is  due  to  the  acquisition  of 
skill  in  interpreting  the  broad  impression  produced  by  exciting  two 
near-by  points. 

The  influence  of  suggestion  and  of  chance  associations  has  been 
brought  out  clearly  by  Solomons4  and  by  Messenger,5  who,  by  syste- 
matically mistraining  an  observer,  were  able  to  induce  a  condition 
in  which  false  judgments  were  the  rule,  and  in  which  there  might 
even  be  a  complete  reversal  of  the  judgments  "one"  and  "two." 
The  fact  is  that  the  impressions  derived  from  two  points  simul- 
taneously excited  appear  in  consciousness  as  a  single,  blended  im- 
pressionj'which,  however,  differs  slightly  from  that  produced  by  the 
excitation  of  one  point;  and  the  more  distant  are  the  two  points, 
the  more  the  difference  in  the  two  impressions  increases.  When  the 
two  points  are  quite  far  apart,  it  becomes  possible  to  single  out 

1  Compare  Czermak,  Sitzgsber.  d.  Wiener  Acad.,  XVII,  Abth.  ii,  pp.  563  f. 

2  See  Hermann's  Handb.  d.  PhysioL,  III,  ii,  p.  382. 

3  Wundt's  Philosophische  Studien,  1898,  XIII,  163. 

4  Psychol.  Rev.,  1897,  IV,  249. 

8  Psychol.  Rev.,  Monog.  Suppl.,  XXII,  1903. 


SENSUOUS  BASIS  OF  DISCRIMINATION  403 

either  of  them  and  devote  attention  to  it  separately,  and  thus  the 
perception  of  two  is  clear.  When  the  points  are  too  near  to  permit 
of  either  being  singled  out  in  this  way,  it  may  still  be  true  that  the 
total  impression  of  the  two-point  stimulation  differs  perceptibly  from 
that  of  one-point  stimulation,  and  these  impressions  may  then  be 
associated  with  the  numbers  one  and  two,  so  that  correct  judgments 
of  the  stimulus  will  be  established. 

In  the  case  of  the  blind,  though  their  skill  in  judging  the  shape 
of  objects  by  touch  is  highly  developed,  it  does  not  appear  that  their 
two-point  threshold  is  specially  low.1  Their  skill  in  touch  judg- 
ments must,  therefore,  be  the  result  of  practice  in  interpreting  the 
total  impressions  derived  from  objects  of  different  size  and  shape. 

§  26.  Since  our  experience  shows  that  localization  of  the  different 
minute  areas  of  the  skin  includes  not  only  the  existence  of  discrim- 
inable  local  signs,  but  also  an  active  process  of  discrimination,  any 
physiological  theory  of  this  class  of  perceptions  must  include  some 
attempt  to  account  for  the  nervous  correlates  of  the  process  of  dis- 
crimination itself.  The  fact  that  discrimination  is  easier  when  the 
stimuli  are  presented  successively  than  when  simultaneously  holds 
good  not  only  in  the  case  here  under  consideration,  but  also  in  many 
others,  such  as  the  discrimination  of  wreights  or  of  tones.  The 
transition  from  one  stimulus  to  another  following  it  may  produce 
a  "shock  of  difference "  2  even  though  the  two  stimuli  blend  when 
applied  simultaneously.  Physiologically  considered,  this  blending 
may  be  related  to  the  convergence  and  summation  of  two  compati- 
ble stimuli  which  was  seen  to  occur  in  the  case  of  reflex  action  (see 
pp.  161  f.).  The  confluence  of  sensory  impulses  from  two  excited 
points  must  be  less  complete  in  the  case  of  perception  than  in  the 
case  of  reflex  action;  otherwise,  it  would  seem,  no  discrimination 
would  be  possible.  To  account  for  the  facts  of  the  two-point 
threshold,  Bernstein3  proposed  a  theory  which  has  much  in  its 
favor.  He  conceived  that  the  sensory  impulses  reaching,  let  us  say, 
the  somesthetic  area  of  the  brain  did  not  impinge  simply  on  a  single 
point,  but  were  distributed  over  a  certain  neighborhood — most  in- 
tensely, however,  to  a  central  point  in  this  neighborhood,  and  less 
and  less  intensely  to  more  and  more  distant  parts  of  the  same.  The 
distribution  of  impulses  from  two  near-by  points  in  the  skin  might 
therefore  overlap  in  the  brain  to  a  greater  or  less  degree.  Where 
the  overlapping  was  but  slight,  there  would  be  two  points  of  maxi- 

1  Haines,  Psychological  Review,  1905,  XII,  207. 

2  See  James,  Principles  of  Psychology,  1890,  I,  495. 

3  Untersuchungen  uber  den  Erregungsprozess  im   Muskel-   und  Nervensystem 
(Heidelberg,  1870).     See  also  Thunberg  in  Nagel's  Handbuch  der  Physiologic, 
1905,  III,  720;  and  Myers,  Textbook  of  Experimental  Psychology,  1909,  p.  235. 


404  PRESENTATIONS  OF  SENSE 

mum  activity  within  the  somesthetic  area,  and  this  condition  would 
favor  discrimination;  where  the  overlapping  was  considerable,  on 
the  contrary,  there  would  be  only  one  point  of  maximum  activity, 
and  therefore  no  possibility  of  a  true  sense  of  double  stimulation; 
though,  if  the  combined  distribution  was  broad,  the  total  impression 
might  be  recognizably  different  from  that  of  a  single  point,  with  its 
narrower  cerebral  distribution.  To  account  for  the  fact  that  two 
points  can  be  discriminated  at  a  much  less  distance  on  some  por- 
tions of  the  skin  than  on  others,  we  make  the  probable  assumption 
that  the  cortical  area  connected  with  a  highly  sensitive  region  of 
the  skin  is  broad,  and  that  connected  with  a  less  sensitive  region 
narrow,  in  comparison  with  the  corresponding  areas  of  the  skin. 
Accordingly,  the  cortical  overlapping  of  impulses  need  be  no  greater 
from  closely  adjacent  points  on  the  finger-tips  than  from  widely 
separated  points  of  the  back.  Such  a  theory  does  not  pretend  to 
explain  the  process  of  discrimination,  but  only  one  of  the  nervous 
correlates  or  prerequisites  of  discrimination;  other  conditions  also 
must  be  met  in  order  that  discrimination  may  occur,  for  two  points 
need  not  be  distinguished  though  separated  by  a  very  considerable 
distance.  On  the  psychological  side,  the  process  corresponds  to 
what,  when  exercised  in  a  more  deliberate  way,  we  call  the  "weigh- 
ing of  data"  before  "making  up  the  mind"  to  an  act  of  judgment. 

§  27.  Closely  connected  with  the  foregoing  is  the  difference  of 
different  parts  of  the  skin  in  furnishing  data  for  discriminating  the 
fact,  the  amount,  and  the  direction  of  motion  in  contact  with  the  body. 
Upon  this  point  the  experiments  of  G.  Stanley  Hall1  are  of  special 
interest.  These  experiments  seem  to  show  that  we  are  more  likely, 
when  in  doubt,  to  judge  motion  on  the  surface  of  the  limbs  to  be 
up  rather  than  down  their  axis;  on  the  breast,  the  shoulder-blades, 
and  the  back,  the  tendency  is  to  judge  motion  to  be  toward  the 
head.  The  discriminative  sensibility  of  the  skin  for  motion  is 
much  greater  than  that  for  separate  touch,  as  determined  by  We- 
ber's experiments.  Thus,  while  at  least  a  distance  of  25  mm.  be- 
tween the  dividers'  points  was  needed  on  the  volar  surface  of  the 
right  arm,  in  order  to  perceive  them  as  two  points,  both  the  fact 
and  the  direction  of  motion  could  be  discriminated  at  an  average 
distance  of  between  6  and  7  mm.  In  judging  the  rate  and  distance 
of  motion  over  the  skin  the  liability  to  error  is  always  great;  but, 
as  a  rule,  distances  rapidly  traversed  are  judged  to  be  relatively 
shorter  than  the  same  distances  more  slowly  traversed.  Inasmuch, 
however,  as  the  judgment  of  motion  on  the  left  arm  was  expressed 
by  reproducing  the  rate  and  distance  with  the  right  hand,  we  have 

1  Motor  Sensations  on  the  Skin,  by  Professor  G.  S.  Hall  and  Dr.  H.  H.  Donald- 
son, in  Mind,  October,  1885,  pp.  557  ff. 


MIXED  AND  TANGLED  SKIN  SENSATIONS         405 

a  double  liability  to  error  involved  in  regulating  the  muscular  move- 
ment of  this  hand  by  means  of  its  series  of  muscular  and  tactual 
sensations. 

Hall  found  the  motor  sensibility  of  different  parts  of  the  surface 
of  the  skin  to  be  different;  but  the  differences  do  not  appear  to 
correspond  to  those  belonging  to  Weber's  sensation-circles.  The 
average  distance,  in  millimetres,  which  a  metallic  point  of  12  mm. 
in  diameter  could  move  over  the  skin  at  a  rate  of  2  mm.  per  second 
before  a  judgment  of  direction  could  be  formed  was  found,  for  one 
subject  of  experiment,  as  follows:  forehead,  0.20;  upper  arm,  0.40; 
forearm,  0.44;  shin,  0.60;  palm,  0.74;  back,  0.85.  Motion  can  be 
produced  so  slowly  as  not  to  be  discriminated  at  all,  even  when  the 
body  in  contact  has  really  moved  from  6  to  12  centimetres.  It  can 
also  be  produced  so  rapidly  as  to  make  it  impossible  to  tell  when  it 
begins  and  when  ends.  Heavy  weights  seem  to  move  faster  than 
light  ones  going  at  the  same  rate;  but  here  other  sensations  are 
called  out  by  the  deep  pressure,  and  combined  with  those  of  con- 
tact. Hall  concludes  that  heat-spots  and  cold-spots  traversed  by 
the  moving  body  are  of  great  service  in  judging  motion  and  its  di- 
rection on  the  skin;  the  cold-spots  more  than  heat-spots,  "be- 
cause of  the  fainter  sensation  and  wider  irradiation"  of  the  latter. 

Further  experiments  with  a  travelling  metallic  point  that  carried 
the  stimulus  of  an  electrical  current  over  the  surface  of  the  skin 
showed  an  astonishing  diversity  of  sensations  developed  at  different 
points  of  the  area  thus  traversed.  Points  of  cutting  pain,  "thrill- 
points,"  "tickle-points,"  "acceleration-points"  (or  places  where  the 
rate  of  motion  seems  suddenly  to  increase  without  any  real  change 
in  the  speed  of  the  moving  metal),  "blind-points"  (or  spots  where 
all  impression  of  contact  is  momentarily  lost),  are  all  to  be  differen- 
tiated. Yet  the  sharp  differentiation  of  these  sensations  is  ren- 
dered difficult  by  the  fact  that  the  various  kinds  are  so  impacted 
and  run  together,  in  a  tangle  of  sensation.  The  experimenters  also 
speak  as  though  many  dermal  sensations  may  thus  be  partially  dis- 
entangled, for  the  description  of  which  language  furnishes  no  ade- 
quate terms.  All  these  facts  agree  exceedingly  well  with  the  theory 
of  local  signs  already  proposed.  These  dermal  signs  are  complex 
"mixtures"  of  feeling,  which  give  to  each  discernible  locality  a 
characteristic  local  stamp.  The  fact  that  our  sensibility  to  motion 
is  so  much  greater  in  each  area  of  the  skin  than  our  susceptibility 
to  the  distance  of  stationary  points  accords  with  the  same  theory. 
Our  ability  to  localize  the  dermal  sensations  is  dependent  upon  the 
degree  and  rate  of  the  changes  in  the  color-tone  of  these  sensations. 
Hall  is  undoubtedly  right  in  holding  that,  by  moving  the  touch- 
ing surface  over  the  surface  touched,  we  do  not  simply  multiply,  but 


406 


PRESENTATIONS  OF  SENSE 


also  diversify,  our  data  for  filling  up  the  dermal  blind-spots  and 
judging  the  nature  of  impressions. 

§  28.  The  localizing  of  sensations  of  temperature  in  the  skin  is, 
in  principle,  the  same  as  that  of  sensations  of  light  pressure  or  of 
motion.  The  former,  however,  are  in  all  our  ordinary  experience 
interwoven  with  the  latter;  they  therefore  have  the  help  of  the  lat- 
ter in  getting  a  place  assigned  to  them  in  the  periphery  of  the  body. 
Goldscheider1  experimented  to  determine  how  far  apart  the  heat- 
spots and  cold-spots  must  be,  respectively,  in  order  that  two  of 
them,  when  stimulated,  may  be  felt  as  two.  Both  kinds  of  sensations 
are  localized,  not  as  points,  but  as  minute  warm  or  cold  drops  in 
contact  with  the  skin.  By  the  following  table,  which  gives  the 
minimum  distances  for  different  areas  of  the  body,  it  appears  that 
the  sense  of  locality  connected  with  the  cold-spots  is  about  twice 
as  fine,  as  a  rule,  .as  that  connected  with  the  heat-spots.  The  dis- 
tances are  given  in  millimetres. 


Part  of  the  body 

Cold-spots 

Heat-spots 

Forehead,  cheek, 

0  8 

3-5 

Breast 

2  0 

4-5 

Abdomen    . 

1-2 

4-6 

Back.     .     .     . 

1  5-2  0 

4-6 

1.5-2.0 

2-3 

Lower  arm 

2-3 

2-3 

Hollow  of  the  ha] 

id  

0.8 

2.0 

Back  of  the  hand 

,  and  upper  and  lower  leg  .     .     . 

2-3 

3-4 

§  29.  Some  basis  seems  to  be  laid  in  the  foregoing  facts  for  a  sys- 
tem of  local  signs  of  the  skin,  that  consist  in  a  mixture  of  color-tones 
and  temperature-sensations.  Yet  sensations  of  heat  or  cold,  in  them- 
selves considered,  differ  chiefly,  if  not  wholly,  in  intensity.  In 
themselves,  therefore,  they  are  not  well  fitted  to  constitute  a  so-called 
"spatial  series"  of  sensations.  If,  for  example,  a  certain  area  of 
the  skin  be  stimulated  simultaneously  by  both  heat  and  cold,  at 
points  too  near  together  to  be  distinguished  by  touch,  the  result  is 
neither  a  modification  of  one  sensation  by  the  other  nor  a  localizing 
of  the  two  sensations  as  lying  closely  side  by  side.2  A  wavering  of 
perception  rather  takes  place,  similar  to  the  strife  of  colors  in  vi- 
sion; the  experience  is  as  though  the  skin  were  being  touched  with 

1  Archiv  /.  Anat.  u.  Physiol.,  Physiolog.  Abth.,  1885,  Supplement-Band,  pp. 
70  ff. 

2  See  Czermak,  Sitzgsber.  d.  Wiener  Acad.,  March,  1855,  p.  500;  confirmed  by 
Klug  and  others. 


NATURE  OF  THE  MUSCULAR  SENSE  407 

a  single  body  alternately  hot  and  cold.  Klug  also  found  that  the 
least  observable  distance  between  two  points  touching  the  skin  at 
the  same  time  depends  upon  their  temperature  relative  to  that  of 
the  skin.  The  fineness  of  our  sense  of  locality,  as  well  as  of  our 
sensitiveness  to  motion,  is  increased  by  exciting  sensations  of  tem- 
perature up  to  the  point  where  pain  intervenes.  But  the  localizing 
of  these  sensations  is  primarily  dependent,  to  a  great  extent,  upon 
their  connection  with  localized  sensations  of  touch.  If  we  bring 
two  parts  of  the  skin,  that  differ  considerably  in  temperature,  into 
contact — for  example,  a  cool  hand  and  warm  forehead,  or  a  cool  hand 
and  a  warm  one — it  is  often  difficult  by  strict  attention  to  the  sensa- 
tions of  temperature  alone  to  tell  which  part  is  cooler,  which  warmer. 
The  difficulty  is  doubtless  largely  due  to  the  fact  that  each  part 
which  feels  the  temperature  of  the  other  is  also  changing  its  own 
temperature  in  the  direction  of  the  temperature  of  the  other.  A 
confusion  of  the  data  for  judgment,  accordingly,  takes  place. 

Any  localization  of  the  sensations  which  occurs  under  such  cir- 
cumstances is  largely  dependent  upon  secondary  considerations, 
and  especially  upon  the  direction  of  the  attention.  We  judge  of 
depth  by  sensations  of  temperature,  indirectly,  and  through  our 
ability  to  remove  or  change  the  intensity  and  locality  of  these  sensa- 
tions by  changing  the  position  of  the  body  in  space  as  related  to 
what  we  know  to  be  hot  and  cold  bodies  or  surrounding  media. 

§  30.  The  specific  sensations  of  the  muscular  sense  constitute 
another  spatial  series  which  combines  with  the  foregoing  in  the 
localizing  of  areas  at  the  periphery,  and  of  external  objects  as 
projected  in  space  and  yet  known  as  in  contact  with  the  body.  In- 
deed, it  is  upon  this  particular  system  of  local  signs  that  the  mind 
is  chiefly  dependent  for  its  data — other  than  the  visual — in  the 
synthetic  construction  of  its  presentations  of  bodies  that  stand  re- 
lated to  each  other  in  three  dimensions  in  objective  space.  Three 
principal  theories  have  been  held  as  to  the  nature  of  the  so-called 
muscular  sensations:  (1)  They  are  to  be  resolved  into  "central  feel- 
ings of  innervation,"  which  differ  only  in  intensity  and  not  in  specific 
quality,  and  which  result  from  the  changes,  initiating  movement  of 
the  bodily  organs,  that  take  place  in  the  brain  as  correlated  with 
impulses  of  the  will;  (2)  they  are  not  specific  sensations,  but  are  due 
to  interpretations  of  those  feelings  in  the  skin  which  originate  on 
account  of  its  changes  of  position,  tension,  etc.,  as  the  underlying 
muscles  are  moved;  (3)  they  are  specific  sensations  dependent  on 
a  nerve-apparatus  of  sense,  which  has  its  end-organs  in  the  muscle- 
fibre,  and  which  is  excited  by  the  contraction  of  the  latter  in  a  man- 
ner dependent  upon  the  kind,  amount,  and  direction  of  the  mus- 
cular movement  taking  place. 


408  PRESENTATIONS  OF  SENSE 

We  have  already  given  certain  reasons  for  preferring  the  last  of 
the  foregoing  views;  other  reasons  are  implied  in  considering  the  nat- 
ure of  what  has  been  called  the  "  feeling  of  innervation  "  or  of '  'active 
energy."  The  muscular  sense,  like  all  the  other  senses  which  con- 
tribute to  our  presentations  of  objects  extended  in  space,  appears 
to  have  its  own  system  of  local  signs.  The  muscular  sensations  are 
qualitatively  (and  not  merely  quantitatively)  different,  according  to 
the  combination  of  the  muscles  moved,  and  according  to  the  exten- 
sion over  the  muscular  area  of  the  stimulus  imparted  to  the  sensory 
nerve-fibres  situated  in  the  muscle  by  the  changing  condition  of  the 
latter  as  it  contracts  and  relaxes.  At  each  step  in  the  flexing  of  the 
leg — for  example — the  muscular  sensations  have  a  specific  quality 
and  value  as  local  signs,  in  our  consciousness,  of  the  position  of  the 
member.  The  same  thing  is  true  of  the  bending  arm,  back,  or 
single  toe  or  finger.  These  sensations  are  indeed  intimately,  and 
even  inextricably,  combined  with  the  spatial  series  of  specifically 
dermal  sensations;  but  in  themselves  they  have  a  different  quality, 
and  are  not  localized  simply  at  the  surface  of  the  body.  As  the  ex- 
tent of  the  circuit  of  motion  gone  through  by  any  limb  increases,  or 
the  intensity  of  the  strain  becomes  greater,  the  quality  of  the  mass  of 
resulting  muscular  sensations  is  perpetually  changing.  These 
sensations  are,  accordingly,  localized  over  a  broader  area  of  the  body 
and  deeper  in  its  substance,  as  it  were.  Every  one  knows  what  new 
mixtures  of  sensation  are  produced  in  consciousness  by  calling  into 
vigorous  exercise  the  unused  more  deeply  lying  muscles  of  the  body. 

The  muscular  sensations  also  assist  the  more  strictly  tactual  in 
discriminating  locality  for  all  cases  where  the  pressure  upon  the 
skin  exceeds  a  certain  small  degree  of  intensity.  In  strong  contact 
or  heavy  pressure  the  sensory  nerves  of  the  underlying  muscle  are 
excited;  we  have  the  feeling,  not  simply  of  being  touched,  but  also 
of  being  pressed.  The  combination  of  these  two  spatial  series 
gives  to  the  mind  a  doubly  constituted  system  of  local  signs;  hence, 
as  the  experiments  of  G.  Stanley  Hall1  show,  our  judgment  of  di- 
rection of  motion  is  quicker  as  the  weight  resting  on  the  skin  is 
increased  up  to  the  limit  where  other  disturbing  sensations  inter- 
vene. The  superior  discriminating  power  which  any  member  of 
the  body  has  when  permitted  to  move — that  is,  to  call  forth  fa- 
miliar series  of  muscular  sensations — is  largely  due  to  the  help 
which  the  local  signs  of  this  system  render  to  the  mind.  When 
the  particular  member  (the  hand)  which  is  capable  of  the  nicest 
tactual  discrimination  is  also  permitted  to  move  over  an  object 
freely,  and  to  acquire  abundant  data  from  all  the  sources  described 
above,  we  have  fulfilled  the  most  advantageous  conditions  for  the 
lMind,  October,  1885,  p.  567. 


JUDGMENT  OF  BODILY  MOVEMENTS  409 

utmost  nicety  of  knowledge  possible  to  "touch,"  in  the  widest 
meaning  of  the  word. 

§  31.  In  point  of  precision,  judgments  of  extent  of  bodily  move- 
ment far  surpass  judgments  of  extent  based  on  cutaneous  sensation 
alone,  but  are  inferior,  in  their  turn,  to  judgments  of  extent  by  the 
eye.  The  perception  of  extent  of  movement  is  one  among  several 
judgments  which  may  be  passed  on  the  movement  of  a  limb,  since 
the  direction,  duration,  and  speed  of  the  movement,  the  resistance 
encountered,  and  the  end-positions  of  the  limb  can  all  be  judged 
with  considerable  accuracy.  The  perception  of  any  one  of  these 
characteristics  of  a  movement  is  usually  confused  by  introducing 
irregularities  into  any  other  of  them.  Thus,  movements  by  different 
limbs,  or  by  the  same  limb  in  different  positions  or  directions,  can- 
not be  so  accurately  compared  in  regard  to  their  extent  as  when 
the  two  movements  to  be  compared  are  as  nearly  as  possible  dupli- 
cates of  each  other  in  all  respects.  Systematic  or  "constant"  er- 
rors often  creep  in  when  judgments  of  extent  are  attempted  in  com- 
paring diverse  movements.  Thus,  a  slow  movement  seems  longer 
than  a  fast  movement  of  equal  extent.1  But,  curiously  enough,  a 
movement  made  against  resistance  seems  no  longer  than  a  free 
movement  of  the  same  extent.2  This  last  result  seems  to  prove 
that  the  extent  of  the  movement  of  a  limb  is  not  judged  in  terms  of 
the  muscular  work  performed  in  executing  it;  while  the  close  in- 
terrelation of  the  speed  of  a  movement  and  its  apparent  extent, 
along  with  other  facts,  has  been  thought  to  indicate  that  extent  was 
judged  in  terms  of  the  duration  of  a  movement.  On  this  point 
opinions  have  differed  widely,  but  there  seems  to  be  no  cogent 
reason  for  singling  out  the  duration  of  a  movement  as  that  on  which 
the  judgment  of  extent  is  based.  It  is  more  probable  that  the  judg- 
ment of  extent,  like  that  of  duration  and  also  of  speed,  is  based  di- 
rectly on  the  entire  complex  of  sensations  which  is  produced  by 
moving  the  limb.3 

§  32.  The  fineness  of  the  spatial  perception  connected  with  the 
"muscle-sense"  is  of  great  significance  in  forming  a  just  concep- 
tion of  "touch"  as  an  organ  of  space-perception.  Seldom,  in  the 
common  use  of  touch  for  discovering  the  form  and  size  of  ob- 
jects— as  in  the  dark — do  we  attempt  to  rely  on  cutaneous  im- 
pressions alone;  we  handle  the  object,  bringing  into  play  sensations 
of  movement  and  position  as  well  as  sensations  from  the  skin.  Such 


Pfliiger's  Archiv  fur  die  gesammte  Physiologic,  1887,  XLVI,  1;  Dela- 
barre,  Ueber  Bewegungsempfindungen  (Freiburg  i.  B.,  1891). 

2Delabarre,  op.  cit.;  Angler,  Zeitschrift  fur  Psychologic,  1905,  XXXIX,  429. 
3  Compare  Hollingworth,  The  Inaccuracy  of  Movement,  1909,  p.  40. 


410  PRESENTATIONS  OF  SENSE 

perceptions  are  not  tactile  alone,  but  "stereognostic."  1  The  per- 
ception of  the  shape  of  objects  simply  laid  or  pressed  on  the  skin 
is  much  inferior  to  the  judgment  based  on  handling  them.  Hence 
there  seems  to  be  sufficient  evidence  for  our  view,2  which  is  also 
that  of  Wundt3 — namely,  that  "touch  space''  in  so  far  as  it  can  be 
conceived  as  independent  of  visual  space,  results  from  a  union,  fusion, 
or  synthesis  of  cutaneous  sensation  with  sensations  of  bodily  move- 
ment and  position. 

§33.  It  is  unnecessary  to  illustrate  further  the  process  by  which 
the  mind's  native  activity  of  discrimination,  with  the  help  of  quali- 
tatively different  sensation-complexes,  or  local  signs,  constructs  its 
field  of  touch.  The  localization  of  certain  points  in  the  area  of  the 
body  which  are  of  marked  local  characteristics,  and  frequently  re- 
current in  experience,  is  the  first  achievement  in  constructing  this 
field.  To  these  landmarks,  as  it  were,  other  points  or  areas,  sub- 
sequently discovered,  are  referred.  One  hand  learns  to  know  the 
other;  the  right  hand  chiefly  explores  the  left  arm  and  side  and  the 
upper  right  leg;  the  left  hand,  the  right  arm  and  side  and  the  upper 
left  leg.  The  finger-tips,  especially  of  the  right  hand,  have  an 
office  similar  to  that  performed  by  the  yellow-spot  of  the  retina; 
they  are  the  centre  or  hearth  of  clear  perceptions  of  touch.  But  in 
order  to  bring  them  to  their  object  they  must  be  moved;  through  this 
motion  fresh  combinations  of  muscular  and  tactual  sensations  re- 
sult. But  long  before  the  entire  field  of  touch  has  been  constructed 
with  any  considerable  approach  to  completeness,  the  eye  has  al- 
ready explored  those  parts  of  the  body  which  are  open  to  its  in- 
spection. It  learns  first  to  know  the  hand,  which  nature  keeps 
constantly  in  motion  before  it.  As  objects  rest  on  the  hand,  it 
notes  the  place  where  they  rest;  with  its  perceptions  of  sight  cer- 
tain combinations  of  tactual  sensations  thus  become  associated.  As 
the  hand  moves  over  other  objects,  or  especially  over  the  other  parts 
of  the  body,  the  eye  marks  its  successive  progress;  combined  sen- 
sations of  muscular  and  tactual  kind  are  thus  associated  with  each 
position  of  the  hand  and  with  each  area  of  the  body  which  it  touches. 
Very  early  in  the  development  of  a  normal  experience  the  eye  comes 
to  be  the  leader  and  critic  of  the  discriminations  connected  with 
the  muscular  and  tactual  sensations.  Its  power  of  rapid  movement 
over  its  total  field,  and  its  delicate  judgment  on  account  of  the 
finely  shaded  complex  local  signs  which  it  calls  forth  with  a  com- 
prehensive simultaneousness,  give  a  great  superiority  to  the  organ 
of  vision  as  a  geometrical  sense.  The  results  of  such  superiority  it 

1  Hoffmann,  Stereognostische  Versitche  (Strassburg,  1883). 

2  Compare  Ladd,  Elements  of  Physiological  Psychology,  1st  ed.,  1887,  417  f. 

3  Physiologische  Psychologic,  6th  ed.,  1910,  II,  517. 


FEELINGS  OF  DOUBLE  CONTACT  411 

constantly  places  at  the  disposal  of  the  more  slowly  moving  and 
less  delicate  sense  of  touch.  For  this  reason,  one  born  blind  can 
never  attain  the  same  quality  (of  "comprehensive  simultaneous- 
ness")  for  his  spatial  intuitions  and  ideas  of  spatial  relations;  even 
the  field  of  touch,  in  spite  of  the  greater  refinement  which  the 
muscular  and  tactual  sensations  of  such  an  unfortunate  person  ac- 
quire through  use,  cannot  possess  this  quality  as  it  is  imparted  by 
the  eye. 

The  familiar  experiments  of  trying  to  estimate  the  size,  shape, 
and  relation  of  objects,  the  amount  and  direction  of  motion,  etc., 
when  blindfold,  show  our  dependence  upon  the  organ  of  sight.  It 
must  not  be  forgotten,  however,  that  the  discriminations  possible 
through  the  muscular  and  tactual  sensations  alone  are  wonderfully 
exact;  and  that  in  certain  circumstances  touch  has  sight  at  a  dis- 
advantage, as  it  were.  Thus  the  player  on  the  violin  who  should 
adjust  his  spacing  of  the  strings  by  the  sensations  of  the  eye,  with 
the  unaccustomed  and  unfavorable  perspective  made  necessary  by 
its  position  in  relation  to  the  left  hand,  would  not  attain  the  art  of 
making  true  and  pure  tones. 

§  34.  Among  the  most  complex  perceptions  of  which  the  skin 
and  muscles  by  their  combined  action  are  capable  are  the  so-called 
"feelings  of  double  contact."  It  is  largely  by  means  of  these  feel- 
ings that  skill  is  acquired  in  the  use  of  tools,  weapons,  and  musi- 
cal instruments.  In  these  cases  the  process  of  projection  goes  so 
far  that  we  seem  to  feel  the  object  with  which  the  implement  is  in 
contact,  not  so  much  in  the  hand  (the  feelings  of  contact  being 
located  there),  by  the  external  means  of  the  implement,  but  rather 
as  ourselves  being  in  the  implement  and  using  it  as  a  sentient  part 
of  the  organism.  The  carver  in  wood  feels  his  chisel  move  through 
the  stuff  he  is  shaping,  and  guides  it  as  unerringly  as  he  would  his 
finger,  so  as  to  lay  it  with  a  given  degree  of  pressure  upon  a  given 
spot.  We  are  all  familiar  with  the  experience  of  feeling  the  ground 
we  are  about  to  tread,  with  a  cane  or  other  stick.  If  the  fingers  be 
lightly  brushed  over  the  hair  when  it  stands  out  from  the  head,  it 
will  be  difficult  to  localize  the  sensations  of  pressure  at  the  scalp 
rather  than  in  the  hair.  We  feel  the  touch  of  our  finger  at  the  end 
of  the  tooth,  where  the  contact  takes  place,  instead  of  where  the 
sensory  nerves  really  receive  the  stimulus  and  convert  it  into  a 
nerve-commotion. 

The  management  of  the  implement  is,  of  course,  really  made 
possible  by  delicate  changes  in  the  shades  of  feeling  called  out  by 
its  changing  pressure  upon  the  nerves  terminating  in  the  skin  and 
muscles  of  the  hand,  and  by  the  accompanying  feelings  of  strain 
and  of  effort  that  result  from  the  movement  of  the  arm  which 


412  PRESENTATIONS  OF  SENSE 

carries  the  hand.  These  feelings  are  aroused  by  the  end  of  the 
implement  which  is  in  contact  with  the  body,  and  are  primarily 
localized  in  that  part  of  the  body;  but  they  are  felt  through  a  more 
artificial  and  elaborate  process  of  localization,  as  though  directly 
dependent  upon  the  other  end  of  the  implement.  Upon  the  aes- 
thetic and  pleasurable  uses  of  these  feelings  of  double  contact  Lotze1 
has  remarked  at  length. 

At  this  point  the  further  discussion  of  the  development  of  our 
presentations  of  sense  in  general  must  be  arrested,  in  order  to  con- 
sider more  in  detail  the  activities  of  the  other  great  "geometrical 
sense." 

1  See  Microcosmus,  i,  pp.  586  ff.  (Edinburgh,  1885). 


CHAPTER  V 

PRESENTATIONS  OF  SENSE;    OR  SENSE-PERCEPTIONS  (CONTINUED) 

§  1.  The  discussions  of  the  last  chapter  as  to  the  data  furnished  in 
the  form  of  complex  sensations,  and  as  to  the  mental  activities  in- 
volved in  the  discrimination,  association,  and  interpretation  of  these 
data,  for  the  localization  and  knowledge  of  objects  of  sense  through 
the  skin  and  the  muscles,  must  have  convinced  us  that  the  problem 
which  nature  solves  with  such  apparent  ease  in  a  practical  way,  is 
exceedingly  difficult — perhaps  impossible — of  a  complete  theoretical 
solution.  Moreover,  the  application  of  the  general  principles  which 
control  the  development  of  our  sense-experience  to  the  particular 
case  of  the  eye  has  many  peculiar  difficulties.  The  physiological 
psychology  of  visual  perception  is,  therefore,  a  much  controverted 
and  very  obscure  domain.  This  fact  is  doubtless  in  part  due  to 
the  amount  of  experimenting  and  speculating  which  has  been  be- 
stowed upon  it;  but  peculiar  difficulties  are  intrinsic  in  the  case  of 
the  eye.  These  are  caused  by  the  great  complexity  of  its  native  ac- 
tivities, and  by  the  speed  with  which  it  reaches  a  generous  maturity! 
of  development.  Nature  has  equipped  this  organ  with  superior 
means  for  furnishing  to  the  mind  a  variety  of  data,  as  respects  both 
quantity  and  quality,  for  the  nicest  discriminations;  it  has  also  pro- 
vided it  with  such  constant  stimulation  as  to  cause  it  to  acquire  an 
incomparable  facility.  The  character  of  its  structure,  functions, 
and  development  is,  therefore,  such  as  to  make  it  difficult  to  dis- 
entangle the  simple  factors  from  those  complex  forms  into  which 
the  synthetic  activity  of  the  mind  has  constructed  them. 

It  is  affirmed  by  one  authority1  that  no  less  than  eight  different 
data,  or  motifs,  are  used  even  in  monocular  vision  by  the  adult  for 
perceiving  the  third  dimension  of  space  and  of  visual  objects  in 
space.  These  are  the  changes  with  respect  to  (1)  extent  and  (2) 
clearness,  of  the  complex  of  the  sensations  of  color  and  light,  as  de- 
pendent on  distance;  (3)  the  perspective  elevation  of  the  bottom 

1  Volkmann  von  Volkmar,  Lehrb.  d.  Psychologic,  II,  p.  84. 
413 


414  PRESENTATIONS  OF  SENSE 

of  distant  objects  above  the  horizon;  (4)  the  covering  of  known  dis- 
tant objects  by  those  placed  nearer;  (5)  the  alterations  of  light  and 
shadow  on  the  curved  surfaces  of  the  object,  according  as  they  are 
nearer  or  more  remote;  (6)  the  perspective  contraction  of  the  retinal 
image;  (7)  the  change  of  the  visor  angle  in  proportion  to  the  dis- 
tance of  the  object;  (8)  the  muscular  sensations  of  the  accommoda- 
tion of  the  eye.  To  these  eight  data,  two  others  at  least  must  be 
added  for  binocular  vision — namely  (9),  the  stereoscopic  double 
images,  and  (10)  the  sensations  arising  from  convergence  of  the 
axes.  These  ten  sets  of  variable  experiences  may  be  combined,  of 
course,  in  an  almost  infinite  variety  of  proportions. 

Moreover,  it  is  not  improbable  that  we  shall  have  to  admit  still 
other  data  as  entering  into  the  complex  perceptions  of  sight.  The 
question  must  be  raised:  Do  not  the  visual  sensations  themselves 
have  a  certain  local  coloring  directly  dependent  upon  the  nervous 
elements  of  the  retina  which  are  excited  by  the  stimuli?  If  we 
answer  this  question  affirmatively,  we  shall  have  a  system  of  local 
retinal  signs  as  constituting  one  of  the  most  primary  of  the  spatial 
series  of  sensations  entering  into  the  space-perceptions  of  this  sense. 

§  2.  Several  of  the  data  just  enumerated,  however,  are  plainly  of 
only  secondary  rank  and  value;  they  do  not  necessarily  enter  into 
every  perception  of  a  visual  object  as  such.  What  does  seem  neces- 
sary to  the  most  elementary  form  of  visual  perception  may  be  stated 
as  follows:  Sensations  of  light  and  color,  differing  in  intensity  and 
quality,  but  simultaneously  present  in  consciousness,  must  be  syste- 
matically arranged  with  reference  to  each  other  by  being  localized  with 
the  help  of  retinal  signs.  For  any  development  of  visual  percep- 
tion as  the  adult  human  being  has  experience  of  it,  we  must  add: 
These  sensations  must  be  associated  with  other  spatial  series  of  mus- 
cular and  tactual  sensations  that  arise  from  accommodation  of  the 
eye  and  from  its  position  and  motion.  The  complexity  of  the  com- 
binations arising  in  the  normal  use  of  the  organ  of  vision  is,  of  course, 
increased  by  the  fact  that  there  are  two  eyes,  and,  therefore,  two 
retinas  with  their  systems  of  retinal  signs,  two  images  of  each  object, 
and  two  sets  of  motions.  But  the  two  eyes  are  (as  we  shall  see  sub- 
sequently) in  a  certain  sense  to  be  regarded  as  one  eye — certainly 
as  constituting  one  organ  of  vision.  So  that,  even  when  one  eye  is 
closed,  the  other  does  not  see  what  it  sees  without  being  influenced 
by  the  closed  and  relatively  inoperative  part  of  the  one  organ.  The 
constancy  with  which  the  eyes  act  together  explains,  in  part,  why 
they  are  one  organ  as  the  two  hands  are  not;  but  the  frequency  with 
which  we  voluntarily  suppress  the  activity  of  one  eye  by  closing  it 
explains,  in  part,  why  they  are  not  one  organ  as  are  the  two  nostrils 
or  the  two  ears. 


DATA  OF  VISUAL  PERCEPTIONS  415 

§  3.  Could  we  select  an  adult  human  being  who  had  never  seen, 
and  proceed  to  develop  his  visual  perceptions,  experimentally,  in 
the  direct  order  of  their  complexity,  we  might  possibly  rely  upon 
his  description  of  his  experience  to  solve  certain  problems  that  now 
seem  unsolvable.  At  present,  however,  it  is  quite  impossible  to 
say  what  the  experience  of  the  subject  of  such  experiments  would 
be.  Nothing  remains,  then,  but  to  employ  the  data  which  physi- 
ological optics  has  secured,  in  order  to  make  a  theoretic  reconstruc- 
tion (confessedly  imperfect  and  doubtful)  of  the  process  that  nature 
is  all  the  while  successfully  completing.  In  this  effort  we  naturally 
follow  the  order  of  nature,  so  far  as  possible;  beginning  with  the 
simplest  conceivable  case  (this  is  substantially  the  course  followed 
by  Wundt),  we  find  three  things  to  be  considered  in  explaining  the 
developed  perceptions  of  sight:  (1)  The  retinal  image  of  the  eye 
at  rest,  and  the  motifs  which  it  furnishes;  (2)  the  single  eye  as 
moved,  and  the  influence  of  these  movements;  (3)  the  conditions 
furnished  by  the  existence  and  relations  of  the  two  eyes  exercis- 
ing their  functions  in  common.  But,  in  reality,  from  the  very  be- 
ginning of  its  activity  the  eye  is  in  motion,  and  acts  as  a  double  organ. 

Corresponding  to  the  three  sets  of  considerations  just  mentioned, 
we  may  speak  of  three  fields  of  vision  which  are  to  be  constructed 
in  the  order  of  their  complexity.  They  may  be  called,  respectively, 
the  retinal  field  of  vision,  the  field  of  monocular  vision,  and  the 
field  of  binocular  vision.  In  the  "retinal  field  of  vision"  we  mean 
to  include  only  such  a  perception — or  mental  spatial  arrangement 
of  sensations  of  color  and  light  as  points  lying  side  by  side — as 
would  be  presented  through  the  excited  expanse  of  nervous  ele- 
ments constituting  the  retina  of  one  motionless  eye,  in  case  there 
had  been  no  previous  vision  with  both  eyes  in  motion.  The  field 
of  monocular  vision,  when  completely  constructed,  includes  all  that 
can  be  seen  with  one  eye  as  the  result  of  its  experience,  devel- 
oped, but  unaided  by  the  other  eye.  The  field  of  binocular  vision 
includes  all  that  can  be  seen  by  both  eyes.  The  first  two  so-called 
"fields  of  vision"  are,  strictly  speaking,  fictitious  and  theoretically 
constructed  in  order  to  explain  the  process  by  which  the  mind 
reaches  the  construction  of  the  third  and  last.  Indeed,  the  ques- 
tion may  be  pressed,  whether  we  can  speak  of  a  purely  "retinal 
field  of  vision,"  and  whether  the  excited  mosaic  of  nervous  elements 
on  which  the  image  is  formed,  without  aid  from  previous  experi- 
ence of  sensations  of  position  and  motion,  could  furnish  any  true 
presentations  of  sight,  or  visual  perceptions  of  objects  as  they  actu- 
ally exist  in  space. 

§  4.  The  "retinal  field"  has  no  clearly  defined  limits,  or  bound- 
ary-lines; it  may  be  described  rather  as  having  its  expanse  of  sensa- 


416  PRESENTATIONS  OF  SENSE 

tions  distinguished  by  a  shifting,  graded  transition  into  a  region  of 
no-sensations.  This  fact  is,  of  course,  due  to  the  constantly  chang- 
ing activity  of  the  nervous  elements  of  the  retina.  Yet  the  sensa- 
tions which  are  massed  in  the  foregoing  experience  constitute  a 
true  spatial  expanse;  that  is,  they  are  not  simply  recognized  as  differ- 
ing in  color-tone,  or  brightness  or  intensity  of  effect,  but  as  having 
true  local  distinctions,  and  as  being  arranged  into  a  system  of  points 
of  color  and  light  lying  side  by  side.  In  other  words,  the  different 
sensations  do  not  fall  together  in  consciousness  so  as  to  resemble 
the  one  sensation  of  smell  produced  by  irritating  simultaneously  a 
number  of  fibres  of  the  olfactory  nerve;  nor  are  they  simply  ana- 
lyzable  into  several  qualitatively  different  factors,  as  is  the  com- 
plex sensation  of  a  musical  clang.  They  are  presented  as  spatially 
systematized.  The  "retinal  field" — at  least,  as  it  appears  in  adult 
vision — may,  then,  be  said  to  be  extended  in  two  dimensions;  and 
the  minima  msibilia  which  compose  it  all  have  local  relations  to 
each  other.  It  cannot  properly  be  said,  however,  to  have  depth 
(as  Stumpf1  and  Hering2  hold  that  it  does);  for  the  different  col- 
ored points  are  not  projected  as  different  in  distance,  nor  can  we 
be  said  to  look  into  the  colored  space  thus  presented  before  the 
mind.  It  is  true  that  the  expanse  of  the  retinal  field  is  not  like 
that  of  a  darkly  colored  wall  or  curtain  placed  in  front  of  the  eye. 
But  the  quasi-appearance  of  depth  is  due  to  constant  change  in 
color-tone  and  brightness  of  the  minute  portions  of  the  field,  which 
has  an  effect  somewhat  like  that  we  get  on  looking  at  a  very  dense 
mist  of  particles  differently  colored  and  drifting.  In  other  words, 
the  secondary  and  derived  data  give  to  it  an  appearance  which  we 
have  learned  to  associate  with  the  perception  of  depth. 

On  the  other  hand,  the  retinal  field  can  scarcely  be  considered 
as  two-dimensional  in  a  way  to  distinguish  it  from  the  three-dimen- 
sional field  of  binocular  vision,  without  implying  a  suggestion  of 
the  third  dimension,  as  though  it  were  itself  seen  projected  in  a 
three-dimensional  space.  The  only  safe  conclusion  from  all  this 
is,  therefore,  that  this  simplest  ( ?)  form  of  adult  visual  experience 
is  so  far  removed  from  the  form  with  which  its  own  development 
began,  as  to  be  no  adequate  representation  of  it.  In  particular, 
it  leaves  the  question  of  the  influence,  and  even  of  the  absolute 
necessity,  of  sensations  of  position  and  motion  for  any  visual  per- 
ception, still  unsolved 


1  Ueber  d.  physiolog.  Ursprung  d.  Raumwrstellung  (Leipzig,  1873).     Stumpf 
holds  that  "Space  is  just  as  originally  and  directly  perceived  as  quality"  (p. 
115). 

2  In  Hermann's  Handb.  d.  Physiol,  III,  i,  pp.  572  f. 


FORMATION  OF  THE  RETINAL  FIELD  417 

§  5.  Further  experiment,  however,  with  this  so-called  "retinal 
field"  serves  to  show  how  complicated  its  apparently  simple  char- 
acter really  is.  In  the  first  place,  even  this  field  is  the  result  of 
the  combined  action  of  the  two  retinas.  If,  with  both  eyes  closed, 
a  "phosphene"  be  produced  in  either  eye  by  pressing  upon  its 
ball,  the  colored  circle  will  be  located  in  the  corresponding  part  of 
the  field;  but  the  character  of  the  entire  field,  as  formed  by  the  ac- 
tivity of  both  retinas,  will  be  changed.  It  is,  of  course,  impossible 
to  suppress  the  action  of  one  retina,  and  thus  examine  a  monocular 
"retinal  field,"  as  it  were.  But  it  may  easily  be  shown  that,  even 
in  vision  with  one  eye  open  and  in  motion,  the  character  of  the  whole 
field  of  vision  is  under  the  influence  of  the  retinal  activity  of  the 
closed  eye.  Let  one  of  the  eyes — both  hitherto  closed  and  motion- 
less— now  be  opened.  Immediately  a  picture  of  all  the  objects 
falling  within  the  field  of  monocular  vision  appears  before  us;  each 
object  seen  with  its  position,  magnitude,  and  spatial  relations  de- 
termined according  to  the  laws  of  visual  perception.  This  monocu- 
lar field  seems  bounded  on  one  side  (the  left  side  if  the  right  eye 
is  opened,  the  right  side  if  the  left)  by  the  rather  dim  outline  of  the 
nose  and  lower  line  of  the  forehead.  What  has  become  of  the  retinal 
field  of  the  closed  eye  ?  It  has  been  submerged  or  overwhelmed  by 
the  field  of  the  open  eye,  on  account  of  the  latter's  stationary  and 
clearly  defined  images  and  strong  arrest  and  fixation  of  attention. 
But  if  a  character  to  arrest  and  fix  the  attention  be  given  to  the  field 
of  the  closed  eye,  it  may  be  made  in  turn  to  overwhelm  that  of  the 
open  eye.  This  can  be  accomplished  by  producing  strong  "phos- 
phenes"  in  the  former.  On  pressing  the  closed  eye  brightly  col- 
ored circles  are  presented  in  the  corresponding  part  of  the  field;  and 
by  using  sufficient  pressure  the  objects  seen  as  projected  in  space 
by  the  open  eye  are  drowned  in  a  shower  of  minute,  vivid  sparks. 

The  "retinal  field"  has  its  character  determined  also  by  as- 
sociated muscular  sensations  dependent  upon  the  movement  of 
both  eyes.  It  will  be  found  impossible  to  make  any  definite  area 
of  this  retinal  field,  which  lies  much  to  the  right  or  left,  to  the  upper 
or  lower  part,  of  its  centre,  a  matter  of  regard  without  detecting 
slight  movements  of  the  eyes  according  to  the  direction  in  which 
the  attention  is  to  be  fixed.  The  value  of  muscular  movements  in 
this  case  cannot  consist  in  their  enabling  a  clear  image  of  objects 
situated  in  different  relations  to  the  eye  to  be  formed  on  its  retina; 
for  with  closed  eyes  no  change  is  occasioned  in  the  retinal  images 
by  motion  of  the  eyes.  Moreover,  it  will  be  found  that  the  extent 
of  this  entire  field  and  its  prolongation,  as  it  were,  in  any  given  di- 
rection are  dependent  upon  the  accommodation  and  motion  of  the 
eyes. 


418  PRESENTATIONS  OF  SENSE 

§  6.  The  foregoing  facts  undeniably  afford  considerable  support 
to  the  "empiristic"  theory  of  visual  perception;  but  they  do  not 
show  that  the  considerations  it  brings  forward  are  entirely  con- 
clusive. It  would  seem  that,  after  excluding  all  the  factors  which 
combine  into  our  ordinary  presentations  of  sight — such  as  double 
images,  accommodation,  convergence  of  the  axes  of  the  eyes,  and 
secondary  helps  by  way  of  shadows,  perspective,  elevation,  etc. — 
a  certain  spatial  quality  still  remains  to  the  simplest  sensations  of 
color  and  light  which  we  are  able  to  reproduce.  It  will  naturally 
be  objected  that  these  sensations  are  the  reactions  of  a  mind  that 
has  had  a  long  previous  experience  in  localizing  visual  sensations 
by  means  of  just  such  helps  as  the  foregoing.  The  question  then 
recurs:  Is  the  fact  that  the  sensations  of  light  and  color,  which  are 
produced  by  the  simultaneous  excitation  of  many  nervous  elements 
of  the  retina,  appear  as  locally  distinct  (even  when  the  eyes  are  closed 
and  motionless)  an  otherwise  unexplained  datum  due  to  an  original 
activity  of  the  mind  under  the  law  of  the  specific  energy  of  these 
nervous  elements;  or  is  it  a  result  of  acquired  experience,  to  be 
explained  by  the  revival  of  images  of  previously  associated  impres- 
sions obtained  when  the  eyes  were  both  open  and  moving?  To 
take  the  former  position  is  to  adopt,  so  far  forth,  the  nativistic 
theory  of  visual  perception;  to  take  the  latter  is  to  espouse  the 
empiristic  opinion.  Either  position  has  its  difficulties.  The  for- 
mer seems  to  us,  however,  nearer  to  the  ultimate  truth. 

§  7.  That  the  sensations  of  light  and  color  occasioned  by  stimu- 
lating different  elements  of  the  retina  have  a  different  value  in 
consciousness,  and  that  the  recognition  of  this  value,  and  the  pres- 
entation of  the  sensations  as  locally  separate  and  arranged  into 
a  spatial  system,  is  native  to  the  mind,  may  be  argued  from  the 
following  among  other  reasons:  The  peculiar  mosaic  structure  of 
the  retina  is  obviously  the  fundamental  cause  for  the  pre-eminence 
of  the  eye  as  a  "geometrical  sense."  It  has  already  been  shown 
(chap.  VIII,  §  15)  that  each  element  of  this  structure  may  be  regarded 
as  an  isolated  sensitive  spot,  which  corresponds,  on  the  one  side,  to 
individual  irritations  from  the  stimuli,  and,  on  the  other,  to  the 
smallest  localized  sensations  of  light  and  color.  But  the  latter  part 
of  this  statement  could  not  be  true  unless  each  of  the  elements  in 
this  nervous  mosaic  had  a  certain  peculiar  representative  value  in 
consciousness.  In  other  words,  sensations  of  light  and  color  are 
localized  in  part,  at  least,  by  means  of  the  specific  local  quality 
which  belongs  to  the  result  of  the  different  points  in  the  retina 
being  simultaneously  irritated.  The  very  construction  of  this  or- 
gan, as  well  as  the  correspondence  between  its  construction  and 
the  nicety  attained  in  its  use  for  local  distinctions,  indicates  that 


VALUES  OF  DIFFERENT  RETINAL  SENSATIONS    419 

the  spatial  quality  of  our  visual  percepts  depends  upon  its  specific 
functions. 

Moreover,  unless  the  series  of  light-  and  color-sensations  had  an 
original  spatial  character,  it  is  difficult  to  see  how  they  could  com- 
bine with  the  other  spatial  series  of  the  eye  into  perceptions  of  ex- 
tended colored  objects.  It  is  difficult  to  see  what  advantage  they 
would  then  have  over  the  series  of  musical  tones  varying  in  pitch. 
Still  further,  it  is  as  impossible  to  prove  experimentally  as  it  is  to 
make  seem  true  to  consciousness  that  the  arrangement  of  the  points 
of  light  and  color  which  appears  before  us  with  closed  and  motion- 
less eyes  is  only  the  residuum,  as  it  were,  of  past  sensations  of  a  mus- 
cular kind.  Such  an  appeal  to  consciousness  could  not  be  made, 
indeed,  with  any  confidence,  if  scientific  analysis  were  able  to  show 
that  the  color-sensations  can  be  perceived  simultaneously,  as  a  sys- 
tem of  points  lying  side  by  side,  without  having  the  characteristics 
of  a  spatial  series.  But  in  view  of  our  inability  to  do  this,  we  only 
account  for  the  facts  of  consciousness  by  admitting  what  the  very 
structure  of  the  organ  suggests,  and  what  general  psychological 
theory  seems  to  confirm,  when  we  hold  that  spatial  perception,  at 
least  in  germinal  form,  is  native  to  the  mind  as  a  synthesis  of  the 
qualitatively  different  sensations  which  result  from  stimulating  simul- 
taneously the  retinal  mosaic  of  nervous  elements. 

The  foregoing  view  is  very  different  from  that  which  assumes 
that  we  have  an  immediate  knowledge  of  the  retinal  image;  or 
that  a  knowledge  of  the  direction  from  which  the  light  falls  upon 
the  retina  is  an  unresolvable  intuition  of  the  mind.  To  such  mis- 
taken statements  it  is  a  sufficient  reply  to  show  that  the  subjective 
image  (or  mental  presentation)  of  the  object  does  not  correspond 
either  to  the  image  on  the  retina  or  to  the  real  object  as  it  is  other- 
wise known  to  exist  in  space.  The  mental  presentation,  for  exam- 
ple, has  no  blind-spot;  it  is  a  different  representation  of  the  real 
object  from  that  offered  by  the  retinal  image,  with  more  inaccu- 
racies than  belong  to  the  latter  as  seen  by  an  observer  looking 
at  it  from  without.  To  the  question,  then,  whether  sensations  of 
light  and  color  would  have  space-form  if  they  came  only  from  an 
excited  but  motionless  retina,  and  were  uncombined  with  other 
sensations  of  a  spatial  series,  we  can  give  only  a  tentative  and  par- 
tial answer.  Doubtless  the  "presentations  of  sense"  formed  by 
combining  such  sensations  alone  would  be  indescribably  different 
from  those  to  which  we  now  ascribe  visual  space-form.  An  animal 
with  a  single  immovable  expanse  of  nervous  elements  susceptible 
to  irritations  from  light  could  not  be  said  to  have  what  we  call 
"vision."  But,  on  the  one  hand,  the  spatial  quality  which  belongs 
to  the  visual  sensations  of  man  cannot  all  be  resolved  into  muscu- 


420  PRESENTATIONS  OF  SENSE 

lar  and  tactual  sensations  of  eye  and  hand;  and  on  the  other  hand, 
sensations  of  light  and  color  do  have  the  quality  which  insures 
their  arrangement  in  consciousness  in  spatial  order.  This  fact  is 
due  to  the  working  of  the  law  of  the  specific  energy  of  the  nervous 
retinal  elements  in  connection  with  the  native  activity  of  mind  in 
synthesizing  these  sensations.  The  law  as  applied  to  the  eye  is 
essentially  the  same  as  that  already  demonstrated  for  the  skin;  the 
activity  assumed  as  native  to  man  is  not  essentially  different  from 
that  ascribed  to  the  lower  animals  in  the  use  of  their  senses.  That 
this  tact  for  the  individual  has  been  largely  won  by  the  development 
of  the  race  is  a  proposition  to  which  our  attitude  is  determined  by 
more  general  conclusions.  But  physiological  optics  cannot  account 
for  the  phenomena  of  vision  without  assuming  both  the  original 
exercise  of  this  tact  and  the  theory  of  local  retinal  signs  as  data 
hitherto  unresolvable  by  its  analysis. 

§  8.  Whatever  may  be  thought  of  the  foregoing  assumptions,  it 
is  certain  that  ordinary  adult  visual  perception  involves  the  motion 
of  the  open  eye — monocular  vision,  of  one  eye  and  binocular  of 
both.  The  sensations  which  accompany  such  motion  must  be  com- 
bined with  sensations  of  light  and  color  to  make  the  complete  pres- 
entations of  sight.  The  consideration  of  the  simplest  case  requires 
that  we  should  recur  to  the  physiology  of  the  eye.  Only  one  small 
spot  in  the  retina  (the  so-called  "fovea  centralist  see  p.  193),  is 
capable  of  giving  a  perfectly  clear  image  of  an  object.  When,  then, 
we  desire  to  see  an  object  clearly,  we  bring  its  image  upon  this 
spot  and  fixate  it  there.  That  point  of  the  object  to  which  the 
centre  of  the  retinal  area  of  clearest  vision  corresponds  is  called 
the  "point  of  regard"  (or  "fixation-point").  In  ordinary  vision, 
then,  the  eye  constantly  changes  its  point  of  regard,  and  so  brings 
successively  upon  its  most  sensitive  area  the  images  of  the  different 
points  of  its  object. 

The  different  changes  of  position  in  the  point  of  regard  are  ac- 
complished by  the  six  muscles  of  the  eyeball.  This  wandering  of 
the  point  of  regard  over  an  object  may  be  considered  as  due  to 
rotating  the  eye  upon  a  pivotal  point,  or  "centre  of  rotation,"  by 
motions  that  have  different  axes  of  rotation.  The  centre  of  rota- 
tion is,  however,  only  theoretically  a  point,  but  is  really  an  inter- 
axial  space.  It  has  been  variously  located  for  normal  eyes  at  about 
13.45-13.73  mm.  behind  the  cornea,  and  1.24-1.77  mm.  behind 
the  middle  of  the  optical  axes.  Of  such  axes  of  rotation,  three  are 
especially  to  be  distinguished — an  antero-posterior,  a  vertical,  and 
a  transverse.  A  line  drawn  from  the  centre  of  rotation  to  the  point 
of  regard  is  called  the  "line  of  regard";  since  each  eye  has  its  own 
centre  of  rotation,  there  are,  in  vision  with  both  eyes,  two  lines  of 


MUSCLES  AND  MOTIONS  OF  THE  EYE-BALL      421 


obl.ssup. 


regard.  A  plane  passing  through  these  two  lines  is  called  "the 
plane  of  regard"  (or  "plane  of  vision").1  In  the  "primary  posi- 
tion" the  head  is  erect  and  the  line  of  regard  directed  toward  the 
distant  horizon.  The  plane  passing  through  the  lines  of  regard 
of  both  eyes  in  this  position  is  the  "primary  plane  of  vision."  In 
this  position  for  most  eyes,  however,  the  line  of  vision  is  inclined 
somewhat  below  the  horizontal  plane. 

Starting  from  the  primary  position,  one  set  of  positions  are  suc- 
cessively assumed  by  moving  the  eye  upon  its  transverse  and  ver- 
tical axes.  When  the  eye  rotates 
round  the  former,  the  line  of  regard 
is  displaced  either  above  or  below; 
it  thus  makes  a  varying  angle  with 
the  line  corresponding  to  its  first 
direction,  and  this  is  called  the 
"angle  of  vertical  displacement" 
(so  Helmholtz),  or  the  "ascensional 
angle."  When  it  moves  about  the 
vertical  axis,  the  line  of  regard  is 
displaced  from  side  to  side,  and 
forms  with  the  median  plane  of 
the  eye  a  varying  angle  called  "the 
angle  of  lateral  displacement."  In 
passing  from  the  primary  position 
to  the  foregoing  secondary  position 
no  rotation  of  the  axis  itself  oc- 
curs. Another  order  of  positions  is 
assumed  by  an  apparent  rotation 
on  the  anteroposterior  axis,  com- 
bined with  lateral  or  vertical  dis- 
placements; this  movement  results 
in  bringing  the  eye  to  an  oblique 
position,  and  is  really  a  torsion  of  the  eye.  The  angle  which  the 
plane  of  regard  makes  with  the  transverse  plane  measures  the 
amount  of  torsion,  and  is  called  the  "angle  of  torsion." 

§  9.  The  law  which  seems  to  govern  the  eye's  movements  of 
torsion — or  combined  movements  sideways,  and  either  up  or  down 
— was  conjectured  by  Listing,  whose  name  it  bears,  and  elaborated 
by  Helmholtz.  Listing's  law  is  stated  by  Helmholtz2  in  the  fol- 

1  For  the  detailed  theory  of  the  movements  of  the  eye,  see  Hering,  in  Her- 
mann's Handb.  d.  Physiol.,  III,  i,  chaps.  IX-XI;  Helmholtz,  Physiolog.  Optik, 
§  27;  Wundt,  Physiolog.  Psychologic  (6th  ed.),  II,  548;  and  Zoth  in  NagePs  Handb. 
d.  Physiol.  (1905),  III,  282  ff. 

2  Physiolog.  Optik,  p.  466. 


FIG.  122. — Diagram  of  the  Attachments 
of  the  Muscles  of  the  Eye,  and  Their 
Axes  of  Rotation— the  latter  being 
shown  by  dotted  lines.  The  axis  of 
rotation  of  the  rectus  externus,  and 
internus,  being  perpendicular  to  the 
plane  of  the  paper,  cannot  be  shown. 


422  PRESENTATIONS  OF  SENSE 

lowing  terms:  "When  the  line  of  regard  passes  from  its  primary 
position  into  any  other  position,  the  torsion  of  the  eye  (as  meas- 
ured by  the  angle  of  torsion)  in  the  second  position  is  the  same  as 
if  the  eye  were  turned  about  a  fixed  axis  standing  perpendicular  to 
both  the  first  and  the  second  positions  of  the  line  of  regard."  The 
same  principle  is  stated  in  different  language  by  Wundt:1  "All 
movements  of  the  eye  from  its  primary  position  take  place  about 
fixed  axes,  each  of  which  at  the  point  of  rotation  stands  at  right 
angles  to  the  plane  which  is  described  by  revolving  the  line  of  re- 
gard; and  all  of  these  axes  lie  in  a  single  plane,  at  right  angles  to 
the  primary  position  of  the  line  of  regard,  at  its  point  of  rotation." 
The  orientating  of  the  eye,  then,  for  every  possible  position  of  the 
line  of  regard,  may  be  referred  to  a  constant  standard. 

More  detailed  statement  of  the  laws  of  the  eye's  motion  in  vision 
is  not  necessary  for  the  purposes  of  physiological  psychology.  It 
needs  only  to  be  noted  that  the  construction  of  the  field  of  monocular 
or  binocular  vision  is  a  synthetic  mental  achievement  dependent  upon 
the  varying  sensations  which  result  from  the  wandering  of  the  point  of 
regard  over  the  outline  of  an  object.  Starting  from  its  primary  posi- 
tion, the  eye  may  come  around,  as  it  were,  by  a  variety  of  circui- 
tous paths,  to  the  fixation  of  any  particular  point  of  its  object.  In 
the  pursuit  of  these  paths  it  develops  various  sensations  which  are 
fitted  to  combine  into  a  spatial  series  of  sensations  that  have  the 
value  of  "local  signs."  Thus  the  field  of  vision  necessarily  has  the 
same  form  as  the  surface  over  which  the  point  of  regard  can  be  made 
to  wander.  Its  construction  is  a  progressive  synthesis  of  the  mind, 
stimulated  and  guided  by  means  which  consist  in  varying  states  of 
consciousness. 

§  10.  Certain  important  consequences  follow  as  to  the  relation 
between  the  lines  of  the  extended  and  objective  "thing"  and  the 
lines  of  the  retinal  image,  as  affording  the  mind  data  for  the  spatial 
ordering  of  the  sensations  that  arise  from  stimulating  the  nervous 
elements  of  the  eye.  Only  those  objects  which  are  seen  by  direct 
vision  (their  images  lying  in  the  line  of  regard  when  the  eye  is  in  its 
primary  position)  appear  in  their  actual  place;  lines  lying  outside 
of  the  vertical  and  horizontal  meridians  of  the  retina,  in  order  to  be 
seen  straight,  must  be  really  bent;  and  all  really  straight  lines  in 
such  positions  are  seen  bent.  This  fact  may  be  proved  in  various 
ways.  If  a  sheet  of  white  paper,  having  a  black  dot  in  its  centre 

1  Physiolog.  Psychologie  (2d  ed.),  II,  pp.  79  f.  In  the  sixth  edition,  the  form  of 
statement  is  somewhat  changed,  so  as  to  indicate  that  the  various  positions  which 
the  eye  assumes  are  the  same  as  they  would  be  if  reached  by  movements  about 
the  axes  so  defined.  The  actual  movements  of  the  eye  from  the  primary  posi- 
tion often  show  considerable  irregularity. 


EFFECTS  OF  ACCOMMODATION  423 

to  serve  as  a  point  of  regard,  be  held  at  right  angles  to  the  line  of 
vision,  with  the  eye  in  its  primary  position  and  constantly  fixed 
upon  this  point,  thin,  straight  slits  of  black  paper  outside  of  the  two 
meridians  will  appear  bent.  Or  if  the  after-images  left  on  these 
meridians  of  the  retina  by  light  falling  through  narrow  and  straight 
slits  be  studied  when  torsion  of  the  eye  takes  place,  these  after- 
images will  themselves  be  found  to  suffer  torsion. 

Besides  the  help  from  sensations  due  to  movements  of  the  eye 
in  fixing  its  point  of  regard,  account  must  be  taken  of  those  which 
may  possibly  result  from  accommodation  of  the  eye  (for  the  mech- 
anism of  accommodation,  see  pp.  188  f).  According  to  Helmholtz:  * 
"There  can  be  no  doubt  that  any  one  who  has  much  observed  his 
own  changes  of  accommodation  and  knows  the  muscular  feeling 
of  the  effort  belonging  to  them  is  in  a  condition  to  tell  whether, 
when  he  fixates  an  object  or  an  optical  image,  he  is  accommodating 
for  a  great  or  small  distance."  There  is  scarcely  greater  doubt  that 
the  significance  of  these  changes  would  not  be  realized  as  indicat- 
ing a  third  dimension  of  space,  were  they  not  combined  with  sen- 
sations belonging  to  the  use  of  both  eyes  in  conjunction  with  the 
organs  of  touch.  Even  adult  judgment  of  distance,  by  accommoda- 
tion alone,  is  extremely  imperfect.  Wundt2  experimented  to  de- 
termine the  niceriess  of  this  judgment  by  regarding  a  black  thread 
stretched  vertically  against  a  white  background,  with  one  eye 
through  an  aperture  in  a  shield.  He  found  that  almost  nothing 
could  be  told  in  this  way  as  to  the  absolute  distance  of  the  thread. 
Its  relative  position,  however,  could  be  discriminated  with  consider- 
able accuracy  by  changes  in  accommodation;  and,  as  might  be  ex- 
pected, with  more  accuracy  when  the  apparatus  was  called  into 
more  active  operation  by  approach  of  the  object  toward  the  eye. 
We  return  to  this  subject  later. 

§  11.  But  all  the  achievements  possible  to  a  single  eye,  when  open 
and  in  motion,  would  not  avail  to  produce  the  presentations  of 
sight  as  our  ordinary  experience  is  familiar  with  them.  Strictly 
monocular  vision  is  for  the  most  part  a  fiction  of  science.  What 
we  can  see  with  one  eye,  after  experience  in  binocular  vision,  de- 
pends upon  what  we  have  been  accustomed  to  see  with  both  eyes. 
Indeed,  what  we  see  at  any  instant  with  one  open  eye  depends,  in 
part,  upon  the  position,  motion,  and  retinal  condition  of  the  other  and 
closed  eye.  A  theory  of  binocular  vision,  however,  requires  the  con- 
sideration of  two  sets  of  data  in  addition  to  those  already  enumerated. 
These  are  the  existence  and  relations  of  the  two  retinal  images,  and 
the  relations  and  laws  of  the  binocular  movements  of  the  two  eyes. 

1  Physiologische  Optik,  633. 

2  Beitrdge  zur  Theorie  d.  Sinneswahrnehmung,  pp.  105-118  (1862). 


424 


PRESENTATIONS  OF  SENSE 


The  fact  that  two  eyes  are  ordinarily  active,  and  that  there  are, 
therefore,  two  images  of  the  object,  is  a  fact  of  the  first  importance 
for  the  theory  of  visual  perception.  Each  eye  is  in  itself,  indeed, 
a  complete  optical  instrument;  each  has  its  own  point,  line,  and 
plane  of  regard,  and  movements  of  rotation,  torsion,  and  accom- 
modation. The  two  eyes,  however,  act  normally  as  one  instrument; 
and  yet  they  cannot  be  regarded  as  mere  duplicates.  The  theory 
of  binocular  vision,  then,  considers  the  two  eyes  acting  as  one.  For 
the  purposes  of  such  theory  it  is  not  important  what  shape  the  two 

retinas  are  regarded  as  having;  they 
are  usually  taken  as  surfaces  with 
the  curvature  of  the  inside  of  a 
sphere  whose  centre  lies  at  a  point 
where  all  the  lines  of  direction  inter- 
sect. It  may  be  assumed,  to  begin 
with,  that  this  point  of  intersection  is 
the  same  for  accommodation  to  all 
distances  of  the  object.  If  the  two 
retinas  were  perfectly  symmetrical 
all  the  nervous  elements  which  com- 
pose the  mosaic  of  each  one  might 
be  regarded  as  situated  at  points 
identical  with  those  occupied  by  the 
nervous  elements  of  the  other.  In 
other  words,  the  surfaces  of  the  two 
retinas  might  be  regarded  as  capa- 
ble of  being  perfectly  superimposed. 
Upon  such  retinas,  when  the  eyes 
were  parallel,  each  single  point  of  an  object  would  have  its  image 
formed  upon  two  "identical"  points  of  the  two  retinas — upon 
points,  that  is,  whose  position  would  be  mathematically  the  same 
with  relation  to  the  centre  of  each  retina. 

But  the  retinas  are  not  symmetrical,  and  the  physiological  centre 
is  not  the  true  mathematical  centre;  moreover,  the  eyes,  to  be  of  use, 
must  act  together  in  other  positions  than  that  called  "primary."  A 
distinction  must  then  be  made  between  corresponding  points  and 
identical  points;  the  former  are  such  as  are  found  by  experiment 
actually,  as  a  rule,  to  act  together  and  to  combine  their  images 
when  simultaneously  stimulated.  But,  further,  in  certain  cases  the 
points  of  the  retinas  which  customarily  act  together  do  not  so  act; 
points  not  exactly  corresponding  sometimes  cover  each  other,  and 
points  usually  corresponding  sometimes  fail  to  cover  each  other. 
Hence,  a  distinction  may  be  made  between  corresponding  points 
and  "covering  points";  the  latter  term  being  used  for  those  points 


FIG.  123.— Diagram  to  illustrate  the 
theory  of  corresponding  retinal 
points.  The  images  of  objects  at 
a"  or  b"  or  c"  will  fall  on  corre- 
sponding points  of  the  retina— a 
and  a1,  6  and  ft1,  c  and  c1— and  be 
seen  single. 


CONDITIONS  OF  BINOCULAR  VISION  425 

whose  impressions,  in  each  individual  case  of  seeing,  are  actually 
referred  to  one  and  the  same  point  of  the  object.1  The  two  points 
of  regard  of  the  two  eyes  are  in  all  cases  identical,  corresponding, 
and  covering. 

Scarcely  more  than  a  reference  to  previous  elaborate  attempts  to 
determine  the  corresponding  points  of  the  two  retinas  is  necessary 
for  our  purpose.  Experiment  shows  that  considerable  reciprocal 
substitution  takes  place  among  the  different  points  of  both  retinas. 
The  eyes  of  most  persons,  if  not  of  all,  are  both  structurally  and 
functionally  incongruous.  When  the  lines  of  regard  lie  parallel  in 
the  plane  of  the  horizontal  meridian  of  the  two  retinas,  the  verti- 
cal meridians  do  not  correspond.  A  vertical  meridian  of  the  left 
eye,  with  its  upper  end  inclined  to  the  left,  may  be  conjoined  with 
a  vertical  meridian  of  the  right  eye  that  has  its  upper  end  inclined 
at  about  the  same  angle  to  the  right.  The  image  of  a  line  which 
lies  on  these  meridians  thus  inclined  appears  in  the  vertical  horizon 
of  the  field  of  vision  and  divides  it  into  a  right  and  a  left  half. 

§  12.  That  objects  are  ordinarily  seen  as  single  when  their  images 
are  formed  on  corresponding  points  of  the  retinas,  and  otherwise  as 
double,  may  be  shown  by  many  familiar  experiments.  If  we  hold 
a  finger  before  the  eyes  and  look,  not  at  it,  but  at  the  wall  or  the 
sky;  or  if  we  point  it  at  some  distant  object,  and  keep  our  eyes 
steadily  fixed  on  the  object — two  transparent  images  of  the  finger, 
rather  than  one  solid  finger,  will  be  seen.  Many  persons  may  have 
difficulty  in  seeing  the  two  images,  but  none  will  fail  to  notice  their 
transparent  character.  Under  these  circumstances  the  wall,  sky, 
or  distant  object  may  readily  be  seen  through  the  finger.  By  ex- 
perimental methods  the  images  of  a  single  object  may  be  dissociated 
and  what  is  really  one  be  seen  as  two;  on  the  other  hand,  images  com- 
ing from  two  objects  may  be  combined  upon  corresponding  points, 
and  thus  what  is  really  two  be  seen  as  one.  It  needs  only  a  little 
skilful  pressure  upon  one  eyeball  to  create  for  us  the  double  of  each 
one  of  a  group  of  friends,  and  to  see  one  body  partially  through  the 
transparent  image  of  another.  If  two  objects  very  similar — for 
example,  the  two  forefingers — be  held  a  little  way  apart  at  about  a  foot 
distant  and  against  a  clear  sky,  three  like  objects,  one  solid  and  two 
transparent,  may  be  made  to  appear  by  combining  the  two  middle 
images  and  dissociating  the  two  on  the  outside.  Two  systems  of 
regularly  recurring  similar  objects — such  as  a  regular  small  pattern 
of  carpet  or  wall-paper,  or  the  diamond-shaped  spaces  of  a  wire 
grating — may  have  all  their  images  combined  by  slipping  them,  as 
it  were,  simultaneously  to  one  side.  There  is,  then,  a  double-seeing 

1  Wundt,  Physiolog.  Psychologie  (6th  ed.),  II,  639. 


426 


PRESENTATIONS  OF  SENSE 


of  what  is  really  single  and  a  single-seeing  of  what  is  really  double; 
but  the  latter  is  much  rarer  than  the  former,  and  usually  occurs 
only  when  brought  about  for  purposes  of  experiment. 

§  13.  It  is  obvious  that  the  relations  of  the  two  images  of  an  ob- 
ject cannot  remain  unchanged  when  the  eyes  are  moved  from  their 
primary  position.  When  the  eyes  are  converged  upon  an  object, 
the  images  which  are  formed  on  the  central  spots  of  the  two  retinas, 

by  rays  coming  from  the  point 
of  regard,  are  exactly  identical 
and  corresponding;  the  object 
in  this  case  is  therefore  seen 
absolutely  single.  Points  of  the 
object  lying  near  to  the  point 
of  regard  in  any  direction,  and 
thus  having  their  images  formed 
close  to  the  centres  of  the  two 
retinas,  are  also  seen  single. 
For  the  points  of  the  retinas 
on  which  the  images  are  then 
formed,  although  not  strictly 
identical,  are  corresponding; 
that  is,  they  have  habitually 
acted  together  in  seeing  objects 
single  by  binocular  vision,  and 
the  slight  incongruousness  of 
the  two  sets  of  images  is  dis- 
regarded, as  it  were,  by  the 

•    j       T>    f       11     ^,'ppfa    Ivmo- 
mma-      *>Ut     &11     ODJ6CCS     lying 

nearer   or    more   remote  than 

,  i  •    ,     n       ,    j    i         ,  i 

the   point    fixated    by    the    eyes 

*^re  liable  to  be  Seen  double* 
„  ,  .  ,  „  ,, 

for  their  images  do  not  fall  on 
«•      corresponding  points  of  the  ret- 

inas. Objects  lying  below  or  above,  or  to  one  side  or  the  other,  of 
the  point  of  regard,  do  not,  as  a  rule,  have  their  images  formed 
on  corresponding  points;  they  may,  therefore,  also  be  seen  double. 
Some  of  these  points,  however,  which  occupy  positions  below  or 
above,  to  the  one  side  or  the  other,  of  the  point  of  regard,  are  seen 
single.  The  sum  of  all  the  points  which  are  seen  single  while  the 
point  of  regard  remains  the  same  is  called  the  horopter. 

§  14.  We  must  now  advance  to  the  consideration  of  the  factors 
which  enter  into  the  construction  of  the  field  of  so-called  "three- 
dimensional  space."  Here  the  value,  and  even  the  necessity,  of 
combining  sensations  due  to  movements  of  the  organism  with  those 


FIG.  124.—  (From  Hering).—  f  f,  the  sash  of  the 
window,  and  p,  the  black  spot  fixated.     On 

the  left  line  of  vision  i  b  lies  a  distant  object, 

and  on  the  right  line  r  e  another  object.  The 
images  of  6  and  e,  as  well  as  the  image  of  p, 
fall  on  the  place  of  direct  vision  and,  there- 
fore,  on  corresponding  points  of  the  two 

retinas. 


VISUAL  PERCEPTION  OF  DEPTH  427 

more  obscure  local  signs  whose  existence  is  required  for  the  explana- 
tion of  the  retinal  field,  become  obvious  beyond  all  doubt. 

The  numerous  factors  which  contribute  to  the  perception  of 
depth  and  relief  may  be  grouped  into  three  classes:  (1)  sensations 
of  the  position  of  the  eyes;  (2)  parallax  between  the  two  eyes  as  due 
to  movements  of  the  head  and  body;  and  (3)  associative  aids,  such 
as  can  be  utilized  to  give  an  appearance  of  relief  to  a  flat  drawing 
or  painting.  We  consider  them  briefly  in  the  order  just  given: 
(1)  Kinesthetic  sensations.  Since,  in  accommodating  the  lens  of 
the  eye  for  different  distances,  the  ciliary  muscle  assumes  various 
degrees  of  contraction,  the  sensory  evidence  of  its  contracted  or 
relaxed  condition  may  give  an  indication  of  the  distance  of  the  ob- 
ject for  which  the  lens  has  been  accommodated.  This  indication 
is  hardly  available,  however,  except  for  distances  within  a  few  feet 
of  the  eye.  Kinesthetic  impressions  of  the  degree  of  convergence 
of  the  eyeballs,  according  to  the  distance  of  the  object  upon  which 
they  were  converged,  should  be  available  for  greater  distances,  per- 
haps even  up  to  100  feet.  As  has  been  said  (p.  423),  experiment 
shows1  that,  when  other  aids  to  accurate  perception  of  depth  are 
excluded,  the  judgments  due  to  this  means  become  very  inaccurate. 
It  will  not,  therefore,  account  for  the  accuracy  ordinarily  attainable. 

(2)  In  considering  the  aids  to  perception  of  depth  grouped  under 
the  head  of  "Parallax,"  we  have  to  distinguish  binocular  parallax 
with  unmoved  eyes  or  head,  from  parallax  (essentially  monocular) 
as  due  to  movements  of  the  head  or  of  the  whole  body.  Since  the 
eyes  look  from  slightly  different  positions,  they  obtain,  as  has  al- 
ready been  shown  (p.  424),  different  views  of  the  same  (tridimen- 
sional)  object.  If  the  visual  object  is  close  to  the  face,  the  differ- 
ence between  the  two  views  is  great;  but  as  the  object  retreats 
further  and  further,  the  view  obtained  by  both  eyes  becomes  more 
and  more  nearly  the  same.  We  should  expect,  then,  that  binocular 
parallax  would  be  of  importance  in  perceiving  distance  and  relief 
when  the  objects  are  near  at  hand;  and  the  actual  importance  of 
this  factor  is  proved  by  means  of  the  binocular  stereoscope.  If 
drawings  or  photographs  are  made  of  an  object  from  two  points  of 
view  that  are  separated  by  the  distance  between  the  two  eyes,  and 
if  then  the  drawing  from  the  more  rightward  point  of  view  is  placed 
before  the  right  eye,  and  that  from  the  more  leftward  before  the 
left  eye,  and  the  two  views  combined  by  aid  of  the  stereoscope,  the 
appearance  of  depth  is  much  intensified.  For  proving  the  impor- 
tance of  binocular  parallax,  the  most  important  stereoscopic  views 

1  Wundt,  Beitrdge  z.  Theorie  d.  Sinneswahrnehmung,  1862;  Hillebrand,  Zeitschr. 
f.  Psychol,  1894,  VII,  97  ;  Bourdon,  La  perception  visuelle  de  I'espace,  1902;  and 
others. 


428  PRESENTATIONS  OF  SENSE 

are  those  which  consist  of  bare  outline  drawings  of  prisms,  pyra- 
mids, dodecahedrons,  etc.,  because  in  such  views  there  is  an  ab- 
sence of  the  associative  factors  which  contribute  to  the  perception 
of  depth.  The  small  amount  of  line-perspective  in  these  drawings 
does  not  alone  suffice  to  make  the  drawings  appear  solid,  except 
when  they  are  binocularly  combined. 

The  "pseudoscopic"  effect  also  is  important  in  this  connection: 
for  if  the  views  appropriate  to  the  right  and  left  eyes  are  interchanged, 
then,  in  the  absence  of  associative  factors,  the  relief  is  reversed;  and, 
even  if  the  associative  factors  are  retained,  as  when  the  right  and 
left  views  of  the  landscape  are  interchanged  on  a  stereoscope  slide, 
a  certain  degree  of  pseudoscopic  effect  is  received.  The  appearance 
of  relief  in  the  absence  of  associative  factors,  and  even,  to  some  de- 
gree, in  opposition  to  these  factors,  is  good  evidence  of  the  real- 
ity of  binocular  parallax  as  an  important  aid  to  the  perception  of 
depth.  On  the  other  hand,  the  diminution  of  the  pseudoscopic 
effect  when  parallax  has  to  contend  with  associative  factors  is  evi- 
dence of  the  value  of  the  latter. 

A  more  exact  conception  of  the  "different  views"  of  the  same  ob- 
ject which  are  obtained  by  the  two  eyes  may  be  reached  as  follows. 
Suppose  two  points  to  lie  in  the  same  line  extending  from  the  eye, 
which  may  be  called  the  line  of  sight.  If  the  eyes  are  directed 
on  the  nearer  point,  the  farther  one  will  be  seen  double;  if  the 
eyes  are  fixed  on  the  farther  point,  the  nearer  will  be  seen  double. 
The  disparate  images  of  the  farther  point,  when  the  nearer  is  fix- 
ated, are  called  "  homonymous, "  because  the  image  which  appears 
to  the  right  is  that  of  the  right  eye;  but  when  the  farther  point  is 
fixated,  the  double  images  of  the  nearer  point  are  called  "heter- 
onymous,"  because  the  image  which  appears  to  the  right  is  that  of 
the  left  eye.  If,  now,  it  can  be  assumed  that  the  difference  between 
homonymous  and  heteronymous  double  images  is  a  difference  which 
affects  perception — though  it  is  not  a  difference  of  which  we  are 
consciously  aware — then  this  difference  would  enable  the  observer 
to  know  whether  a  doubly  seen  point  were  nearer  or  farther  from 
the  eye  than  the  point  on  which  the  eyes  are  fixed. 

The  accuracy  of  the  perception  of  depth,  by  use  of  binocular 
parallax,  is  very  great  for  objects  lying  near  the  eye.  Helmholtz1 
measured  the  accuracy  of  this  judgment  of  distance  by  a  simple  ex- 
periment, in  which  he  fixed  three  pins  upright  in  little  blocks,  and 
endeavored  to  adjust  them  in  a  line  at  right  angles  to  the  line  of 
sight — screening  the  base  of  the  pins  and  the  table  on  which  they 
were  manipulated,  so  as  to  exclude  aids  derived  from  perspective. 
When  the  line  of  the  pins  was  340  millimetres  distant  from  his  eyes, 
1  Physiologische  Optik,  p.  644. 


VISUAL  PERCEPTION  OF  DEPTH  429 

and  the  pins  12  millimetres  apart,  he  found  that  he  never  committed 
an  error  as  great  as  J  millimetre.  Bourdon1  repeating  Helmholtz's 
test,  with  the  line  of  pins  placed  two  metres  away  from  the  eyes, 
found  the  error  (in  judging  whether  the  middle  pin  was  at  the  same 
distance  as  the  two  end  pins)  to  be  always  less  than  2.5  millimetres, 
and  seldom  to  exceed  1.5  millimetres.  These  judgments  of  dis- 
tance are  over  500  times  as  precise  as  can  be  made  by  use  of  con- 
vergence and  accommodation  alone.2 

Parallax  due  to  movements  of  the  head  has  the  advantage  that 
the  head  can  move  through  a  greater  lateral  distance  than  that 
which  separates  the  eyes;  and  thus  can  receive  views  of  an  object 
which  differ  more  than  do  those  of  binocular  parallax.  There  is 
an  offsetting  disadvantage,  in  that  the  two  views  are  not  received 
simultaneously,  and  so  are  not  combined  in  that  direct  and  proba- 
bly instinctive  manner  which  enables  binocular  parallax  to  convey 
an  apparently  immediate  impression  of  the  third  dimension.  But 
there  is  one  way  in  which  the  parallax  due  to  head  movement  is 
probably  utilized — a  way  emphasized  by  Helmholtz,3  and  tested  by 
Bourdon.4  It  is  as  follows:  If  we  suppose  the  head  moved  to  the 
right,  with  the  eyes  remaining  fixed  in  their  sockets,  then  all  objects 
are  displaced  to  the  left  in  the  field  of  view,  and  the  nearer  they  lie 
to  the  eye  the  greater  is  their  backward  displacement.  Since,  how- 
ever, the  eyes  do  not  remain  fixed  in  their  sockets  during  such  a 
head  movement,  but  instead  remain  fixated  on  some  visible  object, 
this  object  must  remain  at  the  centre  of  clear  vision;  while,  on  the 
contrary,  objects  nearer  than  it  move  backward  in  the  field  of  view, 
and  objects  farther  off  than  it  move,  in  the  field  of  view,  in  the 
direction  of  the  head's  movement.  This  motion  of  objects  within 
the  field  of  view  is  very  easily  and  minutely  perceptible,  and  is 
capable  of  giving  a  perfect  indication  of  the  relative  distance  of 
objects  in  the  line  of  sight.  The  question  is,  how  far  this  source  of 
information  is  actually  employed  in  ordinary  perception.  Bourdon 
finds  that,  in  attempting  to  judge  of  distance  with  one  eye,  and  with- 
out the  aid  of  the  associative  factors,  the  observer  does  make  in- 
voluntary sideward  movements  of  the  head,  and  that  his  judgment 
of  distance  is  thus  much  improved. 

(3)  We  now  come  to  consider  the  so-called  "associative"  factors. 
Under  this  head  belong  the  space  in  the  field  of  vision  which  is  oc- 
cupied by  an  object  of  known  size;  the  linear  perspective;  haze  and 
other  atmospheric  effects;  shadows  and  the  covering  of  one  object 

1  Rev.  philos.,  1900,  XXV,  74. 

2  See  O.  Zoth,  in  NagePs  Handbuch  der  Physiologic,  1905,  III,  415. 

3  Physiologische  Optik,  p.  634. 

4  Op.  cit.,  p.  286. 


430  PRESENTATIONS  OF  SENSE 

by  another  and  nearer  object;  and  the  general  make-up  of  the  field 
of  view.  In  comparing  the  distances  of  objects,  all  of  which  are 
over  a  mile  or  two  away,  we  are  dependent  entirely  on  these  associ- 
ative factors.  One-eyed  individuals,  also,  must  be  dependent  on 
them,  and  on  parallax  due  to  movements  of  the  head.  Since  one- 
eyed  individuals  do  not  show  a  noticeable  deficiency  in  the  percep- 
tion of  distance  under  ordinary  conditions,  and  since  a  person  may 
even  become  blind  in  one  eye  without  being  aware  of  the  fact,  it 
is  clear  that  very  good  indications  of  the  third  dimension  are  pro- 
vided by  the  associative  factors,  in  connection  with  movement- 
parallax.  It  is  probable  enough  that  the  factors  here  classed  as 
associative  are  of  much  influence  even  in  normal  binocular  vision. 
The  significance  and  value  of  these  associative  factors  will  be  further 
illustrated — especially  when  we  come  to  consider  certain  classes  of 
the  errors,  and  illusions,  that  are  so  frequent  in  visual  perception. 
Since  these  factors  bring  into  prominence  the  interpretative  function 
of  the  mind  in  all  perception — that  is,  the  presence,  in  the  lowest  and 
earliest  stages  of  the  mental  life,  of  the  working  of  the  so-called 
faculties  of  attention,  discrimination,  association,  etc. — such  errors 
and  illusions  have,  hitherto,  oftener  than  not,  been  laid  to  the  fault 
of  "the  judgment." 

§  15.  A  fact  of  profound  and  far-reaching  psychological  meaning 
comes  to  the  surface  repeatedly  in  considering  the  factors  on  which 
judgment  of  distance  and  relief  is  based.  The  fact  is  this:  We  are 
often  unaware  of  these  factors,  taken  by  themselves,  and  are  even 
unable  to  become  aware  of  them  directly,  though  their  reality  can 
be  demonstrated  by  their  effects.  We  are  unconscious,  for  example, 
of  the  existence  of  two  fields  of  view,  one  due  to  each  eye;  and,  in 
the  case  of  double  images,  we  are  unable  to  tell  by  introspection 
which  belongs  to  the  right  eye  and  which  to  the  left.  It  may  even 
happen  that  an  object  is  visible  only  to  one  eye — as,  for  example, 
when  the  fingers  of  both  hands  are  held  before  the  eyes  in  a  sort  of 
lattice-work — and  yet  we  cannot  tell  with  which  eye  it  is  seen,  or 
whether  it  is  seen  with  both.  Notwithstanding  this  inability  to 
distinguish  between  the  contributions  of  the  two  eyes,  the  facts  of 
stereoscopic  and  especially  of  pseudoscopic  ^vision  show  that  bi- 
nocular perception  of  depth  depends  on  some  sort  of  (physiological) 
distinction  between  the  complex  nervous  impulses  coming  from  the 
two  eyes.  To  this  corresponds  the  distinction  (psychological)  of 
the  complexes  of  resulting  sensations. 

Again,  perception  of  the  distance  of  a  familiar  object  and  also 
perception  of  the  size  of  an  unknown  object,  when  its  distance  is 
known,  are  both  dependent  on  using  the  "visual  size"  of  the  ob- 
ject, or  the  angle  subtended  by  it,  as  an  indication  of  its  real  size 


PERCEPTION  OF  DISTANCE  AND  SIZE  431 

or  of  its  actual  distance;  and  yet  direct  judgment  of  this  visual 
size  is  much  less  certain  and  accurate  than  judgment  of  the  real 
size  or  actual  distance.  These  paradoxes  would  amount  to  genuine 
impossibilities  if  the  whole  process  of  reaching  a  judgment  of  size 
or  depth  went  on  within  the  field  of  consciousness,  and  if  every  part 
of  it  were  accessible  to  attentive  observation. 

Such  experiences  as  these  tend  strongly — and,  may  we  not  say, 
conclusively  ? — to  confirm  our  suspicion  that  innumerable  complex 
"traces"  of  sensations  due  to  native  and  acquired  motor  reactions 
are  so  fused  with  all  the  local  signs  of  the  retina,  as  to  demand  recog- 
nition in  every  satisfactory  theory  of  the  genesis  and  development  of 
the  entire  class  of  visual  perceptions.  The  infant  does  not  even 
initiate,  not  to  say  achieve,  the  process  of  objective  vision  otherwise 
than  through  the  use  of  a  ceaselessly  moving  pair  of  eyes.  And 
that  our  subsequent  analysis,  whether  introspective  or  more  purely 
experimental,  cannot  disentangle  and  reproduce  in  consciousness 
these  fused,  or  synthesized,  factors,  no  more  proves  that  they  did 
not  formerly  exist  than  does  the  similar  inability  of  the  accomplished 
violinist  to  reproduce  all  the  sensations  under  the  guidance  of  which 
he  learned  correct  spacing  and  bowing  for  all  the  different  "posi- 
tions" of  his  instrument,  and  its  most  difficult  and  delicate  work. 
Indeed,  on  the  one  hand,  the  player's  violin  is  as  much  a  seeming 
part  of  his  own  organism  as  is  the  seer's  pair  of  eyes;  and  on  the  other 
hand,  the  complexity  and  depth  below  the  threshold  of  conscious- 
ness, of  the  player's  former  sense-experience  bears  no  resemblance 
to  the  complexity  of  the  lost  art  of  learning  how  to  see  with  the 
average  pair  of  eyes. 

§  16.  And,  indeed,  as  has  already  been  indicated,  the  laws  which 
control  our  estimates  of  visual  magnitudes  are  psychological,  and 
apply  to  all  the  action  of  the  mind  in  constructing  its  sense-data  into 
the  presentations  of  sense.  Yet  more  elaborate  mental  activities, 
such  as  take  place  when  the  distance,  size,  and  contour  of  visual  ob- 
jects are  deliberately  estimated  and  expressed  in  terms  of  an  ac- 
cepted standard,  of  course  imply  more  of  dependence  upon  skill 
acquired  through  experience. 

The  degree  of  fineness  with  which  differences  of  distance  and 
magnitude  can  be  seen,  under  the  most  favorable  circumstances,  is 
limited  by  the  least  observable  differences  in  the  members  of  the 
different  spatial  series  of  sensations  which  compose  the  visual  ob- 
jects. Of  such  series,  those  most  capable  of  exceedingly  fine  differ- 
entiation are  the  local  retinal  signs  and  the  sensations  of  position 
and  motion  accompanying  convergence  of  the  eyes  for  near  dis- 
tances. Different  authorities  assign  different  proportions  to  the 
different  help  which  these  series  render  in  making  the  finest  possi- 


432  PRESENTATIONS  OF  SENSE 

ble  distinctions  of  visual  magnitude.  Hering1  denies  that  any  help 
is  obtained  from  muscular  sensations,  or  "feelings  of  innervation," 
in  comparing  the  size  of  two  minute  objects  near  by,  and  assigns 
all  the  work  of  furnishing  such  data  to  the  "spatial  sense  of  the 
retina."  Lotze,2  who  admitted  the  assistance  of  muscular  sensa- 
tions, nevertheless  held  that  the  fineness  of  the  distinctions  possible 
among  them  is  not  sufficient  to  support  our  ordinary  judgments  of 
the  size,  distance,  and  direction  of  objects.  Wundt3  and  others 
claim  that  it  is  by  gradations  in  the  sensations  of  eye-movements  that 
we  make  the  most  accurate  of  these  estimates;  they  deny  that  any 
"spatial  sense"  (in  Hering's  meaning  of  the  words)  belongs  to  the 
retina.  The  evidence  seems  to  favor  the  view  that  both  classes  of 
sensations  furnish  data  for  all  nice  discrimination  of  visual  extension. 
The  particular  degree  of  accuracy  with  which  minute  differences 
in  the  distance  and  magnitude  of  visual  objects  can  be  perceived 
varies  greatly,  according  to  different  positions  of  the  eyes  and  of 
the  object,  the  amount  of  light,  practice,  etc. — and  all  these,  as  con- 
nected with  individual  peculiarities  of  structure  and  previous  func- 
tion of  the  organs  of  sense.  That  such  estimates  fall  to  some  extent 
under  Weber's  law — in  other  words,  that  the  least  observable  dif- 
ference in  the  length  of  visual  lines  and  surfaces  is  relative  and 
not  absolute — has  already  been  shown  (chap.  Ill,  §  23).  Chodin 
found  the  relative  value  of  the  least  observable  difference,  with  a 
variation  of  the  absolute  vertical  distance  from  2.5  to  160  mm.,  to 
be  as  follows  when  the  lines  lie  in  the  same  direction: 

Absolute  distance..        2.5          5  10          20          40          80         160mm. 

Fraction  of  observ- 
able difference . . 

The  fineness  of  ocular  judgment  is  greater  for  horizontal  dis- 
tances. 

The  measuring  power  of  the  eye  is  much  less  accurate  when 
the  distances  compared  lie  in  different  directions.  In  particular, 
points  separated  by  a  vertical  distance  of  20  mm.  are  estimated  as 
equally  far  apart  with  those  separated  by  a  horizontal  distance  of 
25  mm.4  Most  estimates  of  direction  and  distance  are  compara- 
tively inaccurate  when  only  one  eye  is  used.  A  vertical  line  drawn 
at  right  angles  to  a  horizontal  appears  bent  to  monocular  vision; 
its  apparent  inclination  is  variable,  and  was  found  by  Bonders5  to 
vary  between  1°  and  3°  of  the  angle  within  a  short  time. 

1  Hermann's  Handb.  d.  PhysioL,  III,  i,  pp.  533  f . 

3  Medicin.  Psychologic,  384  f . 

8  Physiolog.  Psychologic  (6th  ed.),  II,  574. 

4  So  Wundt  found,  op.  cit.,  II,  591. 

6  Archiv  f.  Ophthalmologie,  XXI,  iii,  pp.  100  f. 


VISUAL  PERCEPTION  OF  MOTION  433 

§  17.  The  data  or  motifs  already  described  are  the  foundation, 
also,  of  our  perceptions  of  motion,  and  of  our  estimates  of  its  di- 
rection, speed,  and  extent.  It  need  scarcely  be  said  that  all  such 
perceptions  and  estimates  are  relative;  they  imply  the  existence  of 
some  point  which  may  be  regarded  as  fixed,  and  the  application  of 
a  standard  of  measurement.  For  perceptions  of  motion  by  the  eye, 
the  point  of  regard  when  the  organ  is  in  the  primary  position 
furnishes  the  means  of  orientating  ourselves  and  of  placing  the  dif- 
ferent things  of  vision  in  their  right  relations  to  us  and  to  each 
other.  Suppose  the  body  and  head  to  be  erect,  and  the  eyes  mo- 
tionless and  looking  into  the  distance  with  the  lines  of  vision  paral- 
lel; the  perception  of  motion  may  then  arise  in  either  one  of  two  ways. 
Of  these,  by  far  the  most  frequent  is  the  change  of  relative  position 
of  an  object  in  the  field  of  vision  which  is  occasioned  by  its  move- 
ment. What  is  necessary,  however,  is  simply  the  successive  stimu- 
lation of  continuous  points  or  areas  of  the  retina  with  images  that 
are  sufficiently  similar  to  be  perceived  as  one  object.  The  percep- 
tion of  motion  may  also  be  produced  by  the  successive  stimulation 
of  the  same  points  or  areas  of  the  retina  with  images  that  are  too 
dissimilar  to  be  regarded  as  one  object.  One  may  thus  see  motion 
when  neither  the  eyes  nor  any  external  objects  are  really  moved, 
as  is  now  so  familiarly  illustrated  by  the  well-known  performances 
of  kinetoscope  and  kinematograph.  It  is  in  the  latter  way  that  the 
colored  points  of  the  images  formed  by  the  retina's  own  light,  when 
the  eyes  are  closed  and  motionless,  seem  to  be  in  constant  motion. 

The  direction  and  amount  of  motion  perceived  with  the  eyes  is 
measured  off  upon  the  entire  field  of  vision  in  accordance  with  pre- 
vious experience  and  by  means  of  the  data  already  described.  With 
the  eyes  at  rest,  the  retinal  local  signs,  or  space-values  belonging 
to  the  retinal  elements,  furnish  the  more  important  data;  second- 
ary helps,  and  associated  sensations  and  ideas  of  position  and  mo- 
tion, complete  the  perception. 

It  is  assumed,  in  cases  like  the  foregoing,  that  no  sensations  in- 
dicating motion  of  either  the  organ  of  vision,  or  the  head,  or  the 
whole  body,  complicate  the  problem.  But  ordinary  perceptions  of 
motion  are  gained  with  the  eyes  in  motion  out  of  the  primary  posi- 
tion. When  the  eye  and  the  object  both  move  in  such  a  way  that 
the  point  of  regard  remains  fixed  on  the  object,  our  perceptions 
of  motion,  and  estimates  of  its  direction  and  magnitude,  are  de- 
pendent upon  muscular  and  tactual  sensations  occasioned  by  the 
eye's  changes  of  position.  We  know  from  experience  what  kinds 
and  intensities  of  sensations  are  produced  by  keeping  the  point  of 
regard  fixed  on  an  object  which  is  moving  about  at  a  given  rate  in 
a  given  direction.  If  any  of  the  links  ordinarily  belonging  to  this 


434  PRESENTATIONS  OF  SENSE 

chain  of  conscious  experiences  drop  out,  our  measuring  instru- 
ment fails  us  either  partially  or  completely.  The  head,  too,  is  in- 
variably turned  when  we  are  watching  an  object  that  is  moving  in 
any  direction  other  than  straight  forward  or  away  from  us  along  the 
line  of  regard.  The  sensations  originating  in  the  action  of  the 
muscles  and  skin  of  the  head  and  neck  thus  enter  into  our  compu- 
tation; they  must  have  such  a  value  in  consciousness  as  to  inform 
us  about  how  far  the  head  has  gone  from  the  position  with  which 
it  started,  in  order  to  fixate  the  moving  object.  According  to 
Helmholtz,1  the  ordinary  movements  of  the  head  in  vision  follow 
the  same  principle  as  that  followed  by  the  eyes  in  movement;  that 
is  to  say,  the  head  turns  from  its  primary  position  on  an  axis  that 
is  approximately  parallel  to  the  axis  of  the  simultaneous  rotation 
of  the  eyes.  But  Hering2  asserts  that  a  difference  between  the  laws 
of  the  motion  of  head  and  eyes  is  of  essential  significance  for  our 
perception  of  space.  However  this  may  be,  it  is  certain  that  the 
position  and  motion  of  the  head,  as  known  by  its  muscular  and 
tactual  sensations,  must  be  taken  account  of  in  all  ordinary  visual 
perception  of  motion.  The  same  thing  is  true  of  the  position  and 
motion  of  the  entire  body.  Many  of  our  errors  of  sense,  or  false 
perceptions  of  motion — its  existence,  direction,  rate,  and  amount — 
are  dependent  upon  the  principles  of  judgment  governing  such 
data  of  sensations.  We  are  peculiarly  liable  to  error  in  all  cases 
where  the  motions  of  our  own  bodily  organs  are  passive;  in  such 
cases  we  do  not  have  the  ordinary  motifs,  or  data,  at  our  command. 
Objects  are  perceived  at  rest,  either  when,  our  organs  of  vision 
being  themselves  at  rest,  the  images  of  the  objects  do  not  change 
their  position  in  the  field  of  vision,  or  when  sensations  of  motion 
occasioned  by  moving  these  organs  are  such  and  so  great  as  we 
know  by  experience  correspond  to  (or  compensate  for)  the  changes 
in  the  position  of  their  images  which  are  occasioned  by  their  actu- 
ally remaining  at  rest.  But  whenever  we  look  with  moving  eyes 
upon  a  number  of  objects  arranged  in  fixed  position  with  relation  to 
each  other,  a  conflict  between  two  sets  of  data  really  takes  place. 
The  result  with  respect  to  our  perceptions  of  motion  may  depend 
upon  which  of  the  two  is  chiefly  effective  in  arresting  attention. 
When  the  eyes  are  brought  from  the  parallel  position,  which  they 
assume  in  vision  of  remote  objects,  to  convergence  upon  some  near 
object,  the  two  fields  of  view  belonging  to  the  two  eyes  rotate  in 
opposite  directions,  while  the  middle  visual  line  maintains  its  posi- 
tion in  the  median  plane.3  Ordinarily  we  do  not  perceive  this 

1  Physiolog.  Optik,  p.  486. 

2  In  Hermann's  Handb.  d.  PhysioL,  III,  i,  p.  495. 

3  See  Le  Conte,  Sight,  p.  229. 


JUDGMENT  IN  ERRORS  OF  SENSE  435 

rotary  motion  of  the  two  fields  of  vision,  but  consider  the  field  as 
one  and  stationary  and  ourselves  as  changing  our  point  of  regard  in 
it.  By  attention,  however,  we  may  see  that  the  external  objects, 
although  they  really  continue  at  rest,  appear  to  move  as  the  rela- 
tions of  their  double  images  are  changed.  So,  also,  when  the  eye 
or  head  or  body  turns  in  either  direction,  in  order  that  a  new  ob- 
ject may  be  brought  under  regard,  it  is  possible  either  to  perceive 
or  not  to  perceive  the  entire  field  of  objects  sweeping  by;  and  which 
of  the  two  happens  depends  upon  the  direction  in  which  attention 
is  drawn.  When  strictly  attending  to  the  phenomena,  we  cannot 
well  fail  to  regard  everything  as  moving  in  the  opposite  direction 
from  that  in  which  we  know  the  organ  of  vision  to  be  turning. 

§  18.  The  principles  already  laid  down  also  suffice  to  explain 
most  of  the  ordinary  "errors  of  sense,"  as  well  as  certain  extraor- 
dinary experiences  of  a  somewhat  different  kind.  The  right  to 
speak  of  errors  of  sense  has  been  questioned.  It  has  been  claimed 
that  such  errors  belong  rather  to  judgment,  and  that  sense  pure 
and  simple  cannot  err.  The  claim  is  based  upon  a  misunderstand- 
ing of  the  nature  of  perception.  A  very  obvious  difference  exists, 
indeed,  between  a  mistaken  estimate  of  the  distance  of  a  mountain 
through  extraordinary  clearness  of  atmosphere  and  the  seeing  of  a 
square  of  white  paper  as  green  on  a  red  ground,  or  as  yellow  on 
a  blue  ground.  But  the  latter  is  surely  an  "error  of  sense,"  or 
sensation,  in  as  pure  form  as  such  error  is  conceivable.  That  sense 
cannot  err  is  true  only  in  case  we  speak  of  unlocalized  and  unpro- 
jected  sensation,  regarded  as  not  predicating  anything  beyond  itself. 
In  all  presentations  of  sense  a  certain  psychological  judgment  is 
involved;  for  all  such  presentations  imply  association  of  impres- 
sions discriminated  as  similar  or  dissimilar,  and  a  mental  synthesis 
which  is  dependent  upon  attention  and  the  interpretation  of  certain 
motifs  or  data  according  to  past  experiences.  Clear  vision  is  always 
mental  interpretation. 

The  attempt  to  assign  the  relative  amount  of  blame  to  sense  and 
to  intellect,  in  cases  where  our  presentations  of  sense  do  not  rep- 
resent objective  relations  of  things,  assumes  an  ability  to  make  dis- 
tinctions which  we  do  not  possess.  Moreover,  the  distinction,  when 
made  as  the  objection  would  have  it,  will  not  hold.  Innumerable 
experiences  contradict  the  statement  that  immediate  sense-percep- 
tion cannot  err.  When  one  sees  (with  no  power  to  see  otherwise) 
a  gigantic  human  form  through  the  fog,  or  projected  against  the 
scenery  of  a  stage,  and  yet  judges  that  this  form  is  only  of  usual 
size,  the  error  is  not  one  of  judgment,  but  just  the  reverse.  Errors 
of  sense  are  only  special  instances  where  the  mind  makes  its  syn- 
thesis unfortunately,  as  it  were,  out  of  incomplete  data,  instantane- 


436  PRESENTATIONS  OF  SENSE 

ously  and  inevitably  interpreting  them  in  accordance  with  the  laws 
which  have  regulated  all  its  experience.  As  Lotze  has  remarked, 
"The  whole  of  our  apprehension  of  the  world  by  the  senses  is 
one  great  and  prolonged  deception."  Objects  of  sense  are  in  no 
case  exact  copies  of  ready-made  things  which  exist  extra-mentally 
just  as  they  are  afterward  perceived,  and  which  get  themselves 
copied  off  in  the  mind  by  making  so-called  impressions  upon  it; 
they  are  mental  constructions.  In  the  special  case  of  sight  we  have 
seen  that,  in  every  particular — in  its  elements,  its  mode  of  con- 
struction, its  laws  of  change — the  field  of  vision  is  a  subjective  af- 
fair. The  case  is  in  no  respect  essentially  different,  whether  our 
presentations  of  sense  are  so-called  errors  or  true  images  of  things. 
In  both  cases  the  same  data  and  laws  of  the  use  of  these  data  main- 
tain themselves.  Errors  of  sense,  however,  are  distinguished  from 
hallucinations,  because  the  former  result  from  the  activity  of  an 
organism  which  is  normal  in  structure  and  function,  while  the  lat- 
ter do  not. 

§  19.  The  errors  of  visual  perception  are  almost  innumerable; 
they  may  be  classified  in  part,  however,  according  as  they  fall  under 
some  one  or  other  of  the  before-mentioned  principles.  Such  errors 
may  be  called  "normal,"  because  they  are  committed  in  accordance 
with  principles  which  regulate  the  ordinary  activity  of  the  mind  in 
making  its  synthesis  by  the  help  of  the  sense-data  or  motifs  fur- 
nished to  it  through  the  excitement  of  the  organism.  Deceptions 
of  this  class  really  result,  then,  from  the  fidelity  of  both  mind  and 
nervous  system.  Certain  errors  of  sense,  for  example,  are  special 
examples  of  the  working  of  the  laws  which  regulate  the  correspond- 
ence of  the  two  images  in  binocular  vision.  Thus,  near  objects 
erroneously  appear  double  when  the  eye  is  adjusted  for  distant 
vision,  distant  objects  when  it  is  adjusted  for  near  vision;  solid 
things  are  seen  through  other  solid  things;  relations  in  space  in 
general  are  perceived  different  from  the  reality;  and  all  according 
to  the  law  of  the  correspondence  and  non-correspondence  of  the 
two  retinal  images.  Accordingly,  the  inquiry,  Why  is  vision  single 
when  it  is  performed  with  two  eyes  ?  can  demand  and  receive  only 
one  answer.  An  important  condition  of  the  single  vision  of  solid 
objects  is  that  they  shall  be  seen  with  two  eyes.  Whether  anything 
whatever  is  seen  as  two  or  one  does  not  depend,  primarily,  upon  its 
really  being  either  two  or  one,  or  upon  the  existence  of  one  or  two 
retinal  images  of  it  (as  though  such  images  were  directly  perceived) ; 
it  rather  depends  upon  the  appropriate  data  of  sensations  being 
furnished  to  the  mind  for  completing  its  mental  synthesis  of  the 
object.  The  two  eyes  being  simultaneously  affected  in  a  certain 
way,  these  data  are  supplied.  What  is  one  is  seen  as  one,  and 


GEOMETRICAL  OPTICAL  ILLUSIONS  437 

what  is  two  is  seen  as  one,  and  what  is  one  is  seen  as  two — all  in 
essentially  the  same  way. 

§  20.  Under  the  head  of  geometrical  optical  illusions  are  grouped 
a  large  number  of  curious  errors  in  the  perception  of  lines  and  angles. 
Of  the  numerous  figures  which  have  been  found  to  give  rise  to  such 
illusions,  only  a  small  selection  can  be  presented  here;  but  it  may  be 
stated  as  a  general  principle,  that  scarcely  any  geometrical  figure 
is  free  from  illusory  effects;  or,  in  other  words i '.'that  the  apparent 
length  and  direction  of  a  line  is  likely  to  be  affected  by  all  its  visual 
environment.  0 

Among  the  principal  illusions  of  this  general  type  are  such  as 
the  following:  (1)  Specific  illusions.  Vertical  distances  are  per- 
ceived as  greater  than  mathematically  equal  horizontal  distances.1 
This  can  be  seen  in  Figs.  125,  a  and  b;  but  the  reader  can  best 


FIG.  125. 

convince  himself  of  the  fact  by  marking  off  a  vertical  distance  be- 
tween two  points,  and  then — keeping  the  original  two  points  stead- 
ily in  the  vertical  meridian  of  the  field  of  view — locating  a  third 
point  at  what  seems  to  be  the  same  distance  to  the  right  or  left  of 
one  of  the  original  points.  The  error  will  then  be  apparent  on 
measuring  the  two  distances,  or  on  turning  the  figure  so  as  to  inter- 
change horizontal  and  vertical. 

By  a  similar  test  the  upper  half  of  a  vertical  line  can  be  shown  to 
appear  slightly  longer  than  the  lower  half; 2  and  in  general,  when  one 
of  two  equal  figures  lies  directly  above  the  other,  the  upper  appears 
somewhat  larger  than  the  lower.  The  upper  and  lower  halves  of 
a  letter  "S"  or  a  figure  "8"  appear  of  nearly  the  same  size;  but  when 
they  are  inverted  ("g"  and  "g"),  the  actual  difference  between 

1  See  J.  Oppel,  Uber  geometrisch  optische  Tauschungen,  Jahresbericht  d.  physi- 
kal.  Vereins  zu  Frankfurt,  1854-55,  p.  37. 

2  Delboeuf,  Bulletin  de  I'acad.  roy.  de  Belgique.,  2  S£r.,  XIX,  2,  195. 


438  PRESENTATIONS  OF  SENSE 

the  two  halves  becomes  magnified.  There  is  a  somewhat  similar 
illusion  in  dividing  a  horizontal  line  in  halves,1  when  only  one  eye 
is  used,  and  attention  is  directed  to  the  middle  of  the  line;  the  outer 


Fio.  126. 

half  needs  to  be  larger  than  the  inner  or  nasal  half  in  order  that  the 
line  may  appear  equally  divided.  Not  all  individuals,  however,  are 
subject  to  this  illusion,  and  it  is  always  slight  in  amount. 

(2)  One-dimensional  illusions.  In  distinction  from  the  illusions 
already  mentioned,  which  seem  to  be  specifically  connected  with 
certain  parts  of  the  retina  or  of  the  field  of  view,  there  is  a  large 
class  of  more  general  errors  which  are  likely  to  be  committed  in 


I 


FIG.  127.     (From  Ebbinghaus.) 

the  judgment  of  lengths.  They  may  be  classed  as  one-dimensional 
illusions,  since  they  occur  within  a  single  straight  line,  and  are  not 
due  to  the  influence  of  figures  in  other  parts  of  the  field  of  view. 

The  best-known  of  these  one-dimensional  illusions  is  commonly 
expressed  by  saying  that  filled  or  divided  space  appears  greater 
than  empty  or  undivided  space.  This  rule,  however,  is  subject 
to  certain  qualifications.  It  is,  first,  rather  the  filled  or  empty  ap- 


FIG.  128. 

pearance  which  counts,  than  the  amount  of  light  received  from  the 
spaces  in  question;  for  a  black  line  on  a  white  sheet  appears  longer 
than  an  equal  distance  laid  off  between  two  points  (Fig.  126), 
though,  objectively,  it  is  the  black  line  which  is  empty.  In  the  same 

1  Kundt,  Poggendorf's  Annalen,  1863,  CXX,  118. 


GEOMETRICAL  OPTICAL  ILLUSIONS  439 

way,  it  is  not  the  most  finely  divided  space  which  seems  greatest, 
but  the  space  of  which  the  division  is  most  obtrusive  (Fig.  127). 
These  facts  indicate  that  the  illusion  is  dependent  on  the  figure 
which  we  are  led  to  see  rather  than  on  the  mere  distribution  of 
retinal  stimulation.  There  are  further  qualifications,  in  detail, 
to  the  rule  that  divided  space  appears  greater  than  undivided;  for 
example,  a  single  division  of  a  line  at  its  middle,  or  a  division  into 
three  parts,  of  which  the  middle  one  is  much  larger  than  that  at 
either  end,  causes,  if  anything,  the  opposite  illusion  (Fig.  128). 

When  subdivision  of  a  line  causes  it  to  appear  longer,  the  parts 
into  which  it  is  divided  (or  some  of  them)  themselves  appear  shorter 


FIG.  129. 

than  isolated  lines  of  the  same  length.1  If,  on  the  contrary,  the 
division  is  such  as  to  make  the  whole  line  appear  shorter,  the  parts 
then  appear  long  in  comparison  with  isolated  lines  of  the  same  length. 
For  example,  if  a  line  is  divided  into  a  large  central  piece  and  two 
small  end-pieces,  the  whole  line  appears  shortened,  and  the  middle 
part  lengthened ;  and  this  illusion  is  intensified  by  leaving  the  middle 
part  blank,  as  in  Fig.  129,  in  which,  by  a  combination  of  these 
effects,  the  whole  of  the  shorter  line  appears  shorter  than  the  vacant 
middle  part  of  the  longer  line.2  If,  however,  the  middle  part  is 
relatively  small,  it  appears  shorter  than  an  equal  isolated  line,  while 
the  whole  line  appears  too  long.  By  cutting  out  equal  distances 
from  the  middles  of  short  and  of  long  lines,  as  in  Fig.  130,  an 
effect  of  contrast  is  produced.3 

1  Ebbinghaus,  Grundzuge  der  Psychologic,  1908,  II,  59. 

2  Compare  the  very  similar  illusions  investigated  by  H.  J.  Pearce,  Psychol. 
Rev.,  1904,  XI,  143. 

3  Miiller-Lyer,  Archiv  /.  Physiol,  1889,  Suppl.,  p.  263;   Zeitschrift  f.  Psychol., 
1896,  IX,  1. 


440  PRESENTATIONS  OF  SENSE 

Very  similar  to  these  one-dimensional  illusions  of  visual  percep- 
tion are  some  which  occur  in  the  comparison  of  distances  by  touch, 
when  lines  are  applied  to  the  skin,  or  by  touch  and  the  muscle-sense, 
when  the  finger  is  drawn  along  a  line,  the  divisions  being  made 
in  such  a  way  as  to  be  appreciated  by  touch.1  And  much  the  same 
illusions  occur  in  the  comparison  of  short  intervals  of  time,  when 
these  are  marked  out  by  sounds.  It  appears  quite  probable,  there- 
fore, that  these  one-dimensional  illusions  are  not  specifically  de- 
pendent on  the  processes  of  vision,  nor  even  on  the  processes  of 
space-perception.  One  may  say  that  the  process  of  comparison 
is  affected,  in  all  these  cases,  with  the  same  sort  of  difficulty  and 
source  of  confusion;  or  one  may  say  that  the  same  sort  of  configura- 


FIG.  130. 


tion  is  introduced  into  the  percept  in  all  these  cases,  and  that  the 
illusions  are  incidental  to  the  configuration. 

No  fully  satisfactory  theory  has  yet  been  worked  out  to  cover 
and  explain  all  the  facts.  The  illusions  which  have  been  men- 
tioned in  visual  perception  of  lines  are  rather  slight,  and  even  in- 
constant from  one  individual  to  another;  but  they  are  made  more 
striking,  and  harder  to  escape,  by  adding  to  the  one-dimensional 
figure  other  lines  and  angles. 

1  Compare  Robertson,  Psychol.  Rev.,  1902,  IX,  549;  Pearce,  Archiv  /.  d.  ges 
Psychol,  1903,  I,  31;  Psychol.  Rev.,  1904,  XI,  143. 

It  should  not  be  understood,  however,  that  the  same  illusions  occur  in  all  cases 
in  touch  as  in  vision.  Of  the  illusions  to  be  mentioned  on  succeeding  pages,  that 
of  Miiller-Lyer  holds  of  touch,  but  that  of  Poggendorf  is  reversed  (Robertson). 
In  an  extended  study  of  the  illusion  of  rilled  and  empty  space  in  touch  (Arch. 
f.d.  ges.  Psychol.,  1910,  XVI,  418),  Cook  finds  (1)  that  when  adjacent  areas 
of  the  skin  are  simultaneously  excited,  a  filled  space  is  overestimated  by  com- 
parison with  an  empty,  but  (2)  that  when  the  stimuli  are  applied  successively 
to  the  same  area,  the  opposite  illusion  results;  and  (3)  that  when  the  stimuli 
are  applied  successively  to  different  areas,  no  illusion  occurs  in  the  comparison 
of  filled  and  empty  space.  Analysis  of  the  total  impression,  or  isolated  attention 
to  the  particular  elements  to  be  compared,  is  favored,  in  the  case  of  touch,  by 
successive  presentation.  In  general,  it  would  seem  that  touch  is  rather  more  sub- 
ject to  illusions  of  this  sort  than  is  vision;  for  analysis  is  easier  in  vision. 


THE  MULLER-LYER  ILLUSION 


441 


(3)  The  Miiller-Lyer  illusion.  The  most  striking  illusion  in 
length  of  lines  is  that  first  announced  by  Miiller-Lyer  in  1889,  and 
called  by  him  a  "confluxion"  effect,  in  opposition  to  "contrast." 


FIG.  131.— The  Mtiller-Lyer  Illusion. 


Fig.  131  presents  the  "confluxion"  illusion  in  a  common  form, 
but  there  are  numerous  variants.  A  continuous  series  of  transi- 
tional forms  can  be  constructed,  leading  from  this  arrow-head 


FIG.  132.— Modified  Mdller-Lyer  Figure. 


figure  to  the  one-dimensional  illusion  of  Fig.  129:  thus,  the  hori- 
zontal line  may  be  omitted,  and  only  the  arrow-heads  left — the 
distances  between  the  points  of  the  arrows  being  the  object  of  com- 


FIG.  133. — Confluxion  and  Contrast. 


parison;  the  angle  of  the  arrow-heads  may  then  be  diminished  to 
zero,  so  giving  the  one-dimensional  figure.  Another  transitional 
series  can  be  made  by  substituting  for  the  arrow-heads  rectangles 
(with  or  without  one  side  open),  and  then  narrowing  the  rectangle 


442 


PRESENTATIONS  OF  SENSE 


FIG.  134.— Reversible  MOller-Lyer  Illusion. 

to  the  width  of  the  "line"  in  the  one-dimensional  figure  (Fig.  132). 
From  the  continuity  of  these  series,  there  can  be  little  doubt  that  the 
same  principle  enters  into  both  extremes  of  the  series;  on  the  other 

hand,  the  introduction  of  figures  in  two 
dimensions  certainly  intensifies  the  illu- 
/    sory  effect. 

*  The  Miiller-Lyer  figure,  like  the  one- 
dimensional  figure,  and  even  more,  pro- 
duces a  compound  or  reduplicated 
illusion.  One  of  the  two  lines  to  be 
compared  seems  shorter  than  it  is,  and 
the  other  longer  than  it  is,  and  therefore 
the  comparison  of  the  two  is  subject  to  a 
double  illusion.  Further,  either  of  the 
two  principal  lines  of  the  Miiller-Lyer 
figure  is  influenced  by  two  oblique  lines 
at  each  end,  and  any  one  of  these  four 
oblique  lines  is  enough  to  produce  a 
moderate  degree  of  the  illusion.  In 
other  words,  the  sides  of  an  acute  angle 
seem  shorter  than  they  are,  and  the  sides 

/  of  an  obtuse  angle  longer  than  they  are. 

*  Since,  however,  the  same  sort  of  illusion 

occurs    when    rectangular    figures,    or 
FIG.  135.— The  Angle  illusion,     curves,  or  even  parts  of  a  single  straight 

line,  are  substituted  for  the  arrow-heads, 

a  more  comprehensive  statement  of  the  illusion  may  take  the  fol- 
lowing form:  When  a  point  is  apprehended  as  part  of  a  compact 
figure  (as  an  arrow-head,  a  rectangle,  or  a  short  stretch  of  line),  it 
becomes  attracted,  or  displaced,  toward  the  centre  of  that  figure.1 

1  Compare  the  similar  formulations  by  Judd,  Psychol.  Rev.,  1899,  VI,  241;  by 
Pearce,  Psychol.  Rev.,  1904,  XI,  143;  by  Benussi,  in  Meinong's  Untersuchungen 
zur  Gegenstandstheorie  und  Psychologic,  1904,  p.  303;  and  by  Smith  and  Sowton, 
Brit.  Journ.  of  Psychol.,  1907,  II,  196. 


CENTRAL  FACTOR  IN  ILLUSIONS 


443 


§  21.  A  few  words  of  explanation  are  needed  with  reference  to  the 
formula  just  given.  Neither  " figure"  nor  "centre"  is  to  be  under- 
stood in  a  strict  geometrical  sense.  By  "figure"  is  meant  any  part 


Fro.  136.— Modified  Angle  Illusion. 

of  the  total  presentation  (whether  this  be  visual,  tactile,  or  audi- 
tory) which,  usually  because  of  its  compactness,  is  apprehended  as 
possessing  a  certain  degree  of  unity  and  isolation.  By  "centre  of 
the  figure"  is  to  be  understood  something  analogous  to  the  centre 
of  gravity,  but  it  is  a  subjective  centre  of  gravity,  depending  rather 
on  the  figure  as  apprehended  than 
on  the  exact  geometrical  relations 
within  the  stimulus.  For  example, 
when  the  inner  circle  in  a,  Fig. 
133,  is  compared  with  the  outer 
circle  in  6,  an  illusion  occurs  which 
is  probably  a  reduplication  of  the 
one-dimensional  illusion  of  Fig.  129; 
diameters  drawn  through  the  cir- 
cles would  give  the  one-dimensional 
figure.  Both  a  and  b  are  most 
readily  apprehended  as  rings,  and 
the  centre  of  attraction  lies  within 
the  ring  and  not  at  the  geometrical 
centre  of  the  circles.  In  c  of  the 
same  figure,  on  the  other  hand,  the 
ring  is  not  compact,  and  the  inner 
circle  is  apprehended  as  an  inde- 
pendent figure;  its  centre  of  attrac- 
tion lies  at  its  geometrical  centre, 
and  the  circumference  is  drawn 
inward;  and  thus  comparison  of  FlQ.  137.— Double  Angle  illusion, 
the  inner  rings  of  b  and  c  leads  to 

a  "contrast"  illusion  similar  to  that  in  the  one-dimensional  Fig. 
130.     It  appears,   therefore,   that  both   the  "contrast"   and    the 


444 


PRESENTATIONS  OF  SENSE 


"confluxion"  illusions  are  covered  by 
the  comprehensive  formula  of  the  pre- 
ceding paragraph.  Which  of  the  two 
effects  shall  result  depends  on  the  sub- 
jective grouping  of  parts. 

The  importance  of  some  such  mental 
or  "central"  factor  as  apprehension  in 
the  causation  of  these  illusions  is  indi- 
cated by  the  fact  that  opposite  illusions 
can  sometimes  be  got  from  the  same  ob- 
jective figure,  according  to  the  way  in 
which  its  parts  are  combined.  Thus,  in 
Fig.  134,  a  given  objective  distance  ap- 
pears now  longer  and  again  shorter  than 
another  objectively  equal  distance,  ac- 
cording as  attention  is  fixed  on  the  fig- 
ures in  black  or  on  the  figures  in  white. 
§  22.  (4)  Illusions  of  angles  and  the 
direction  of  lines.  The  apparent  direc- 
tion of  a  line  is  easily  influenced  by  the 
FIG.  138.— Poggendorf  Figure,  presence  of  other  lines  in  its  neighbor- 
hood. The  general  rule  applying  to 

these  cases  is  often  stated  in  the  following  form:  Acute  angles  are 
over-estimated,  and  obtuse  angles  under-estimated.  This  formula 
is  to  be  regarded  less  as 
an  explanation  of  the  large 
class  of  illusions  to  which 
it  is  applied  than  as  a  con- 
venient designation  of  the 
class  itself.  These  figures 
seldom  require  the  ob- 
server to  make  a  direct 
estimate  of  the  size  of  an 
angle,  either  in  degrees 
or  in  comparison  with  an- 
other angle;  he  has  rather 
to  compare  the  directions 
of  two  lines  or  to  locate 
the  prolongation  of  a  line 
when  it  is  interrupted 
The  error  committed  is  as 

it  would    be   if   the    acute  FIG.  139.— ZSllner  Figure. 

angles  of  the  figure  were 

over-estimated;  but  the  over-estimation  of  the  angle  is  not  a  di- 
rectly observed  fact.     Indeed,  when  a  direct  comparison  of  angles 


ILLUSIONS  OF  ANGLES 


445 


is  required,  under  conditions  which  make  comparison  difficult — 
such  as  omission  of  the  vertex,  or  unequal  length  of  the  sides  of 
the  angles — then  quite  other  sorts  of  errors  appear.1  This  makes  it 
appear  doubtful  whether  the  over-estimation  of  acute  angles  is, 


FIG.  140a.— Hering  Figure. 


FIG.  140&.— Hering  Figure. 

in  reality,  a  universal  principle  which  can  be  freely  invoked  for  the 
explanation  of  these  illusions. 

In  the  "angle  illusion"  (Fig.  135),  the  acute  angle  at  the  vertex 
is  said  to  be  over-estimated,  because  the  true  prolongation  of  one 
side  falls  within  the  apparent  divergence  of  the  sides.  The  illusion 
appears  to  depend  in  part  on  the  disparity  in  length  of  the  two  lines 
which  form  the  angle;  if  both  are  treated  alike,  as  in  Fig.  136,  the 
effect  is  reduced,  made  uncertain,  and  may  even  be  reversed. 

1  Judd,  op.  cit.,  A.  H.  Pierce,  Studies  in  Auditory  and  Visual  Space  Perception 
(New  York,  1901),  p.  271. 


446 


PRESENTATIONS  OF  SENSE 


Reduplication  of  the  angle  illusion  gives  such  effects  as  are  seen 
in  Fig.  137,  or,  in  more  pronounced  form,  in  Fig.  138,  which 
last  presents  the  long-famous  Poggendorf  illusion.  Various  degrees 
of  this  illusion  are  produced  by  dissecting  the  figure  and  presenting 
its  parts  separately;  thus  the  illusion  is,  if  anything,  strengthened  by 
leaving  only  the  exterior  obtuse  angles  between  the  interrupted 
oblique  line  and  the  parallels,  while  it  is  much  diminished  or  even 
reversed  by  leaving  only  the  acute  angles.1 

By  multiplication  of  the  same  elements,  the  figures  of  Zollner 
(139)  and  of  Hering  (140  a  and  6),  and  other  variants,  are  pro- 
duced. The  same  errors  in  the  apparent  prolongation  of  the  ob- 
lique lines  can  be  observed  here,  but,  in  addition,  the  parallels  are 


:*:*:*:*.v;v;v; 

FIG.  141.— The  Twisted  Cord  Illusion  (Fraser). 

affected  in  their  apparent  direction,  and  straight  lines  may  even  ap- 
pear bent.  These  illusions  are  interpreted  in  the  following  way,  as 
coming  under  the  rule  of  over-estimation  of  acute  angles:  If  the 
angles  between  the  parallels  and  the  intersecting  oblique  lines  are 
exaggerated,  and  if  the  resulting  illusion  affects  chiefly  the  paral- 
lels, then  these  will  appear  swung  around  toward  right  angles  with 
the  obliques,  and  so  the  Zollner  illusion  will  result.  Further,  since 
the  over-estimation  of  the  angles  is  greater,  the  smaller  the  angles, 
the  parallels  will  appear  bent  when  the  angle  of  the  intersecting 
lines  varies;  and  thus  the  other  figures  are  provided  for. 

It  is  a  curious  fact  that  when  the  oblique  lines  in  the  Zollner 
figure  are  made  exceedingly  oblique,  while  at  the  same  time  the 
principal  lines  are  indistinct,  an  illusion  opposite  to  that  of  Zollner 
is  produced.  This  may  be  called  the  "  twisted  cord  illusion,"  since 
the  figure  (No.  141)  produced  is  similar  to  the  drawing  of  a  cord 

1  Jastrow,  Amer.  Journ.  of  PsychoL,  1892,  IV,  381;  A.  H.  Pierce,  Studies  in 
Auditory  and  Visual  Space  Perception  (New  York),  1901,  p.  257. 


ILLUSIONS  OF  AREAS  447 

made  by  twisting  together  a  black  and  a  white  strand.1  The 
illusion  is  strengthened  by  placing  the  "cord"  on  a  checkered  back- 
ground in  such  a  position  that  each  of  the  obliques  is  augmented 
at  each  of  its  ends  by  parts  of  the  squares  of  the  background.  By 
varied  application  of  this  principle,  Fraser  has  produced  very  strik- 
ing illusions.  Since  the  deflection  of  the  "cord"  is  in  the  direction 
of  the  oblique  lines,  the  illusion  may  be  called  one  of  "conflux- 
ion,"  and  in  distinction  to  it,  the  Zollner  illusion  may  be  called  one 
of  contrast. 

§  23  (5)  Illusions  of  area.  The  judgment  of  area,  like  that  of 
angles  and  directions,  is,  introspectively,  less  direct  and  confident 
than  the  judgment  of  length.  If  the  areas  of  the  several  surfaces 
in  Fig.  142  are  compared  (without  the  aid  of  calculation),  much 
uncertainty  is  likely  to  be  felt;  but  the  figures  will  probably  appear 
unequal,  though  all  have  very  nearly  the  same  area.  In  general, 
compactness  of  form  seems  to  diminish  the  apparent  area  of  the 


O 


FIG.  142. — Comparison  of  Areas. 

surface.  Another  cause  must  be  sought  for  the  illusion  of  Fig. 
143.  Here  the  judgment  of  area  is  perhaps  confused  by*  the  ob- 
trusive inequality  of  the  adjacent  sides,  or  by  the  evident  tendency 
of  the  lower  figure,  if  continued  upward,  to  enclose  the  upper  figure. 
§  24.  The  field  of  geometrical  illusions  has  proved  exceedingly 
fertile  in  theories,  but  rather  barren  of  scientific  agreement.  A 
twofold  division  may  be  made  into  theories  which  appeal  to  pe- 
ripheral factors  and  those  which  appeal  to  central  factors.  The 
chief  peripheral  factors  are  those  which  relate  either  to  the  retina 
or  to  the  eye  movements.  On  the  one  hand,  it  is  impossible  to 
found  any  comprehensive  theory  purely  on  peculiarities  of  the  retina. 
On  the  other  hand,  the  eye-movement  theory  has  been  applied 
to  these  illusions  with  much  detail  and  much  apparent  success. 
The  general  conception  underlying  this  theory  is  that  the  eye 
measures  lengths,  angles,  etc.,  by  moving  over  them,  and  that  move- 
ments of  greater  extent  or  energy,  or  those  which  involve  more  effort, 
lead  to  the  appearance  of  greater  magnitude.  Since,  for  example, 
vertical  movements  of  the  eyes  require  more  effort  than  horizontal 

1  Fraser,  Brit.  Journ.  of  Psychol.,  1908,  II,  307. 


448 


PRESENTATIONS  OF  SENSE 


movements,  vertical  lines  seem  longer  than  equal  horizontals;  di- 
vided space  impedes  the  movement  of  the  eye  over  it,  and  so  appears 
the  greater;  acute  angles  call  for  abrupt  changes  in  movement,  and 
so  are  felt  as  large  in  comparison  with  obtuse  angles,  which  require 
but  a  slight  deviation  in  the  eye's  direction.  In  the  Muller-Lyer 
figure,  the  readiest  explanation  is  that  the  eye  is  led  to  err  in  its 
fixations,  and  so  comes  to  measure,  not  the  distances  between  the 
points,  as  required,  but  rather  the  distances  between  the  figures 
adjacent  to  the  points. 

Actual  photographic  records  of  the  eye's  movements  during  ex- 
amination of  the  Muller-Lyer  figure  do  not,  however,  as  will  be 


FIG.  143.— An  Illusion  of  Area  (Jastrow). 

seen  later  in  the  chapter,  always  substantiate  this  interpretation. 
They  are  more  favorable  to  the  statement  that  the  figure  with 
inward-turned  obliques  impedes  the  eye's  movement,  making  it 
"slow  and  hesitating,"  whereas  the  figure  with  outward-pointing 
obliques  allows  the  eye  free  play  for  "extensive  and  energetic" 
movement.1  Unfortunately,  hesitating  and  impeded  movements 
are  required  by  the  theory  also  for  explaining  the  over-estimation 
of  divided  space;  so  that  the  two  explanations  are  inconsistent. 

Supporters  of  the  eye-movement  theory  have  often  brought  for- 
ward the  observation  that  steady  fixation  diminishes  the  illusions, 
whereas  free  and  rapid  roaming  of  the  eyes  over  the  figures  favors 
the  illusory  effect.  On  the  other  hand,  opponents  of  the  theory 
have  reported  that  a  momentary  exposure  of  the  figures — too  brief  to 

t^     J  Wundt,  Physiol.  Psychol,  6th  ed.,  1910,  II,  583. 


CENTRAL  THEORIES  OF  ILLUSIONS  449 

permit  the  eyes  to  move  from  one  point  to  another — does  not  de- 
stroy the  illusions  but  rather  favors  them.  These  two  observations, 
in  appearance  contradictory,  are  in  reality  much  the  same  thing;  for, 
as  will  be  set  forth  more  fully  in  later  paragraphs  on  this  subject, 
rapid  movement  of  the  eyes  over  a  figure  amounts  to  obtaining  a 
series  of  brief  exposures — the  figure  being  invisible  during  the  move- 
ments, and  being  seen  only  during  the  brief  fixations  of  the  eye  which 
intervene  between  the  movements.  It  would  seem,  then,  that  the 
absence  of  the  sensory  effects  which  result  from  the  eyes  in  motion 
is  one  of  the  factors  most  directly  responsible  for  such  geometrical 
illusions.  Nor  is  it  by  any  means  fair  to  disregard  the  sensations 
of  position  and  tendencies  to  movement,  or  memory  images  of  past 
movements,  which  may  exist  in  the  absence  of  actual  movement. 
From  this  point  of  view,  the  question  of  the  relation  of  eye  move- 
ments to  these  illusions  is  lost  in  the  general  problem  of  the  visual 
perception  of  space. 

§  25.  Of  "central"  theories  of  the  source  of  these  illusions,  the 
most  comprehensive  are  the  perspective  theory,  the  dynamic  theory, 
and  what  may  perhaps  be  called  the  "  confusion"  theory. 

The  perspective  theory  starts  from  the  undoubted  facts  that  even 
simple  line  drawings  readily  suggest  objects  in  three  dimensions; 
and  that  the  introduction  by  suggestion  of  the  third  dimension 
must  lead  to  changes  in  the  apparent  length  and  direction  of  the 
lines  composing  the  figures.  The  application  of  this  principle  to 
some  of  the  figures  is  readily  made.  Thus,  vertical  lines  in  the 
field  of  view  very  often  represent  lines  extending  away  from  the  ob- 
server and  foreshortened  by  perspective;  such  lines  are  interpreted 
in  accordance  with  their  objective  length,  and  therefore  as  longer 
than  equal  horizontal  lines  of  the  field  of  view,  which  are  not  sub- 
ject to  foreshortening.  In  accordance  with  this  tendency,  vertical 
lines  in  these  simple  figures  appear  longer  than  equal  horizontal 
lines.  The  application  of  the  principle  of  perspective  to  many  of 
the  illusions,1  however,  is  much  less  direct,  and  finds  little  support 
in  introspection. 

§  26.  The  dynamic  theory  is  the  work  of  Lipps,2  and  is,  without 
doubt,  the  most  interesting  and  even  fascinating  of  all.  Lipps 
urges  that  account  must  be  taken  of  the  inner  activity  of  the  observ- 
ing subject,  even  though  this  activity  may  not  come  to  conscious- 
ness as  anything  separate  from  the  figures,  but  may  rather  be  pro- 

1  For  the  application  of  this  theory  to  many  other  illusions,  see  Thie*ry,  in 
Wundt's  Philos.  Studien,  1895,   XI,    307,  603;    1896,   XII,  67;    and  Filehne, 
Zeitschr.  /.  PsychoL,  1898,  XVII,  15. 

2  Raumesthetik   und  geometrische   Tduschungen,  1897;   Zeitschrift  /.   PsychoL, 
1898,  XVIII,  405;  1905,  XXXVIII,  241. 


450 


PRESENTATIONS  OF  SENSE 


jected  into  the  figures,  or  "felt  in"  them.  Always  there  is  present, 
in  the  apprehension  of  a  figure,  an  "expansion"  of  the  mental  grasp 
to  include  all  of  the  figure,  and  an  opposed  bounding  or  limiting 
activity,  by  which  the  figure  is  distinguished  from  surrounding  space. 
These  activities  are  felt  as  belonging  to  the  figure,  and  their  balance 
determines  the  apparent  length  of  a  line.  But  the  presence  of 
other  lines,  as  in  the  Miiller-Lyer  figure,  leads  to  expansions  and 
limitations  which  are  appropriate  enough  as  applied  to  the  whole 
figure,  but  not  as  applied  to  the  particular  lines  which  are  to  be 
compared.  In  many  cases,  besides  these  expanding  and  limiting 
tendencies,  other  tendencies,  drawn  from  the  dynamics  of  nature, 
are  felt  in  the  figures.  Thus,  a  vertical  line  suggests  a  struggle 


Fia.  144.— A  Test  of  the  Dynamic  Theory. 

against  gravity,  and  the  gravity-feeling  in  it  affects  its  apparent 
length.  The  same  conceptions  are  applied  by  Lipps  to  the  aes- 
thetics of  simple  forms,  and  to  architecture. 

A  difficulty  with  the  dynamic  theory  is  that  it  tends  to  run  to 
fanciful  explanations.  It  explains  too  easily  and  too  much;  it  could 
often  be  just  as  well  applied  if  the  illusions  were  the  opposite  of 
what  they  are;  for  the  dynamics  of  these  figures  is  usually  quite  am- 
biguous. For  example,  a  vertical  line  may  be  thought  of  as  stand- 
ing upright  or  as  hanging  downward;  its  relations  to  gravity  are 
opposite  in  the  two  cases;  if  gravity  tends  to  compress  it  in  the  first 
case,  it  tends  to  stretch  it  in  the  second,  and  therefore  opposite 
illusions  ought  to  result  from  the  two  ways  of  looking  at  it.  In 
Fig.  144,  therefore,  there  should  appear  quite  a  strong  illusion  as 
between  the  two  vertical  lines,  and  this  should  be  reversed  on  re- 
versing the  figure. 

§  27.  The  "confusion  theory"  might  also  be  called  the  attention 
theory.  It  takes  its  start  from  the  evident  fact  that  the  illusion 
figures  are  so  constructed  as  to  make  difficult  the  isolation  of  the 
particular  feature  which  is  to  be  judged.  It  is  difficult  to  fix  atten- 
tion on  this  one  feature,  thrusting  aside  all  complicating  features. 


CONFUSION  THEORY  OF  ILLUSION  451 

If  this  difficulty  is  only  partially  overcome,  judgment  will  be  based 
on  a  partial  confusion  of  complicating  features  with  the  feature 
which  is  ostensibly  judged.  Thus,  in  attempting  to  compare  the 
areas  of  variously  shaped  surfaces  (Figs.  142  and  143),  one  feels  the 
difficulty  of  isolating  the  bare  feature  of  area  from  the  more  ob- 
trusive feature  of  compactness;  and  the  errors  committed  go  to 
show  that  the  isolation  has  not  been  fully  carried  out.  In  the  Miiller- 
Lyer  and  other  "confluxion"  illusions,  it  is  much  easier  to  take  ar- 
row-heads, etc.,  as  units,  and  judge  the  distance  between  them,  than 
to  isolate  the  exact  points  which  mark  the  ends  of  the  distances  to 
be  compared;  and  the  errors  committed  seem  to  show  that  the  iso- 
lation is  not  fully  carried  out.  The  nature  of  the  "confusion"  in 
the  cases  of  "contrast"  illusions  is  not  so  readily  pointed  out;  and 
in  the  vertical-horizontal  and  other  specific  illusions,  special  causes 
would  probably  have  to  be  invoked.  Nor  is  it  entirely  easy  to  ap- 
ply this  conception  to  the  illusions  of  angle  and  direction,  though 
the  judgment  of  these  matters  must  be  admitted  to  be  rather  indi- 
rect and  indefinite,  and  such,  therefore,  as  might  probably  be  sub- 
ject to  confusion.  The  confusion  theory  has  at  least  this  advantage, 
that  it  almost  certainly  affords  the  true  explanation  of  some  of  the 
illusions; — and  this  is  more  than  can  be  said  of  the  other  central 
theories.  But  reference  to  confusion  takes  us  only  a  little  way,  and, 
beyond  that,  leaves  various  possibilities  of  explanation  open  in 
each  particular  case.  What  is  needed  is  to  point  out  the  nature  of 
the  confusion  in  each  class  of  illusions;  and  this  cannot  yet  be  done 
with  any  certainty. 

Rather  in  favor  of  the  confusion  theory,  further,  is  the  fact  that 
practice  in  making  the  required  judgments,  in  such  cases  as  the 
Miiller-Lyer  and  Poggendorf  figures,  produces  a  gradual  decrease 
and  ultimate  disappearance  of  the  illusion;  and  this  result  follows, 
even  though  the  observer  does  not  know  the  nature  of  his  errors.1 
The  probable  explanation  of  this  practice  effect  is  that  the  observer 
is  conscious  of  the  difficulty  of  isolating  the  feature  to  be  judged, 
and  therefore  devotes  himself  to  this  isolation.  The  skill  which  he 
thus  acquires  in  thrusting  aside  complicating  features  of  the  figures 
is  in  part  a  specific  aptitude  in  dealing  with  a  particular  figure,  and 
may  not  be  transferred  promptly  to  another  figure  or  even  to  a 
changed  position  of  the  same  figure,  yet  facility  in  dealing  similarly 
with  another  figure  is  more  easily  acquired  because  of  the  previous 
practice.  That  the  practice  effect  truly  consists  in  the  acquiring 
of  skin_iDLJsolatiori  is  indicated  by  the  observation  of  Benussi2 — • 
namely,  that  thejllusion  is  muj£h,m.pre_oj.ijckly  overcome  when  prac- 

1  Judd,  Psychol  Rev.,  1902,  IX,  27;  Lewis,  Brit.  Journ.  of  PsychoL,  1908,  II,  294. 
3  Op.  cit. 


452  PRESENTATIONS  OF  SENSE 

tice  is  governed  from  the  outset  by  a  clearly  formulated  intention 
to  isolate  a  certain  feature  of  the  figure;  while,  on  the  contrary, 
practice  with  the  definite  intention  of  always  grasping  the  figure  as 
a  whole  leads  not  to  the  usual  decrease,  but  to  an  actual  increase 
of  the  illusion.  In  concluding  the  whole  matter,  we  may  confidently 
assert  that  the  geometrical  illusions,  or  most  of  them,  are  chiefly 
due  to  central  rather  than  peripheral  factors;  and  that  these  central 
factors  are  intimately  bound  up  with  the  tendency  to  apprehend 
figures,  and  compact  parts  of  figures,  as  wholes.  It  is,  in  most 
cases,  if  not  in  all,  as  wholes,  or  units  of  "  apperception,"  that  we 
have  been  accustomed  to  regard  them.  And  this  tendency  may, 
therefore,  well  enough  be  the  chief  cause  of  the  illusion. 

§  28.  The  necessity  of  appealing  to  certain  obscure,  but  effective 
central  factors,  as  mingled  with,  or  even  dominating  the  peripheral 
factors,  is  felt  in  trying  to  account  for  those  errors  of  sense  which 
occur  in  connection  with  the  strife  and  prevalence  of  contours, 
and  the  binocular  mixing  and  contrast  of  colors.  If  a  well-defined 
image  of  some  contour,  such  as  a  sharp-marked  limit  between  two 
differently  colored  surfaces,  be  formed  on  one  retina,  and  on  the 
corresponding  points  of  the  other  the  image  of  a  uniform-colored 
background,  then  only  the  former  will  be  visible.  This  is  called 
the  "prevalence  of  contours."  But  if  the  contours  of  the  images  of 
two  differently  colored  objects  run  on  the  retina  so  as  to  cross  only 
in  one  place,  then  sometimes  one  color  and  sometimes  the  other  will 
prevail  and  get  itself  perceived  at  that  place.  This  is  called  "the 
strife  of  contours."  If  two  squares  of  red  paper  and  two  of  blue, 
all  of  equal  size  and  brightness  and  without  any  distinguishing 
marks,  be  laid  side  by  side  at  equal  distances,  and  their  images 
then  combined,  the  color  of  the  middle  one  of  the  binocular  images 
will  at  first  be  sometimes  redder  and  sometimes  bluer  than  that  of 
the  two  side  images,  but  in  no  case  exactly  like  either  of  them.  By 
steadily  looking  it  is  said  to  be  possible  to  mix  the  colors  of  the  two 
objects  in  a  binocular  image  which  is  reddish  blue  (or  violet).1  This 
is  called  "  the  binocular  mixing  of  colors."  If  such  a  deception  can 
be  secured,  and  made  subject  to  the  more  or  less  voluntary  direc- 
tion of  attention,  it  is  manifest  that  the  mixing  of  colors  on  which  it 
depends  must  be  largely  central,  and  not  wholly  dependent  upon 
the  retinas  of  the  two  eyes.  If  a  white  stripe  be  placed  upon  a  black 
surface  and  divided  into  two  images,  the  right  one  of  which  is 
formed  by  looking  at  one  half  through  blue  glass,  the  left  by  look- 
ing through  gray  glass,  then  the  right  image  will  be  seen  blue,  but  the 
left  will  be  seen  yellow.  This  is  called  "  binocular  contrast  of  colors." 

1  So  Hering  asserts,  "Physiolog.  Optik,"  in  Hermann's  Handb.  d.  Physiol,  III, 
i,  p.  592.  Binocular  mixing  of  colors  has  been  denied  by  some  authorities. 


BINOCULAR  MIXING  AND  CONTRAST  453 

The  peculiar  perception  of  lustre  is  due  to  a  struggle  between 
the  two  fields  of  vision  which  results,  not  in  combining  the  black 
images  of  one  field  with  the  white  images  of  the  other  so  as  to  pro- 
duce an  equal  tint  of  gray,  but  in  a  rapid  alternation  of  the  two. 
Very  smooth  bodies,  when  they  reflect  the  light  perfectly,  do  not 
appear  lustrous.  But  when  the  surface  of  such  bodies — as,  for 
example,  the  surface  of  a  sheet  of  water — becomes  ruffled  by  rip- 
ples, it  becomes  lustrous.  The  perception  of  lustre  may  be  pro- 
duced by  combining  two  stereoscopic  pictures  of  an  object  which 
are  alike  in  contour,  but  one  of  which  is  black  with  white  lines  where 
the  other  is  white  with  black  lines.  Two  such  pictures  not  com- 
bining to  produce  an  equal  tint  of  gray  over  the  whole  surface,  the 
images  of  the  separate  points  on  the  two  retinas  enter  into  a  strug- 
gle with  each  other;  and  the  rapid  alternation  of  the  prevalence, 
first  of  one  and  then  of  the  other,  gives  rise  to  the  appearance  of 
lustre. 

Similar  conclusions  seem  forced  upon  us  by  our  experience  with 
certain  of  the  most  common  and  persistent  optical  illusions  of  mo- 
tion. The  fact  that  a  steady  succession  of  images  (as  in  the  case 
of  watching  a  fall  of  water),  passing  over  a  particular  region  of  the 
retina  for  a  long  time,  sometimes  ceases  to  be  perceived  as  a  motion, 
and  that  the  image  of  a  stationary  body  on  the  same  retinal  region 
may  appear  to  be  moving  in  the  opposite  direction,  has  been  ex- 
plained by  "Thomson's  law."  This  law  refers  the  phenomena  to 
the  principle  of  fatigue.  Recent  investigations,  however,  seem  to 
show  that  the  explanation  is  incorrect.  They  bring  out  the  remark- 
able result  that  the  same  elements  of  the  retina,  when  stimulated 
simultaneously,  may  give  rise  to  impressions  both  of  motion  and 
of  rest.  For  this  result  some  unknown  law  of  cerebral  action  would 
seem  to  afford  the  only  possible  explanation.1 

§  29.  The  fact  that  things  are  seen  upright  and  in  correct  rela- 
tions horizontally,  by  means  of  data  furnished  through  inverted  reti- 
nal images,  as  well  as  all  illusions  and  errors  that  are  connected  with 
this  normal  fact,  implies  yet  more  maturity  of  experience.  Why  do 
we  see  the  upper  part  of  the  object  by  means  of  the  lower  part  of  the 
retinal  image,  and  vice  versa  ?  and  why  do  we  see  the  right  side  of 
the  object  by  means  of  the  left  side  of  the  retinal  image,  and  vice 
versa?  Such  questions  have  often  been  propounded  as  psycho- 
logical puzzles  of  special  difficulty.  The  only  answer  possible  fol- 
lows, obviously,  from  the  foregoing  principles.  Strictly  speaking, 
we  neither  see  the  external  object  nor  the  retinal  image;  the  field 
of  vision  is  a  subjective  affair,  and  is  like  neither  of  these  two.  The 
presentation  of  visual  sense  is  normally  dependent  upon  the  retinal 
1  See  Bowditch  and  Hall,  Journal  of  Physiology,  III,  pp.  299  f. 


454  PRESENTATIONS  OF  SENSE 

image  for  the  data  from  which  it  is  constructed;  the  image  is  de- 
pendent upon  the  external  object  for  its  formation  by  rays  of  light 
reflected  from  the  object  and  converged  upon  the  nervous  elements 
of  the  retina.  The  different  parts  of  the  object  as  seen  are  pri- 
marily localized  simply  with  reference  to  each  other  by  means  of 
local  retinal  signs  and  of  complex  sensations  produced  by  motion 
of  the  eyes.  But  as  yet  the  field  of  vision  has  no  locality  in  ob- 
jective space;  no  part  of  it  can  be  said  to  be  either  up  or  down, 
either  right  or  left.  The  use  of  such  terms  of  position  implies  an 
association  of  localized  sensations  of  sight  with  those  of  touch  and 
of  the  muscular  sense,  in  giving  us  a  picture  of  the  relation  of  the 
different  parts  of  the  body  to  each  other,  and  of  the  entire  body  to 
the  ground,  the  sky,  and  the  various  parts  of  surrounding  objects. 
When  the  eyes  are  moved  downward,  the  lower  parts  of  the  body 
and  objects  situated  on  the  ground  successively  come  into  the  field 
of  vision;  when  the  eyes  are  moved  upward,  the  near  ground  and 
lower  parts  of  objects  successively  disappear  from  the  field  of  vision, 
and  remoter  or  higher  objects  come  to  view.  Seeing  objects  to  the 
right  or  to  the  left  is  accomplished  by  motion  of  the  eyes  in  the  cor- 
responding direction.  Right  is  the  direction  in  which  the  right 
hand  is  placed  from  the  middle  of  the  body;  left  is  the  direction  in 
which  the  left  hand  is  found.  The  massive  feelings  of  touch  and 
muscular  sensation  keep  us  informed  of  the  general  relation  of  our 
bodies  to  the  earth  and  to  objects  on  its  surface.  The  head  is  the 
upper  part,  or  part  farthest  away  from  the  ground;  the  feet  are  the 
lower  part,  or  members  of  the  body  in  contact  with  the  ground. 
Thus  we  come  to  use  terms  for  localized  sensations  of  sight  which, 
in  this  use  of  them,  have  no  primary  reference  whatever  to  the  field 
of  vision  in  itself  considered. 

§  30.  The  above  view  of  the  relation  of  erect  vision  to  the  in- 
verted retinal  image  is  substantiated  by  the  experience  of  one  look- 
ing through  a  microscope.  Since  the  ordinary  microscope  itself 
gives  an  inverted  image  of  the  object,  the  second  inversion  by  the 
eye  gives  an  image  on  the  retina  which  is  erect  with  respect  to  the 
object,  though  inverted  with  respect  to  the  usual  retinal  image  of 
an  object.  There  is,  however,  no  distortion  or  internal  change 
(except  magnification)  in  the  spatial  relations  within  the  image. 
Nor  is  there  any  change  in  the  eye  movements  necessary  to  bring 
any  part  of  the  image  to  the  area  of  clear  vision;  for  these  movements 
all  involve  a  movement  of  the  fovea  toward  a  point  of  the  optical 
image  on  the  retina,  and  the  latter  movement  remains  the  same  no 
matter  what  inversions,  or  even  distortions,  the  rays  of  light  may  have 
undergone  on  their  way  to  the  retina.  In  other  words,  the  eye- 
movement  value,  as  it  may  be  called,  of  the  various  retinal  points  is 


UPRIGHT  AND  INVERTED  VISION  455 

unaltered  by  changes  in  the  rays  of  light  on  their  way  to  the  retina. 
Therefore,  the  observer,  looking  through  the  microscope,  obtains 
an  internally  correct  view  of  the  object,  and  experiences  no  difficulty 
in  directing  his  eyes  to  any  point  of  it.  His  difficulty  begins,  how- 
ever, when  he  attempts  to  move  the  object  under  the  microscope, 
so  as  to  bring  into  view  a  fresh  part  of  it;  for  now  the  long-standing 
connection  between  the  retina  and  the  movements  of  the  hand  plays 
him  false;  and  he  is  sure  to  move  the  object  to  the  left  when  he  should 
move  it  to  the  right,  and  up  when  he  should  move  it  down.  No  long 
experience  with  the  microscope,  however,  is  needed  to  overcome  this 
difficulty  by  establishing  a  new  association  between  retinal  direc- 
tions and  appropriate  hand  movements.  This  new  association  is 
operative  only  within  the  special  situation  of  looking  through  a 
microscope;  it  gives  way  at  once  to  the  usual  association  on  removing 
the  eye  from  the  instrument.  Much  the  same  effect  of  practice 
is  noted  in  handling  objects  while  viewing  them  through  a  mirror; 
though  here  the  inversion  is  not  complete. 

An  interesting  experiment,  along  the  line  of  these  observations, 
was  performed  by  Stratton.1  While  covering  one  eye,  he  placed 
before  the  other  an  optical  instrument  which,  like  the  microscope, 
gave  an  inverted  image  of  objects,  and  so  an  upright  image  on  the 
retina.  No  light  was  allowed  to  enter  the  eye  except  through  this 
inverting  instrument,  and  this  instrument  was  worn  continuously  in 
the  daytime,  both  eyes  being  covered  when  the  instrument  was  re- 
moved for  sleep.  The  experiment  was  continued  for  a  week,  with  the 
result  that  movements  of  the  hands  and  body,  which  on  the  first  day 
suffered  great  confusion,  became  rapidly  associated  with  the  novel 
visual  relations,  so  that  before  the  end  of  the  week  all  bodily  move- 
ments were  correctly  and  promptly  executed  under  the  direction 
of  the  eye;  and,  whereas  at  first  everything  had  appeared  upside 
down,  this  appearance  also  gave  place  to  a  normal  appearance  after 
a  few  days.  When  the  instrument  was  removed  at  the  end  of  the 
week,  a  new  period  of  confusion,  false  movements,  and  inverted  ap- 
pearance supervened  before  the  old  association  was  re-established. 
This  experiment,  as  well  as  the  experiences  with  the  microscope  and 
mirror,  are  interesting  as  revealing  a  remarkable  looseness  or  flexi- 
bility in  the  association  between  the  spatial  perceptions  of  different 
senses. 

§  31.  In  concluding  this  brief  survey  of  the  very  fruitful  field 
afforded  by  so-called  "errors  of  sense,"  as  bearing  upon  any  attempt 
to  frame  an  adequate  theory  of  visual  perception,  we  mention  the 
following  inferences  as  those — it  seems  to  us — which  are  most  amply 

1  "Vision  without  Inversion  of  the  Retinal  Image,"  Psychol.  Rev.,  1897,  IV, 
341,  463. 


456  PRESENTATIONS  OF  SENSE 

supported.  It  should  be  said,  in  a  preliminary  way,  that  the  great 
variety  of  special  and,  too  often,  exclusive  and  partisan  theories 
adopted  by  different  observers,  is  what,  under  the  circumstances, 
might  be  expected.  For  explanation  in  general  is,  for  this  sort  of 
phenomena,  an  extremely  intricate  affair.  And  this  is  true,  even 
when  the  phenomena  themselves  seem  to  be  of  the  utmost  simplicity 
in  character.  A  variety  of  theories,  therefore,  may  well  enough  ap- 
peal to  certain  facts  of  experience,  or  rather,  to  certain  aspects  of 
common  facts,  to  sustain  their  special  contentions.  Perhaps,  too, 
they  are  all  needed;  or  at  least,  they  may  all  be  said  to  contain  cer- 
tain of  the  elements  indispensable  to  a  complete  explanation.  The 
difficulty  with  nearly  all  of  them,  however,  is  that  they  fail  to  take 
sufficient  account  of  the  equally  important  facts  on  which  other 
rival  theories  are  based.  Moreover,  it  must  not  be  forgotten  that 
essentially  the  same  errors  of  sense  may  in  differing  individual  cases, 
or  under  slightly  differing  circumstances,  require  a  somewhat  differ- 
ent set  of  explanations.  The  nervous  mechanism  engaged,  and 
the  mental  development  acquired,  in  the  growth  of  perception 
through  the  use  of  the  eyes,  and  the  associations  of  this  growth  with 
the  senses  of  touch  and  hearing,  are  variable  to  a  certain  extent 
which  cannot  be  determined  a  priori.  It  is  a  well-known  fact  that 
the  mental  pictures  and  conceptions  of  spatial  extensions  and  of  the 
relations  of  objects  in  space,  are  not  by  any  means  the  same  with 
all  adults.  The  claim  has  even  been  made  that  they  are,  in  general, 
somewhat  markedly  different  with  the  two  sexes.  And  certainly, 
in  this  regard,  the  accomplished  geometrician,  or  microscopist,  or 
astronomer,  is  at  a  world-wide  remove  from  the  savage  or  the  newly- 
born  infant. 

§  32.  At  the  same  time  we  repeat,  with  increased  confidence, 
that  the  most  recent  investigations  in  this  line  tend  to  confirm,  in 
its  essential  features,  the  theory  of  visual  perception,  especially  on 
the  side  of  the  apprehension  of  spatial  qualities  and  spatial  relations, 
which  has  been  advocated  in  the  preceding  sections;  and  which 
may — at  least,  for  most  of  its  more  important  connections — be  as- 
sociated with  the  great  names  of  Lotze,  Helmholtz,  and  Wundt. 
Its  essential  features,  as  we  are  inclined  to  adopt  it,  while  confess- 
ing the  many  obscurities  and  difficulties  which  still  accompany  any 
attempt  at  completing  it,  are  the  following: 

First:  The  data,  or  factors,  on  the  basis  of  which,  so  to  say,  the 
construction  of  visual  space  proceeds,  have  all  the  characteristics 
required  by  the  so-called  "local  signs"  (compare  p.  386).  They 
have  these  characteristics  in  the  highest  degree.  They  are  series  of 
sensations,  which,  as  series,  are  of  like  quality;  which  admit  of 
easy,  rapid,  and  frequent  repetition  in  a  varying  order  of  arrange- 


CONCLUSIONS  AS  TO  VISUAL  PERCEPTION        457 

ment;  and  which,  by  the  very  nature  of  the  organism  and  the  condi- 
tions of  mental  development,  are  adapted  to  combine  with  other 
series  of  like  characteristics.  Of  such  series  the  most  important  are 
(1)  the  retinal  signs,  and  (2)  the  entire  complex  of  tangled  and  ob- 
scure sensations  which  accompany  and  follow  the  attaining  and 
holding  of  any  position,  by  movement  of  the  eyes. 

Second:  As  to  the  retinal  signs,  they  may  most  properly  be  spoken 
of  as  "native" — at  least,  in  the  sense  that  their  existence  and  dis- 
criminable  character  must  be  assumed  as  "given"  (datum)  in  any 
attempt  to  explain  the  initial  steps  of  visual  perception.  There  is 
scarcely  less  doubt  that  the  motor  reactions  which,  of  necessity, 
evoke  the  first  complex  sensations  of  position  (even  of  eye-strain) 
and  of  motion,  are  native,  in  the  sense  that  they  are  innate  powers 
and  are  not  learned  by  the  infant.  Indeed,  it  is  by  no  means  cer- 
tain that  the  resulting  sensations  may  not  have  from  the  first  a  cer- 
tain value  as  local  signs,  comparable  in  a  way  to  that  of  the  associ- 
ated local  signs  of  the  retina. 

In  this  connection,  attention  should  be  called  to  the  fact  that  the 
"centre  of  clear  vision"  is  not  a  point,  but  an  area  within  which 
many  points  can  be  distinguished;  this  makes  it  difficult  to  accept 
any  theory  which  regards  visual  space  as  organized  about  a  single 
point.  The  analogy  of  latitude  and  longitude  in  geography,  or 
of  co-ordinate  geometry,  is  tempting  to  the  psychologist  but  is  rather 
misleading.  The  organization  is  indeed  partly  of  this  sort,  as  is 
shown  by  the  fact  that  an  object  seen  in  indirect  vision  attracts  the 
eye  to  make  a  movement  toward  it;  and  that  this  movement  is 
about  sufficient  in  extent  to  bring  the  object  to  the  centre  of  clear 
vision.  To  regard  visual  space  as  a  system  of  "polar  co-ordinates," 
the  position  of  every  point  in  the  field  of  view  being  determined  by 
its  direction  from  the  "origin,"  or  centre  of  clear  vision,  and  by  its 
distance  from  that  origin,  is  not,  however,  an  adequate  account  of 
visual  space.  The  "origin"  is  here  not  really  a  point;  and  the  sub- 
division of  the  field  is  much  finer  than  can  be  accounted  for  in  this 
way.  Neighboring  and  probably  also  distant  points  in  the  field 
must  be  related  directly  to  one  another,  and  not  simply  to  the  centre 
of  clear  vision.  The  actual  organization  of  visual  space  is,  in  gen- 
eral, from  a  mathematical  point  of  view,  much  less  simple  and  clean- 
cut  than  a  system  of  polar  co-ordinates.1 

Third:  As  between  nativistic  and  empiristic  theories,  the  balance 
of  evidence  seems  to  favor  the  former.  Certainly  both  types  of 
eye  movement  are  innate  powers,  and  not  learned  by  the  infant. 
Their  native  character  does  not,  perhaps,  prove  that  spatial  con- 

1  See  Dodge,  Psychol  Rev.,  Monograph  Suppl.  XXXV  (1907),  p.  64. 


458  PRESENTATIONS  OF  SENSE 

sciousness  is  innate,  but  it  does  show  that  the  spatial  organization 
of  the  retina  and  of  its  nervous  connections  is  partly  innate.  Bio- 
logically, it  would  be  difficult  to  believe  that  a  fixed  form  of  organiza- 
tion, such  as  that  of  visual  space,  should  be  newly  acquired  by  every 
individual.  This  consideration  applies  with  most  force  to  two- 
dimensional  space,  but  also  with  some  probability  to  the  third  di- 
mension. In  so  far  as  distance  from  the  eye  is  definitely  related  to 
binocular  parallax,  the  reactions  of  the  eyes  and  limbs  to  the  distance 
of  objects  might  well  be,  in  a  measure,  instinctive.  They  certainly 
are  instinctive  in  some  animals;  thus  the  chick,  on  emerging  from 
the  egg,  correctly  gauges  the  distance  of  a  grain  at  which  he  pecks; 
and  human  infants,  in  spite  of  what  has  sometimes  been  alleged 
to  the  contrary,  show  little  tendency  to  reach  for  the  moon  or  for 
anything  that  is  definitely  beyond  their  reach. 

Fourth:  The  incalculable,  but  enormous  influence  on  sense-ex- 
perience which  lies  back  of  all  the  phenomena  obtained  for  scien- 
tific treatment,  whether  from  the  physiological,  the  psycho-experi- 
mental, or  the  purely  introspective  point  of  view,  must  never  be  lost 
out  of  account.  Indeed,  these  residua  of  past  experiences,  if  we 
may  so  call  them,  are  doubtless  in  many  cases  the  determining  causes 
of  the  character  of  the  new  experience.  They  consist  of  obscure 
and  scarcely  recognized  sensations,  images  of  previous  sensations, 
motor  tendencies  and  impressions,  fusions  of  unanalyzable  elements, 
flighty  and  flitting  syntheses  that  have  scarcely  the  quality  of  even 
an  instinctively  formed  judgment;  and — perhaps,  above  all, — 
workings  of  the  organism  which  do  not  result  in  any  effect  that  rises 
" above  the  threshold"  of  consciousness.  But  it  is  just  such  un- 
recognized, and  largely  unanalyzable,  factors  as  these,  which 
chiefly  determine,  not  only  our  conduct  under  the  direction  of  sight, 
but  also  our  seemingly  most  logical  conceptions  and  deliberate 
judgments  concerning  visual  objects. 

Fifth:  To  deny  the  influence  of  "central"  factors,  and  the  ne- 
cessity of  a  theory  of  perception  which  gives  them  their  full  value, 
would  be  to  controvert  everything  which  we  know  about  the  nature 
of  the  nervous  mechanism  and  its  relations  to  the  awakening  and 
development  of  mental  life.  We  have  already  said  that  only 
psychical  factors  can  be  fused,  or  united,  or  consciously  judged  to- 
gether in  any  sort  of  relation,  in  a  mental  product.  Of  this  declara- 
tion, the  physiological  correlate  is,  of  course,  the  recognition  of 
the  influence  of  the  cerebral  mechanism  in  every  sort,  and  every  in- 
dividual experience,  of  sense-perception.  And  we  only  return  once 
more  to  the  psychological  point  of  view  when  we  recognize  anew  the 
abundant  evidence  that  attention,  active  discrimination,  varied 
forms  of  conscious  feeling  other  than  sensation,  and  innumerable 


GRAPHIC  RECORDS  OF  EYE    MOVEMENTS        459 

forms  of  association,  all  enter  into  our  experience  of  objects  as 
known  by  the  organism  employed  in  vision. 

§  33.  In  all  the  older  discussions  there  was  little  exact  knowledge 
of  the  actual  movements  made  by  the  eyes  in  their  mastery  of  the 
details  of  spatial  perception.  Even  the  laws  summarized  under 
the  name  of  Listing  were  based  upon  a  study  of  the  positions  reached 
by  movement  rather  than  of  the  movements  themselves.  Recently, 
several  observers  have  obtained  actual  registration  of  these  move- 
ments; and  the  results  must,  assuredly,  be  harmonized  with  any 
theory  of  visual  space-perception. 

The  simple  experiment  of  comparing  the  observations  of  the  move- 
ments of  the  eyes,  as  seen  by  another  observer,  with  those  which 
our  consciousness  seems  to  assure  us  are  made,  is  sufficient  to  show 
how  uncertain  is  the  latter  kind  of  evidence  in  support  of  the  facts. 
In  reading  a  line  of  print,  for  example,  one  may  seem  to  one's  self 
to  be  moving  the  eyes  smoothly  along  the  line,  while  objective  ob- 
servation shows  that  the  actual  movements  are  a  series  of  jerks, 
separated  by  brief  periods  of  rest.  The  jerky  character  of  this 
movement  is  difficult,  or  impossible  to  correct;  whereas,  in  follow- 
ing a  moving  object  the  eyes  themselves  move  in  a  fairly  steady 
way  and  at  a  speed  regulated  by  the  speed  of  the  object. 

§  34.  The  needed  method  of  obtaining  an  accurate  graphic 
record  of  the  eye  movements  was  first  successfully  applied  by  Dela- 
barre,1  and  was  also  employed  with  success  by  Huey.2  These  ex- 
perimenters obtained  a  record  by  attaching  a  light  disk  to  the  sur- 
face of  the  eyeball;  and  from  the  disk  a  thread  ran  to  a  light  lever 
which  wrote  upon  a  moving  surface.  This  method  had  the  disad- 
vantage of  weighting  the  eye  and  so  producing  possible  distortions 
in  its  movements. 

A  much  improved  device  for  photographing  the  eye  while  in 
motion  has  more  recently  been  accomplished  by  several  observers.3 
This  method  consists  in  general  of  obtaining  a  photographic  record 
of  the  movements  of  a  beam  of  light  reflected  from  the  cornea  itself, 
or  from  some  object  attached  to  the  cornea.  Among  the  important 
facts  established  by  this  method  are  one  or  two  relating  to  the  fixa- 
tion of  the  eye.  When  the  eye  "rests  upon  an  object,"  the  fovea, 
or  centre  of  clear  vision,  has  a  diameter  of  one  or  two  degrees,  and 
what  falls  outside  of  this  area  is  seen  with  diminishing  clearness  as 

1  American  Journal  of  Psychology  (1898),  IX,  572. 

3  Ibid.,  p.  575;  and  (1900)  XIII,  282. 

3  For  a  more  detailed  account  of  the  methods  and  results  of  these  experiments, 
see  Dodge  and  Cline,  Psychol.  Rev.  (1901),  VIII,  145;  Stratton,  Phil  Stud.  (1902), 
XX,  336,  and  Psychol  Rev.  (1906),  XIII,  82;  and  Judd,  Psychol.  Rev.,  Mono- 
graph Supplement  XXIX  (1905). 


460  PRESENTATIONS  OF  SENSE 

we  move  toward  the  periphery  of  the  visual  field.  In  other  words, 
the  fovea  is  not  a  mathematical  point;  and,  consequently,  fixation 
of  the  eye  is  itself  not  a  perfectly  precise  and  rigid  affair.  Within 
this  area  there  is  a  constant  slow  (exploring  ?)  movement  of  the  eye ; 
and  if  the  eye  is  momentarily  turned  away  from  any  point  in  the 
area,  on  bringing  it  back  to  examine  the  same  point  its  new  posi- 
tion is  not  usually  the  same  as  the  old.1 

§  35.  In  regard  to  speed,  the  movements  of  the  eye  fall  into  two 
main  classes.2  Of  the  first  type  are  the  movements  by  which  an 
object,  first  seen  in  indirect  vision,  is  brought  to  the  centre  of  clear 
vision.  This  is  the  simple  reactive  movement  of  the  eye  to  periph- 
eral stimulation,  with  its  psychological  accompaniment  of  a  trans- 
ference of  attention.  Its  speed  depends  upon  the  extent  of  the  move- 
ment, being  more  rapid  for  longer  than  for  shorter  movements;  it 
varies  from  .029  seconds  for  movements  of  5  degrees  to  .100  for 
movements  of  40  degrees.3  In  reading,  however,  the  extent  of  this 
movement  is  often  less  than  5  degrees,  and,  in  time,  not  more  than 
one-fiftieth  of  a  second.  The  speed  is  not  readily,  or  at  all,  sub- 
jected to  voluntary  control.  The  function  of  this  first  type  of  eye 
movement  is  simply  that  of  carrying  the  eye  from  one  point  of 
fixation  to  another,  with  the  least  possible  loss  of  time.  No  clear 
vision  results  from  the  stimulation  of  the  retina  during  these  jumps 
of  the  eye — nothing,  in  ordinary  circumstances,  but  a  brief  and 
featureless  blur,  which  has  no  value  for  purposes  of  visual  percep- 
tion, and  which  is  usually  unobserved.4 

The  second  main  type  of  movement  is  the  "pursuit"  movement, 
which  occurs  when  a  moving  object  is  followed  by  the  eye.  This 
is  determined  by  the  speed  of  the  object,  but  neither  very  swiftly, 
nor  very  slowly,  can  moving  objects  be  followed  by  the  eye.  This 

1  McAllister,  Psycholog.  Rev.,  Monograph  Supplement  XXIX  (1905),  p.  17. 
The  reader  can  readily  satisfy  himself  of  the  truth  of  these  statements  by  what  is 
called  the  "after-image  method"  of  studying  eye  movements — a  method  long 
in  use,  and  in  many  respects  valuable.     An  after-image  resulting  from  fixation 
of  a  bright  object  moves  with  the  eye  and  represents  a  fixed  point  of  reference 
in  the  retina.     If  a  sharp  after-image  is  first  secured,  and  then  the  attempt  is 
made  to  look  steadily  at  a  point  on  the  wall,  the  after-image  will  seem  to  move 
about  over  the  wall,  indicating  the  unsteadiness  of  fixation  (see  Dodge,  Psychol. 
Rev.,  Monograph  Supplement  35, 1907).     A  variation  of  this  experiment  shows 
that  in  repeatedly  looking  at  the  same  object,  there  is  a  certain  latitude  in  the 
fixations  of  the  eye.     If  a  bright  light,  such  as  the  rising  or  setting  sun,  is  looked 
at  repeatedly  for  a  second  or  two,  the  eye  being  turned  to  some  other  object  be- 
tween the  fixations,  quite  a  group  of  after-images  of  the  sun  will  accumulate; 
and  the  extent  of  the  group  will  show  the  variability  of  repeated  fixations. 

2  Dodge,  Amer.  Journ.  of  Physiol.,  VIII,  301. 

3  See  the  table  given  by  Dodge  and  Cline,  op.  cit.,  145. 

4  Dodge,  Psychol.  Rev.,  1900,  VII,  454. 


SPEED  OF  EYE   MOVEMENTS  461 

second  type  of  movements  of  the  eye  differs  from  the  first  type,  not 
only  in  speed,  but  also  in  function.  Pursuit  movements  cannot  be 
executed  at  will;  they  occur  only  in  the  presence  of  a  moving  ob- 
ject. Without  them,  however,  clear  vision  of  moving  objects  is 
impossible;  as  can  easily  be  shown  by  casting  a  shadow  on  a  page 
of  print,  steadily  fixating  the  shadow,  and  then  either  moving  the 
page  from  side  to  side  before  the  fixed  eye,  or  causing  the  shadow  to 
move  back  and  forth  over  the  page  and  al- 
lowing the  eye  to  follow  it.  In  either  case, 
the  print  becomes  unreadable  through  blur- 
ring. The  same  effect  is  produced  when  the 
eye  passes  rapidly  from  one  to  another  of  a 
few  bright  spots  standing  out  against  a  dark 
background.  From  these  and  similar  expe- 
riences, we  reach  the  conclusion  that,  in  read- 
ing, we  obtain  fairly  clear  retinal  impressions 
only  during  the  fixation  pauses  of  the  eye, 
and  that  the  practical  valuelessness  of  the  Movement  in  Tracing  the 

•j         *    ui        •         i      j  Ai_  i  j?        Outline  of  a  Circle.  (Strat- 

periods  or   blurring  leads  to  the  neglect  or      ton.) 
them.     The  same  kind  of  jerky  movement 

of  the  eyes,  with  periods  of  fixation  between  periods  of  blurring, 
occurs  when  we  seem  to  ourselves  to  be  sweeping  our  eyes  slowly 
over  any  geometrical  figure,  or  over  any  scene.  This  process  has 
been  carefully  worked  out  in  the  case  of  one  reading  a  printed  page.1 
The  eye  moves  across  the  line  of  a  newspaper  with  a  series  of 
jumps,  fixating  from  3  to  8  points  in  a  line,  and  with  a  speed  de- 
pending on  the  mentality  of  the  reader.2  The  accompanying  fig- 
ure (No.  145),  taken  from  Stratton,3  shows  how  the  eye  "moves 
around"  a  circle  which  has  been  placed  before  it. 

§  36.  In  general,  it  must  be  noted  that  the  power  of  making 
spatial  distinctions  greatly  exceeds  the  accuracy  with  which  dis- 
tances can  be  measured  by  the  moving  eye.  A  circle  of  an  inch  in 
diameter,  placed  five  feet  from  the  eye,  is  practically  a  unit  to  fixa- 
tion, although  it  contains  a  multitude  of  distinguishable  points, 
and  a  variety  of  perceptible  lines,  angles,  and  figures.  Spatial  dis- 
tinctions in  indirect  vision  are  finer  than  the  adjustment  of  eye  move- 
ments to  points  in  indirect  vision.  The  eye  can  measure  the  dis- 
tance between  two  points  much  more  exactly  than  it  can  jump  from 
one  to  another.4  It  can  distinguish  forms  much  more  accurately 
than  it  can  follow  them  by  its  movements. 

1  Compare  Huey,  American  Jour,  of  Psychol.  (1900),  XIII,  283;  and  Dearborn, 
The  Psychology  of  Reading  (New  York,  1906). 

2  Ruediger,  The  Field  of  Clear  Vision  (New  York,  1907). 

3  Philos.  Studien  (1902),  XX,  336,  and  Psychol.  Rev.  (1906),  XIII,  82. 

4  McAllister,  op.  cit.,  p.  17;  Dearborn,  Psychol.  Rev.,  1904,  XI,  297. 


462  PRESENTATIONS  OF  SENSE 

These  experiments  also  show  that  we  can  no  longer  appeal,  as 
Helmholtz  and  Wundt  formerly  did,  to  the  supposed  orderly  pro- 
cession of  adjacent  points  of  the  field  of  view  through  the  area  of 
clear  vision  as  the  eye  is  moved.  There  is  no  such  procession;  and 
nothing  objective  passes  in  such  review.  As  a  specific  instance  of 
the  difficulty  connected  with  this  same  view,  let  us  consider  a  prob- 
lem which  is  fundamental  in  the  perception  of  two-dimensional 
space.  Such  a  space  is  characterized,  as  Helmholtz  pointed  out,1 
by  the  circumstance  that,  given  any  two  points  in  it,  A  and  B,  it 
is  possible  to  enclose  A  by  a  circle  or  any  other  closed  line,  so  that 
passage  from  A  to  B  can  only  occur  by  crossing  the  line.  Now 
Helmholtz  endeavors  to  show  that  experience  could  teach  us  this 
peculiarity  of  the  visual  field,  since,  whenever  we  turn  the  eye  from 
A  to  B,  we  always  observe  the  fixation  point  to  cross  the  separating 
line.  As  a  matter  of  fact,  no  such  observation  could  ordinarily 
be  made,  for  the  separating  line  would  simply  disappear,  in  a  brief 
unobservable  blur,  when  the  eye  jumped  from  A  to  B.  If  the  ex- 
perience taught  anything,  it  would  apparently  be  that  movement 
from  A  to  B  did  not  take  the  eye  across  the  line  encircling  A.  So 
elementary  a  spatial  relation  as  that  of  a  point  or  line  lying  between 
two  other  points  could  not  be  observed  in  the  course  of  eye  move- 
ments of  the  common  jump  type;  for  the  eye  would  not  show  us  the 
middle  point  in  the  course  of  its  movement  between  the  extremes. 
The  eye  might,  of  course,  fixate  first  one  of  the  outer  points,  next 
the  middle  point,  and  finally  the  other  extreme;  but  it  might  also 
pass  directly  from  one  extreme  to  the  other,  and  then  to  the  middle. 
No  fixed  ordering  of  the  points  in  the  field  of  view  could  result 
from  such  experiences.  To  be  sure,  when  objects  move  about 
within  the  field,  the  eye  may  follow  them,  and  during  such  rela- 
tively slow  movements  observe  the  sequence  of  stationary  objects 
over  which  the  moving  object  passes;  but  the  moving  object  may 
take  any  course,  and  the  experience  so  gained  would  seem  unfitted 
to  give  rise  to  a  definite  arrangement  of  points  in  the  visual  field, 
and  so  to  a  well-ordered  field  of  space.  All  such  experiences,  in 
conjunction  with  others  of  the  most  varied  kind,  doubtless  con- 
tribute to  our  knowledge  of  the  spatial  relations  of  objects;  but  the 
point  is  that  the  quasi-mathematical  correspondence  which  Helm- 
holtz conceived  to  exist  between  eye  movements  and  visual  space 
breaks  down  utterly  before  the  fact  that  the  jump  of  the  eye — the 
only  type  of  eye  movement  which  shows  any  approach  to  the  mathe- 
matical precision  required  by  the  theory — is  too  rapid  to  admit  of 
perception  during  its  execution,  and  amounts  simply  to  a  sudden 
shift  of  clear  vision  from  one  object  to  another. 

1  Physiolog.  Optik,  pp.  533  ff. 


VALUE  OF  OBJECTIVE  MEASUREMENTS  463 

§  37.  Once  again,  then,  in  estimating  the  bearing  of  the  recent 
experiments  to  determine  more  exactly  the  movements  of  the  eyes, 
upon  our  theory  of  sense-perception,  is  it  necessary  to  return  to  the 
original  point  of  view  from  which  all  theories  must  take  their  start. 
Only  psychical  factors  can  be  built  into  mental  products.  Objective 
measurements  of  visual  movements  are,  therefore,  of  little  value 
for  theoretical  purposes,  except  as  they  give  sure  indication  of  the 
present,  or  past,  value  of  the  quantitative  and  qualitative  changes 
in  the  psychical  factors  that  are  correlated  with  these  movements. 
A  closer  examination  of  these  very  experiments  shows  that,  on  the 
whole,  they  do  not  destroy,  or  even  greatly  depreciate,  our  estimate 
of  this  value.  We  have  seen,  for  example,  that  when  the  eyes  seem 
to  us  to  be  at  complete  rest  while  regarding  attentively  the  field 
covered  by  the  area  of  clear  vision,  they  are  really  going  through 
a  succession  of  exceedingly  minute  and  slow  movements,  as  though 
ceaselessly  " exploring"  this  field.  Something  similar  may  be 
brought — at  least  vaguely — into  consciousness,  by  closing  the  eyes 
and  watching  the  behavior  of  the  retinal  field  as  correlated  with  the 
changing  complex  of  sensations.  Neither  the  field  itself,  nor  any 
of  the  minutest  subdivisions  of  it — try  as  hard  as  one  will  to  fixate 
it  rigidly — can  be  kept  from  ceaseless  motions;  and  by  directing 
attention  to  the  psychical  accompaniments  of  these  motions,  they 
can  themselves  be  regularly  felt.  Still  further,  the  jerky  and  dis- 
continuous character  of  the  movements  of  the  eyes,  with  the  inter- 
vening pauses  for  momentary  fixation,  when  reading  print  or  sur- 
veying a  geometrical  figure,  or  a  landscape,  corresponds  precisely 
with  the  correlated  mental  experience.  Neither  in  reading  print 
(nor  in  hearing  speech),  nor  in  looking  at  any  object  whatever,  do 
we  become  consciously  aware  of  more  than  a  percentage  of  what, 
objectively  considered,  passes  through  the  field  of  vision.  Our 
mental  apprehension  is  as  jerky,  as  imperfect  and  full  of  blurred  or 
blind  spots,  as  are  the  movements  of  the  eyes.  As  we  have  just 
seen,  even  the  area  of  clearest  vision  requires  to  be  diligently  ex- 
plored with  slowly  moving  eyes,  in  order  to  become  the  more  thor- 
oughly apprehended. 

Still  further,  it  needs  again  to  be  insisted,  that  we  are  examining 
by  relatively  simple  and  coarse  methods,  the  swift  and  complex 
processes  of  nature,  where  most  of  what  there  is  to  be  examined  has 
long  ago  fallen  below  the  threshold  of  consciousness,  and  has  be- 
come quite  impossible  to  revive  in  anything  approaching  its  orig- 
inal form.  To  employ  again  an  illustration  already  used  once  before: 
By  aid  of  delicate  tactual  and  muscular  sensations,  which  he  once 
followed  comparatively  slowly  and  on  the  watch,  as  it  were,  to  re- 
produce them,  the  accomplished  violinist  has  acquired  the  art  of 


464  PRESENTATIONS  OF  SENSE 

spacing  and  bowing  on  the  violin.  But  he  cannot  recall  or  describe 
those  sensations;  and  the  very  effort  to  do  so  in  any  particular  in- 
stance would  paralyze  his  art.  Neither  can  the  experimenter  de- 
termine their  original  quality  as  local  signs,  or  specific  value,  by 
measuring  the  movements  of  the  ringers  and  wrist  of  the  left  arm, 
or  the  sweep  and  the  pressure  of  the  right  arm.  But  let  paralysis 
impair  the  sensations  of  the  skin  and  muscles  of  either  of  these 
members,  and  the  art  of  the  violinist  instantly  disappears.  Al- 
though, however,  the  accomplished  player  can  determine  almost 
exactly  the  number  of  thousands  of  vibrations  of  his  string,  by  mov- 
ing his  finger  a  minute  fraction  of  an  inch,  his  art  is  simple  and 
coarse  compared  with  that  with  which  Nature  endows  the  common- 
est pair  of  eyes. 

§  38.  The  nature  of  the  "sense-data"  which  the  mind  has  at  its 
disposal  for  constructing  its  presentations  of  sense,  and  the  psycho- 
physical  laws  which  are  followed  in  the  process  of  construction, 
have  been  explained  in  such  detail  that  little  need  be  added  con- 
cerning the  development  of  visual  perception.1  Visual  space  pre- 
sents itself  to  us  as  a  coherent  complex  of  sensations  of  light  and 
color  systematically  arranged.  The  arrangement  implies  certain 
native  activities  of  the  mind  in  connection  with  and  dependence 
upon  the  action  of  the  nervous  organism;  but  it  also  implies  an 
immense  influence  from  experience.  It  is  extremely  difficult,  if  not 
wholly  impossible,  to  distinguish  with  confidence  the  limits  which 
must  be  drawn  between  what  is  native  and  what  is  learned.  The 
seeing  of  colors  is  undoubtedly  a  far  more  simple  and  primary  act 
than  the  seeing  of  colored  objects  as  situated  in  relation  to  each 
other  in  objective  space.  A  colored  surface,  or  a  system  of  color- 
sensations  related  to  each  other  as  side  by  side  in  space-form,  re- 
sults in  experience  from  the  weaving  together  of  several  spatial 
series  of  sensations.  Such  a  surface  may  theoretically  be  conceived 
of  as  presented  to  the  mind  through  the  activity  of  the  nervous 
elements  belonging  to  the  retina  of  a  single  motionless  eye.  The 
motifs  or  data  which  the  mind  would  have  for  constructing  such  a 
surface  must  be  found  in  the  series  of  sensations  of  light  and  color 
as  varying  in  intensity  and  quality  according  to  the  locally  distinct 
nervous  elements  which  are  simultaneously  excited.  The  evidence 
seems,  on  the  whole,  favorable  to  the  assumption  that  some  indefi- 
nite picture  of  visual  space  might  be  gained  wholly  through  the 
excitation  of  a  motionless  nervous  mosaic  (like  the  retina)  sensitive 
to  light. 

But  visual  space,  as  experience  makes  it  known  to  us,  requires 

1  On  this  subject,  compare  Ladd,  Psychology,  Descriptive  and  Explanatory, 
pp.  321-375;  487-495;  and  A  Theory  of  Reality,  chap.  IX. 


DEVELOPMENT  OF  VISUAL  PERCEPTION  465 

binocular  vision  with  moving  eyes.  The  firm  spatial  connection  of 
all  the  parts  requires  that  a  system  of  lines  of  direction  should  be 
fixed,  prescribing  the  objective  points  at  which  the  sensations  pro- 
duced by  exciting  together  the  different  pairs  of  the  covering  points 
of  the  retina  must  appear  in  visual  space.  To  establish  such  spatial 
connection,  both  eyes  must  move  in  their  conjoined  action  as  a 
single  organ  of  vision.  By  this  action  the  field  of  binocular  vision 
is  built  up  in  an  order  of  experience  which,  on  the  whole,  consists 
in  the  successive  mastery  of  more  and  more  complex  problems. 
For  the  process  of  learning  to  localize,  the  one  centre — the  point 
of  starting  and  the  goal  of  return — is  the  area  of  clearest  vision  of 
the  retina  (the  yellow-spot),  to  which  the  point  of  regard  in  the  ob- 
ject corresponds.  With  the  point  of  regard  fixed  in  the  primary 
position  of  the  eye,  the  first  and  most  essential  means  is  gained  for 
orientating  objects  in  the  field  of  vision.  The  meridians,  horizontal 
and  vertical,  and  the  locations  of  different  points  in  the  surface  of 
the  field  of  vision  thus  presented  to  the  ruind,  afford  the  compara- 
tively simple  problems  furnished  by  the  primary  position.  In  this 
way  a  central  area,  determining  lines,  and  finally  a  continuous  sur- 
face are  fixed,  to  which  may  be  referred  all  the  directions  and  loca- 
tions of  the  binocular  points  and  lines  of  regard  in  the  secondary 
positions  of  the  eye. 

In  constructing  the  field  of  binocular  vision  with  moving  eyes,  the 
general  principle  seems  to  be  observed  that  by  motion  the  relative 
space-values  of  the  retinal  elements  are  not  changed;  but  their  ab- 
solute values — that  is,  the  complex  which  is  formed  by  combining 
all  these  muscular  and  tactual  sensations  with  the  local  signs  of  the 
retina — are  changed  in  equal  sense  and  measure.  What  moving  the 
eyes  does  for  the  retinal  images,  moving  the  head  and  body  does 
for  the  presentations  of  sense  as  constructed  in  binocular  vision; 
it  alters  the  absolute  values  of  the  complex  of  sensation  as  related 
to  objective  space,  while  keeping  the  relative  values  belonging  to 
the  different  positions  of  the  eyes  unchanged. 

The  visual  perception  of  depth  involves  a  later  and  more  complex 
training  from  experience  than  the  perception  of  two-dimensioned 
extension.  To  solve  at  all  adequately  the  problem  of  depth,  bin- 
ocular vision  with  moving  eyes,  and  its  resulting  combination  and 
separation  of  the  double  images  of  objects,  seems  necessary.  The 
existence  and  assistance  of  those  secondary  helps,  which  are  so 
important  in  perceiving  the  solidity  and  distance  of  objects,  imply 
a  further  development  of  experience.  In  all  these  advances,  how- 
ever, the  course  of  acquisition  is  not  in  separate  straight  lines  that 
run  parallel  or  converge,  as  it  were.  More  complex  experience, 
when  obtained,  modifies  what  is  really  more  simple  and  primary. 


466  PRESENTATIONS  OF  SENSE 

What  we  see  in  monocular  vision  with  an  open  eye,  and  even  what  we 
see  with  both  eyes  closed  and  motionless,  depends  upon  what  we 
have  learned  to  see  with  both  eyes  in  varied  movement  and  avail- 
ing themselves  of  all  possible  secondary  helps.  It  also  depends 
upon  what  we  have  learned  to  know  of  the  nature  and  probable  posi- 
tion and  shape  of  manifold  objects  of  which  the  eye  has  already  at- 
tained the  mastery.  How  simple  the  visual  data  which  the  mind 
may  have  learned  to  interpret  into  terms  of  complicated  visual  ob- 
jects, placed  in  definite  spatial  relations,  is  amply  proved  by  every 
illustrated  lecture,  and  by  the  phenomena  of  visual  dreams.1  Not 
infrequently — indeed,  habitually — what  the  eyes  present  in  the  form 
of  visual  images,  strictly  so  called,  are  the  barest  schemata  of  what 
the  mind  sees  by  way  of  interpreting  these  images. 

§  39.  Finally,  brief  mention  must  be  made  of  the  connections 
which  are  constituted,  in  the  development  of  our  perception  of  ob- 
jects as  having  the  qualities  and  relations  of  space-form,  by  the 
joint  action  and  mutual  assistance  of  eye  and  hand.  With  the 
sense-presentations  of  one  of  these  senses  the  images  of  objects  as 
known  by  the  other  become  most  intimately  related.  It  is  a  misuse 
of  terms,  however,  and  involves  the  entire  subject  in  confusion,  to 
speak  of  this  joint  product  as  a  "sense-perception."  It  is  rather  to 
be  spoken  of  as  a  mental  image  or  concept.  The  visual  presenta- 
tion of  an  object — as,  for  example,  a  ball,  a  pen,  a  table — may  re- 
call its  tactual  presentation.  We  readily  interpret  one  into  terms 
of  the  other — sight  into  terms  of  touch,  and  touch  into  terms  of 
sight.  But  all  the  perceptions,  as  such,  of  spatial  properties  and 
relations,  whether  gained  by  eye  or  hand,  are  kept  quite  distinct 
and  separable  in  the  mind.  No  such  synthesis  takes  place  between 
the  spatial  series  of  the  one  sense  and  the  spatial  series  of  the  other 
sense  as  takes  place  between  the  spatial  series  of  the  same  sense. 
And  all  the  properties  and  relations  of  bodies  as  known  in  space- 
form  are  given  by  each  of  these  senses.  The  view  which  makes  the 
sense  of  sight  dependent  upon  the  sense  of  touch  and  the  muscu- 
lar sense  for  the  construction  of  its  spatial  objects  is  erroneous. 
While  feeling  the  pen,  we  can  image  how  it  would  look;  when 
seeing  it,  how  it  would  feel.  We  can  image  how  much  exertion 
would  be  required  to  reach  a  mountain  which  appears  to  the  eye  so 
far  away,  or  how  a  mountain  would  look  at  a  distance  of  so  many 
miles  as  measured  by  the  exertion  required  to  walk  there.  But  the 
true  presentations  of  the  visual  objects  and  tactual  objects  do  not 
mix  in  one  combined  perception.  They  unite  only  in  one  image  or 
idea  of  the  object. 

1  On  the  latter  subject  compare  Ladd,  Mind,  New  Series,  I,  p.  299. 


VISUAL  AND  MUSCULAR  PERCEPTION  467 

§  40.  Interesting  experiments  have  been  conducted  to  determine 
the  degree  of  accuracy  with  which  perceptions  of  distance  by  sight 
can  be  translated,  as  it  were,  into  terms  of  the  tactual  and  mus- 
cular sense.  Some  of  these  experiments  show  the  amount  of  har- 
mony which  can  be  obtained  between  optical  localizing  and  localiz- 
ing with  the  finger.  Helmholtz1  made  use  of  a  vertical  thread 
which  he  tried  to  locate,  as  seen  in  monocular  vision,  by  hitting  it 
with  a  pencil's  point;  Donders,2  of  a  very  small  induction-spark, 
which  was  to  be  touched  with  the  index-finger.  The  result  of  50 
experiments,  made  for  distances  along  the  same  line  of  regard  vary- 
ing between  60  and  610  mm.,  when  only  the  spark  itself  was  seen  in 
perfectly  dark  surroundings,  showed  that  the  distance  was  over- 
estimated 34  times,  under-estimated  12,  estimated  right  4  times. 
The  greatest  errors  were  +35  and  — 34  mm.;  the  mean  error 
10.6  mm.  When  the  surroundings  were  visible  and  the  electrodes 
seen  with  open  eyes,  the  eyes  then  closed,  and  the  finger  reached 
to  the  estimated  distance,  the  greatest  errors  were  +30  and  — 12 
mm.,  and  the  mean  variable  error  9.8  mm.,  for  distances  from  80  to 
630  mm.  The  exact  localizing  of  the  point  of  regard  in  terms  of 
touch  is  more  difficult  the  farther  the  object  is  removed  and  the 
less  assistance  is  had  from  secondary  helps.  Localizing  in  the 
same  way  when  the  object  lies  out  of  the  line  of  regard  is  still  more 
inaccurate.  In  29  experiments,  where  the  spark  to  be  localized 
was  flashed  at  a  distance  of  210-600  mm.  to  one  side  of  this  line, 
the  greatest  errors  were  +120  and  — 68  mm.,  with  a  mean  error 
of  about  34  mm. 

The  problem  of  comparing  the  judgments  of  linear  extension 
made  by  the  eye,  the  hand,  and  the  arm,  and  of  determining  their 
relative  accuracy,  has  more  recently  been  examined,  experimentally, 
at  considerable  length  by  Jastrow.3  His  method  was  to  present  a 
definite  length,  varying  from  5  mm.  to  120  mm.,  to  the  retina,  the 
skin  (by  application  of  a  pair  of  points,  or  by  motion  of  a  single 
point),  to  the  forefinger  and  thumb  (by  being  held  between  the 
two),  or  to  the  arm  when  in  free  movement  and  guiding  a  pencil  to 
express  its  estimate.  The  subject  of  experiment  was  required  to 
get  a  clear  perception  of  the  given  distance  by  one  of  these  organs 
(called,  in  such  case,  the  " receiving  sense"),  and  then  either  si- 
multaneously or  successively  express  this  perception  through  the 
same  or  some  other  one  of  these  organs  (the  "expressing  sense"). 
In  this  manner  it  was  discovered  that,  if  the  eye  is  both  receiving 

1  Physiolog.  Optik,  p.  650. 

2  Archiv  /.  Ophthalmologie,  XVII,  ii,  p.  55. 

3  Article  on  "The  Perception  of  Space  by  Disparate  Senses,"  in  Mind,  October, 
1886,  pp.  539-554. 


468  PRESENTATIONS  OF  SENSE 

and  expressing  sense,  small  lengths  will  be  under-estimated  and 
large  lengths  exaggerated,  the  point  where  no  error  is  made  being 
at  about  38  mm.;  whereas,  if  the  hand  is  both  receiving  and  ex- 
pressing, small  lengths  will  be  exaggerated  and  large  lengths  un- 
der-estimated, the  "indifference-point"  being  at  about  50  mm.; 
but  the  arm  exaggerates  all  lengths  within  the  limits  of  the  experi- 
ments. When,  however,  the  eye  expresses  and  the  other  organs 
receive  the  impression,  all  lengths  are  greatly  under-estimated;  but 
if  the  hand  is  the  expressing  sense,  all  lengths  are  greatly  exagger- 
ated. The  arm  as  expressing  sense  exaggerates  all  lengths  received 
by  the  eye,  and  under-estimates  all  received  by  the  hand. 

The  relative  accuracy  of  the  three  senses,  whether  receiving  or 
expressing,  or  both,  stands  in  the  order  of  eye,  hand,  arm — the  hand 
being  only  slightly  better  than  the  arm.  The  degree  of  confidence 
felt  in  the  estimate  made  is  naturally  greatest  where  the  accuracy 
is  greatest.  Inasmuch  as  "the  expressing  sense  gives  the  charac- 
teristic properties  to  the  curve  of  error," 1  the  question  arises  whether 
all  the  phenomena  cannot  be  accounted  for  by  a  special  applica- 
tion of  the  law  of  habit  in  connection  with  the  normal  action  of  the 
sensory  apparatus.  Each  sense,  when  expressing  the  estimate, 
tends  to  approximate  it  in  size  toward  those  dimensions  which  it  is 
most  accustomed  to  judge  accurately. 

All  the  foregoing  results  show  plainly  that  the  interpretation  of 
visual  distance  in  terms  of  the  tactual  and  muscular  sense  is  a  mat- 
ter of  complex  experience,  and  is  not  usually  more  than  very  im- 
perfectly attained.  It  bears  little  comparison  with  the  nicety  of 
the  spatial  perceptions  belonging  to  each  one  of  the  two  senses 
concerned  when  interpreting  its  own  specific  data  in  corresponding 
terms,  as  it  were. 

§  41.  In  closing  this  subject,  the  one  psychological  truth  of  pre- 
eminent value  which  has  been  most  obviously  demonstrated  should 
be  stated  again.  Perception  is  the  result  of  an  extremely  complex 
activity  of  the  psychical  subject,  Mind;  it  involves  the  synthesis  of 
a  number  of  sense-data  according  to  laws  that  are  not  deducible 
from  the  nature  of  the  external  objects,  or  of  the  physiological  ac- 
tion of  the  end-organs  and  central  organs  of  sense.  An  analysis  of 
these  data  themselves  is  not  sufficient  to  explain  perception.  The 
descriptions  of  Physiological  Psychology  can  do  no  more  than 
enumerate  these  data,  show  their  dependence  on  external  stimuli, 
and  the  value  which  they  have  as  motifs  for  the  perceiving  subject; 
and  then  understand  the  laws  of  this  synthesis  as  the  permanent 
modes  of  the  behavior  of  the  psychical  subject.  The  object  of 
sense-perception,  the  presentation  of  sense,  is  not  an  extra-mental 

1  Ibid.,  p.  549. 


PERCEPTION  A  MENTAL  ACHIEVEMENT  469 

entity  made  up  outside  of  the  mind  and  borne  into  or  impressed 
upon  it  through  the  avenues  of  sense.  It  is  a  mental  construction. 
The  field  of  vision  is  a  subjective  affair,  and  so  is  the  field  of  touch. 
The  same  psychical  subject  which  reacts  upon  the  stimulation  of 
the  nervous  organs  of  sense  in  the  form  of  sensations,  by  its  activity 
in  synthesizing  these  sensations,  constructs  the  objects  of  sense. 
The  fundamental  fact  is  the  presence  and  activity  of  the  subject, 
known  as  Mind. 


CHAPTER  VI 
TIME-RELATIONS  OF  MENTAL  PHENOMENA 

§  1.  So-called  "presentations  of  sense"  appear  in  consciousness, 
not  only  as  having  spatial  qualities  and  relations,  but  also  as  oc- 
curring either  simultaneously  or  successively  as  respects  Time-form. 
The  clearest  experience  of  the  manner  in  which  our  sensations  are 
located  in  this  framework  of  time,  as  it  were,  is  gained  by  attention 
to  the  successive  tones  of  a  melody,  or  to  the  rhythm  of  visual 
or  muscular  impressions  which  accompanies  a  regularly  recurrent 
motion  of  some  member  of  the  body.  What  is  true  of  the  presenta- 
tions of  sense  is  also  true  of  all  mental  phenomena,  such  as  the  re- 
produced images  of  sense,  the  pure  creations  of  fancy,  and  the 
thoughts.  All  these  have  that  form  of  occurrence  and  relation  which 
we  call  "Time." 

Physiological  Psychology,  however,  can  no  more  give  an  ultimate 
explanation  of  this  time-form  which  belongs  to  all  mental  phenom- 
ena than  of  the  space-form  which  objects  of  sense  acquire  as  the 
result  of  a  mental  synthesis.  Experimental  science  cannot  explain 
"  time."  Nothing  is  accomplished  toward  comprehending  the  ori- 
gin of  the  mental  representation  of  time  by  indicating  the  speed, 
number,  and  order  of  the  various  series  of  conscious  experiences. 
Successive  presentations  of  sense  or  successive  ideas  do  not  of 
themselves  constitute  a  mental  presentation  or  idea  of  succession. 
The  idea  that  a  follows  or  precedes  b  is  not  the  idea  of  a  nor  the 
idea  of  b;  neither  is  it  the  idea  of  a  +  b  or  of  a  —  b.  Experimental 
science  can  explain  the  order  of  succession;  but  in  doing  this  it 
implies  the  idea  of  succession,  and  this  idea  is  not  itself  a  succes- 
sion, or  an  order  of  succession,  or  a  compound  of  successive  ideas.1 

Many  thousands  of  experiments  have  been  made  (since  the  work 
of  Bonders  in  1868),  with  the  use  of  the  most  complicated  and  deli- 
cate machinery,  in  order  to  fix  the  amount  of  time  required  for  the 
various  processes,  both  nervous  and  mental,  which  are  the  condi- 
tions of  our  conscious  life.  These  experiments  have  succeeded  in 
bringing  many  interesting  facts  to  light.  But  the  laws  thus  estab- 
lished beyond  all  reasonable  question  are  remarkably  few;  more- 

1  Compare  Volkmann  von  Volkmar,  Lehrb.  d.  Psychologic  (3d  ed.),  II,  pp.  11  f. 

470 


EXPERIMENTS  IN  REACTION-TIME  471 

over,  they  are  nearly  all  merely  restatements  in  more  definite  form 
of  already  familiar  generalizations.  That  a  kind  of  sluggishness  or 
inertia,  which  the  stimulus  must  overcome,  belongs  to  all  the  senses, 
and  that  they  often  continue  to  act,  when  once  roused,  after  the  ex- 
citing cause  is  withdrawn;  that  different  sensations  following  each 
other  too  quickly  tend  to  confuse  or  destroy  each  other;  that  no 
one  can  see  or  think  more  than  about  so  rapidly,  but  that  this  rate 
varies  with  different  individuals  and  with  the  same  individual  at 
different  times;  that  it  takes  more  time  to  perceive  or  think  where 
the  objects  are  complex,  and  are  either  too  small  or  too  large  or 
too  closely  alike;  that  it  takes  time  to  will  or  choose,  less  time  to 
act  when  we  know  what  to  expect,  and  more  time  to  move,  in  re- 
sponse to  a  particular  sensation,  some  part  of  the  body  which  we 
are  not  accustomed  to  connect  with  that  sensation;  that  practice 
increases  the  speed  of  our  mental  and  bodily  action,  and  that  fatigue 
and  certain  drugs  diminish  it — all  these  statements  were  matters  of 
common  observation  long  before  experimental  psychology  began  its 
use  of  scientific  methods. 

§  2. *  It  is  not  necessary  to  describe  the  construction  of  the  ma- 
chines1 which  have  been  used  in  experimenting  upon  the  time-rela- 
tions of  mental  phenomena,  or  the  methods  of  using  them  employed 
and  commended  by  different  observers.  The  general  problem 
is  in  all  cases  essentially  the  same — namely,  to  produce  certain 
definite  impressions  upon  the  organs  of  sense,  to  secure  a  definite 
result  in  the  form  of  motion  of  some  part  of  the  body  as  a  sign 
that  the  impressions  have  been  received  (and,  perhaps,  interpreted 
and  mentally  combined),  and  to  measure  with  extreme  accuracy  the 
interval  between  peripheral  stimulation  and  resulting  motion. 

The  electrical  current  is  ordinarily  used  to  mark  both  the  in- 
stant when  the  external  sense-stimulus  acts  on  the  organ  and  that 
when  the  resulting  motion  occurs.  The  stimulus  may  consist  in 
the  flash  or  crackle  of  an  electric  spark,  the  appearance  of  one 
or  more  colors  or  figures,  or  letters  or  words,  the  sounding  of  a 
bell  or  a  falling  ball,  etc.;  the  motion  may  be  with  the  finger  press- 
ing a  key,  or  the  foot  or  hand  closing  or  breaking  a  circuit,  or  the 
vocal  organs  calling  into  a  tube,  etc.  The  one  difficult  matter 
which  marks  the  success  or  the  comparative  failure  of  any  series 
of  observations  is  the  arrangement  of  the  experiments  and  their 
tabulated  results  so  as  to  analyze  the  different  elements  of  the  com- 
plex process  involved.  Such  experiments  need  to  be  repeated 
many  times  upon  the  same  individual,  so  as  to  eliminate  the  vari- 

1  For  an  excellent  account  of  these  machines,  see  Titchener,  Experimental 
Psychology,  Quantitative  (1905),  chap.  Ill;  and  for  the  nervous  mechanisms  in- 
volved, nearly  every  chapter  of  part  I  of  this  book  may  be  consulted  anew. 


472       TIME-RELATIONS  OF  MENTAL  PHENOMENA 

able  factors  of  bodily  condition,  attention  or  distraction  of  mind, 
practice,  etc. ;  they  need  also  to  be  repeated  with  many  individuals, 
so  as  to  calculate  accurately  the  so-called  personal  equation. 

§  3.  The  interval  between  the  instant  when  the  external  stimulus 
begins  to  act  upon  the  end-organ  of  sense  and  the  resulting  move- 
ment of  some  member  of  the  body  has  been  called  "physiological 
time"  by  Hirsch  and  others,  and  "reaction-time"  by  Exner.  The 
latter  term  is  preferable.  "Reflex  time"  is  a  similar  conception; 
but  this  term  refers  to  the  time  of  organized  reflexes,  whereas  re- 
action-time refers  to  reactions  which  are  not  instinctively  connected 
with  the  stimuli  used.  The  reflex  time  varies  greatly,  as  has  al- 
ready been  stated  (p.  167),  in  different  reflexes;  in  some  it  is  shorter 
than  the  quickest  reaction-time,  but  in  others  not. 

Reaction-time  is  "simple"  when  all  the  elements  which  tend  to 
complicate  the  processes  involved  in  the  reaction,  and  so  to  lengthen 
the  time  required  by  it,  have  been  as  far  as  possible  eliminated. 
Reaction  obtained  in  response  to  a  single  sensation  of  known  qual- 
ity, the  instant  of  whose  appearance  is  expected,  by  executing  a 
single  natural  and  easy  motion,  best  fulfils  the  conditions  of  sim- 
plicity. It  is  therefore  requisite,  for  all  experiments  of  this  sort, 
that  the  average  simple  reaction-time  of  each  individual  experimented 
upon  shall  be  determined;  and  also  the  effect  of  practice,  exhaustion, 
and  other  influences  upon  this  interval.  But  even  the  simplest 
reaction-time  is,  of  course,  a  very  complex  affair. 

Donders1  distinguished  no  less  than  twelve  different  processes 
as  entering  into  "physiological  time"  (or  simple  reaction-time) — 
and  this  without  interpolating  any  purely  psychical  elements,  as 
occupying  separate  periods,  into  the  entire  interval.  The  analysis 
of  Exner2  is  more  pertinent  to  our  purpose.  Exner  finds  seven 
elements  in  all  reaction-time:  (1)  An  action  of  the  stimulus  on  the 
end-organ  of  sense  preparatory  to  excitation  of  the  sensory  nerve; 
(2)  centripetal  conduction  in  this  nerve;  (3)  centripetal  conduction 
in  the  spinal  cord  or  lower  parts  of  the  brain;  (4)  transformation 
of  the  sensory  into  the  motor  impulse;  (5)  centrifugal  conduction 
of  the  impulse  in  the  spinal  cord;  (6)  centrifugal  conduction  in 
the  motor  nerve;  (7)  setting-free  of  the  muscular  movement.  Of 
these  seven  factors,  however,  the  fourth  is  most  interesting  to  psy- 
chology. It  may  properly  be  called  "psycho-physical"  as  distin- 
guished from  more  purely  physiological  time.  The  other  six  ele- 
ments (with  the  exception  of  the  first,  on  account  of  difficulties 
inherent  in  the  experiments)  have  been  determined  with  some  de- 
gree of  defmiteness.  It  is,  then,  theoretically  possible  to  ascertain 

1  Archiv  f.  Anat.,  PhysioL,  etc.,  1868,  p.  664. 
8  See  Hermann's-  Handb.  d.  PhysioL,  II,  ii,  p.  271. 


INERTIA  OF  THE  END-ORGANS  473 

the  amount  of  these  six  and  subtract  them  from  the  entire  reaction- 
time;  the  remainder  would  be  the  interval  occupied  by  the  central 
cerebral  processes  (that  is,  by  No.  4).  Thus  Exner  assumed  62 
metres  per  second  as  the  probable  rate  of  conduction1  in  both  sen- 
sory and  motor  nerves ;  and  in  the  spinal  cord,  8  for  the  sensory  and 
11-12  for  the  motor  process.  In  this  way  he  calculated  that  about 
0.0828  sec.  is  the  "  reduced  reaction-time,"  or  interval  occupied 
within  the  cerebral  centres  in  transforming  the  sensory  into  motor 
impulses — in  the  special  case  of  reaction  from  hand  to  hand,  where 
the  whole  reaction- time  is  0.1337  sec.  The  uncertainties  of  all 
such  calculation,  however,  occasion  the  demand  for  other  methods 
of  determining  the  strictly  "psycho-physical"  portion  of  reaction- 
time. 

§  4.  As  bearing  on  the  foregoing  problem  of  analysis,  it  must  be 
remembered  that  any  psycho-physical  theory  of  the  time  relations 
of  mental  phenomena  requires  that  account  be  taken  of  the  inertia 
of  the  nervous  system.  As  composed  of  moving  molecules,  it  nec- 
essarily requires  some  time  to  be  started  by  the  action  of  a  given 
stimulus,  then  reach  its  maximum  of  activity  in  a  particular  direc- 
tion, then  subside  into  a  negative  condition  with  respect  to  this 
direction  (called  "Anklingen"  and  "Abklingen"  of  the  nervous  ex- 
citement, by  the  German  investigators).  This  statement  follows  as 
a  necessary  assumption  from  the  physical  nature  of  the  nerve-fibres 
and  nerve-cells,  since  inertia  is  a  property  of  every  material  mechan- 
ism. It  is  difficult,  however,  to  justify  the  assumption  experi- 
mentally, or  to  fix  the  exact  amount  of  time  consumed  by  the  inertia 
of  different  parts  of  the  nervous  system.  Experiment  demonstrates 
no  stadium  of  latent  excitation  for  the  motor  nerve,  such  as  is 
about  y^  sec.  for  the  muscle  when  electricity  is  used.  The  case 
is  different,  however,  with  the  end-organs  of  sense.  They  do  ex- 
hibit a  certain  sluggishness,  and  this  is  one  reason  why  only  so  many 
sensations  in  a  given  unit  of  time  can  be  produced  by  their  successive 
irritation. 

The  result  of  the  inertia  of  the  end-organs,  as  determining  the 
number  of  separate  excitations  of  which  they  are  capable  in  a 
second,  varies  for  the  different  senses.  The  nerve-endings  of  touch 
probably  exceed  all  others  in  the  promptness  with  which  they  re- 
spond to  stimulus  and  then  return  to  a  relative  equilibrium.  But 
the  number  of  separate  sensations  of  this  sense  which  can  be  pro- 
duced during  a  given  interval  depends  in  a  remarkable  way  upon 
the  quality  and  intensity  of  the  stimulus,  the  place  where  it  is  ap- 
plied, etc.  The  results  of  different  experimenters  therefore  differ 

Compare  p.  132.  Recently,  by  improved  methods,  Piper  (Pfliiger's  Archiv, 
1908,  CXXIV,  591)  has  obtained  120  metres  per  second  in  human  nerves. 


474       TIME-RELATIONS  OF  MENTAL  PHENOMENA 

widely.  Preyer  thought  that  27.6-36.8  stimulations  (per  second) 
of  the  skin  fused  into  one  continuous  sensation;  but  Valentin  put 
the  limit  at  480-640,  and  von  Wittich1  succeeded  in  observing 
a  vibrating  or  discontinuous  sensation  corresponding  to  about 
1,000  separate  excitations  in  this  unit  of  time.  Hearing  can  re- 
ceive nearly  as  many  separate  sensations  in  a  second  as  can  touch. 
The  noise  of  the  electric  spark  has  been  heard  with  one  ear  only, 
as  separate  sensations,  at  intervals  of  0.00205  sec.;  but  hardly  or 
not  at  all  at  intervals  of  0.00198  sec.  The  number  of  possible  sensa- 
tions of  sound  may  then  be  placed  at  about  500  per  second.  E.  H. 
Weber  noticed  that  we  can  tell  whether  two  watches  are  ticking 
exactly  together  much  better  when  both  are  held  near  the  same  ear 
than  when  one  is  held  at  each  ear. 

The  smallest  interval  for  sensations  of  sight,  when  the  two  stimuli 
act  on  the  same  place  of  the  retina,  is  still  greater.  In  ordinary 
daylight,  rotating  disks  whose  surface  is  part  white  and  part  black 
become  gray  (that  is,  the  sensations  fuse)  when  they  attain  a  motion 
of  about  24  per  second.  It  can  be  told  which  of  two  images  of  elec- 
tric sparks  that  are  0.011  mm.  apart  on  the  retina  occurs  first,  if 
the  difference  in  the  time  of  their  occurrence  is  0.044  sec.  If  the 
two  sparks  are  seen  as  one  with  an  apparent  motion,  its  direction 
can  be  distinguished  when  the  two  ends  of  the  line  of  motion  are 
only  0.014-0.015  sec.  apart.  But  if  one  stimulus  strikes  the  fovea 
centralis  and  the  other  a  point  of  the  retina  6  mm.  off,  the  smallest 
interval  for  distinct  perception  is  increased  to  0.076  sec.2  Within 
certain  limits  these  intervals  are  independent  of  the  intensity  of  the 
light,  when  it  falls  on  the  retina  near  its  centre;  but  (compare  p.  333) 
the  intensity  and  quality  of  the  sensations  are  connected  with  the 
time  during  which  the  stimulus  acts.  The  law  for  the  "time- 
course"  of  such  retinal  excitations  has  been  stated  and  defended  by 
Fick,3  as  known  by  the  name  of  "Talbot's  principle":  If  any  place 
of  the  retina  is  periodically  excited  with  light  of  given  intensity,  for 
a  certain  time  a,  and  then  left  unexcited  for  a  time  6,  and  if  the 
time  a  +  b  is  less  than  about  0.04  sec.,4  then  the  sensation  becomes 

continuous,  with  a  strength  corresponding  to  the  excitation 


a  +  b. 

1  For  his  remarks  on  Preyer's  experiments,  see  the  article  in  Pfliiger's  Archiv, 

II,  pp.  329  ff. 

2  Compare  Exner,  in  Hermann's  Handb.  d.  PhysioL,  II,  ii,  pp.  256  f.;  and  Sitzgs- 
ber.  d.  Wiener  Acad.,  LXXII,  pp.  156  f. 

3  Archiv  /.  Anat.,  PhysioL,  1863,  pp.  739  f.;  and  Hermann's  Handb.  d.  Physiol, 

III,  i,  pp.  212  f. 

4  This  time,  called  the  "action  time  of  light,"  is  the  time  during  which  the 
stimulus  must  act  in  order  to  produce  its  maximum  effect,  in  point  of  apparent 
intensity.     Talbot's  principle  may  be  restated  by  saying  that  the  apparent 


MEASUREMENT  OF  SMALLEST  INTERVAL         475 

The  measurement  of  the  smallest  interval  for  sensations  of  smell 
and  taste  cannot  be  made  with  satisfactory  exactness  on  account  of 
the  nature  of  the  stimuli  of  these  senses.  Little  is  known  which 
goes  beyond  ordinary  experience  concerning  after-tastes  analogous 
to  the  after-images  of  the  eye.  One  experimenter  (Bidder)  thought 
that  the  sensation  continued  after  the  tongue  had  been  so  carefully 
dried  off  that  no  particles  of  the  tastable  substance  were  left  remain- 
ing; but  of  this  we  can  scarcely  be  sure.  It  may  be  that  certain 
substances  leave  their  after-taste  because  their  tastable  particles  are 
dissolved  later;  or  because  their  effect,  being  weaker,  is  at  first 
suppressed  by  particles  of  stronger  quality.1 

§  5.  When  the  successive  sensations  are  of  different  senses,  the 
" smallest  interval"  between  them,  and  so  the  number  possible  in 
a  second,  varies  still  more.  The  following  table2  exhibits  the  re- 
sults obtained  by  several  different  observers: 

Sec. 

Between  two  sensations  of  sound  (electrical  sparks) 0.002 

Between  two  sensations  of  light  (direct  electrical  excitation  of  same 

retinal  spot) 0.017 

Between  two  sensations  of  touch  (impact  on  finger — Mach)     ....  0.0277 
Between  two  sensations  of  light  (at  fovea  centralis,  by  optical  images)     .  0 . 044 
Between  two  sensations  of  light  (at  periphery  of  retina,  by  optical  images)  0 .049 
Between  sensation  of  sight  and  sensation  of  touch  (sight  following)   .      .   0 . 05 
Between  sensation  of  sight  and  sensation  of  hearing  (sight  following)      .  0.06 
Between  two  sensations  of  noises  (each  heard  by  one  ear)       .      .      .      .  0 . 064 
Between  sensation  of  sight  and  sensation  of  touch  (sight  preceding)        .  0.071 
Between  two  sensations  of  light,  one  at  the  periphery  and  the  other  at 

the  centre  of  retina 0.076 

Between  sensation  of  sight  and  sensation  of  hearing  (sight  preceding)     .  0.16 

§  6.  The  point  of  starting  for  determining  experimentally  all 
the  problems  which  concern  the  durations  and  relations  in  time  of 

intensity  of  a  light  which  acts  on  the  retina  for  less  than  its  action  time,  is  di- 
rectly proportional  to  the  time  during  which  it  acts.  The  action  time  varies, 
however,  with  the  intensity  of  the  stimulus,  being,  according  to  McDougall 
(British  Journal  of  Psychology,  1904,  I,  151),  as  long  as  one-fifth  of  a  second 
when  the  stimulus  is  so  weak  as  to  be  barely  perceptible,  and  decreasing  to  0.03 
sec.  when  the  intensity  of  the  stimulus  is  sufficiently  increased.  According  to 
Kunkel  (Pfliiger's  Archiv,  IX,  206),  the  action  time  varies  for  light  of  different 
colors.  The  action  time  for  sound  varies  with  the  intensity,  and  apparently 
also  with  the  pitch.  With  intense  stimuli,  the  maximum  subjective  intensity  was 
reached,  in  Sander's  work  (Psycholog.  Studien,  1910,  VI,  34),  between  0.6  and 
1.0  sec.;  with  weak  stimuli,  Kafka  (ibid.,  1906,  II,  292)  found  that  the  maximum 
effect  might  not  be  reached  under  1.5  sec.  The  subjective  intensity  rises  at  first 
rapidly,  then  more  slowly,  till  the  maximum  is  reached.  The  rise  is  more  rapid 
for  high  than  for  low  tones. 

1  Compare  von  Vintschgau,  in  Hermann's  Handb.  d.  Physiol,  III,  ii,  p.  221. 

3  By  Exner,  in  Hermann's  Handb.  d.  Physiol,  II,  ii,  p.  262. 


476       TIME-RELATIONS  OF  MENTAL  PHENOMENA 


mental  phenomena  is,  therefore,  gained  by  fixing  the  so-called 
" simple  reaction-time."  In  its  very  simplest  form  the  question 
may  now  be  stated  as  follows:  How  long  an  interval  will  elapse, 
under  the  most  favorable  circumstances,  between  the  instant  when 
some  end-organ  of  sense  is  stimulated  and  the  instant  when  mo- 
tion follows  as  the  result  of  recognizing  the  fact,  in  consciousness, 
that  such  stimulation  has  taken  place  ?  As  has  already  been  indi- 
cated: To  shorten  the  reaction-time  as  much  as  possible,  the  sub- 
ject must  know  what  place  of  the  sensory  organism  is  to  be  hit  by 
the  stimulus,  and  about  when  to  expect  it;  he  must  also  be  called 
upon  to  react,  in  one  and  the  same  easy  and  natural  way,  in  all 
cases,  as  soon  as  he  knows  that  he  is  hit  at  all. 

Under  the  foregoing  conditions  the  simple  reaction- time  varies 
usually  within  only  rather  narrow  limits.  It  does  vary,  however, 
from  one  moment  to  another  in  the  same  individual,  so  that  a  series 
of  reactions  must  be  taken,  and  the  average  and  variability  com- 
puted. It  varies  also  from  one  individual  to  another.  Besides 
these  variations,  the  reaction-time  shows  many  interesting  differences 
according  to  the  exact  conditions  surrounding  the  experiment;  such 
are  the  character  of  the  stimulus,  the  nature  of  the  reacting  move- 
ment, the  "central"  or  mental  conditions,  etc. 

§  7.  Reaction-time  varies,  first,  with  the  sense-organ  which  is 
stimulated.  Some  of  the  determinations  of  the  older  observers, 
which  have  been,  in  general,  confirmed  by  later  work,  are  brought 
together  in  the  following  table : 1 


Observer. 

Optical  stimulus. 
Sec. 

Acoustic  stimulus. 
Sec. 

Stimulus  of  touch. 
Sec. 

Hirsch   ..... 

0  200 

0  149 

0  182  (hand). 

Hankel  

0  225 

0.151 

0.155 

Bonders      ,     . 
VonWittich     .     .     . 
Wundt         ...      . 
Exner    

0.188 
0.194 
0.175 
0  .  1506 

0.180 
0.182 
0.128 
0  .  1360 

0.154  (neck). 
0.130  (forehead). 
0.188 
0.1276  (hand). 

Auerbach    .... 
Von  Kries  .... 

0.191 
0.193 

0.122 
0.120 

0.146 
0.117 

We  conclude,  then,  that  under  the  most  favorable  circumstances 
the  reaction-time  can  scarcely  be  reduced  to  ^  of  a  second,  while 
it  rarely  rises  much  above  T\  of  a  second.  Reaction  to  sound  and 
to  touch  differ,  on  the  average,  but  little,  whereas  reaction  to  light 
is  distinctly  slower. 

1  Taken  from  the  article  of  Kries  and  Auerbach,  Archiv  /.  Anat.  u.  PhysioL, 
Physiol.  Abth.  (1877),  pp.  359  f. 


VARIATIONS  IN  REACTION-TIME  477 

Stimuli  to  the  other  senses  have  been  less  employed  in  reaction- 
time  experiments;  and,  accordingly,  the  times  are  less  well  estab- 
lished. For  warmth  and  cold,  the  times,  are  somewhat  longer  than 
for  touch,  and  longer  for  warmth  than  for  cold.  An  approximate 
figure  for  reaction  to. cold  is  0.15  sec.  and  to  warmth  0.18  sec.1  It 
should  be  noted  that  temperature  stimuli  cannot  be  so  directly  ap- 
plied to  the  end-organs  as  can  light,  sound,  and  pressure  on  the 
skin;  the  temperature  of  the  skin  must  first  be  changed.  Hence 
the  reaction-time  to  warmth  and  cold  varies  widely  according  to 
the  mode  of  application  of  the  stimulus.  The  times  given  above 
were  obtained  by  bringing  a  warm  or  cold  piece  of  metal  in  contact 
with  the  skin.  If  radiant  heat  is  applied  by  bringing  a  candle  close 
to  the  skin,  then,  though  the  final  impression  may  be  almost  painful, 
its  development  is  so  slow  that  the  reaction-time  rises  to  half  a 
second.2  The  fact  that  reaction  to  warmth  is  slower  than  to  cold 
is  believed  to  indicate  that  the  warmth-receptors  lie  deeper  in  the 
skin  than  the  cold-receptors  (see  p.  181). 

Reaction  to  painful  stimuli  is  especially  slow.  The  fact  that  the 
sensation  of  cutaneous  pain  is  slow  in  developing  can  be  readily 
observed  introspectively;  a  painful  stimulus,  if  not  applied  precisely 
to  a  pain  spot,  is  apt  to  evoke  first  a  sensation  of  touch,  and  then, 
after  an  appreciable  interval,  a  sensation  of  pain.  If  a  pain  spot 
is  directly  excited,  by  a  strong  stimulus,  two  sensations  of  pain  are 
felt,  though  at  an  appreciable  interval.  The  reason  for  this  prob- 
ably is  that  the  strong  stimulus  excites  the  nerve-fibres  directly, 
as  well  as  the  slow-acting  end-organs  of  pain.  A  weak  stimulus 
apparently  excites  only  the  end-organs;  accordingly  Thunberg 
finds  3  that  pain-reaction  requires  over  a  second  when  the  stimulus 
is  weak,  but  drops  abruptly,  at  a  certain  strength  of  stimulus,  to 
0.40  sec. 

The  reaction  to  taste  is  slow,  and  varies  with  the  part  of  the  tongue 
to  which  the  stimulus  is  applied,  and  with  the  particular  taste 
aroused.  The  following  figures  are  given  by  Kiesow,4  the  stimuli 
being  applied  to  the  tip  of  the  tongue: 

To  salt 0.307  sec. 

To  sweet 0 . 446  sec. 

To  acid 0 . 536  sec. 

To  bitter 1.082  sec. 

1  Goldscheider,  Archiv  f.  (Anat.  w.)  Physiol.  (1887),  p.  469;    Von  Vintschgau 
and  Steinach,  Pfliiger's  Archiv  (1888),  XLIII,  152. 

2  Tanzi,  cited  from  Sherrington  in  Schafer's  Textbook  of  Physiology,  1900,  II, 
963. 

3  Nagel's  Handbuch  der  Physiologic,  1905,  III,  710. 

4  Zeitschrift  /.  PsychoL,  1903,  XXXIII,  453. 


478       TIME-RELATIONS  OF  MENTAL  PHENOMENA 

The  reaction  to  bitter  is  prompter  when  the  back  of  the  tongue 
is  stimulated.  The  slowness  of  reaction  to  taste  may  probably  be 
due,  in  part,  to  the  impossibility  of  applying  the  stimulus  directly 
upon  the  taste-buds.  Sensations  of  taste,  as  Kiesow  notes,  are  slow 
and  gradual  in  their  development,  and  the  reaction  to  the  first  ap- 
pearance of  the  specific  sensation  is  a  reaction  to  very  faint  stimuli, 
and  "sensorial"  (see  p.  485)  in  character,  and  therefore  slow. 

The  reaction  to  smell  is  still  slower,  though  how  far  this  is  due 
to  the  relative  inaccessibility  of  the  end-organs  is  uncertain.  Times 
of  0.2  to  0.8  sec.  have  been  obtained.1 

The  reaction-time  to  cutaneous  stimuli  varies  with  the  part  of 
the  skin  excited.  This  would  be  expected,  on  account  of  the  un- 
equal distances  of  different  parts  from  the  brain,  and  the  consequent 
unequal  times  which  must  be  consumed  in  transmission  by  the  nerve.2 
However,  the  observed  differences  are  not  all  accounted  for  by  this 
difference  in  distance.  Reaction  to  a  stimulus  applied  to  the  fore- 
head is  more  delayed  than  to  a  stimulus  applied  to  the  hand  (Ex- 
ner).  Dolley  and  Cattell3  measured  the  time  of  reaction  to  stimuli 
applied  to  various  parts  of  the  skin,  and  found  many  differences 
which  could  not  be  accounted  for  by  the  lengths  of  nerve  traversed, 
but  only  by  differences  in  the  closeness  of  cerebral  connection  be- 
tween sensory  and  motor  nerves.  Thus,  the  reaction  of  the  right 
hand  to  a  stimulus  applied  to  that  hand  is  quicker  than  to  a  stimulus 
applied  to  the  left  hand;  and  the  foot,  though  it  always  gives  slower 
reactions  than  the  hand,  is  relatively  quick  when  the  stimulus  is 
applied  to  the  foot  or  leg.  In  general,  it  appears  that  the  cerebral 
path  from  the  skin  of  a  given  member  to  the  muscles  of  that  same 
member  is  either  especially  short,  or  especially  open. 

§  8.  It  has  been  argued  that  the  apparent  difference  in  the  reac- 
tion-times of  different  senses  is  due  to  difference  in  the  intensity  of 
the  stimuli  applied.  Increasing  the  strength  of  the  stimulus  de- 
creases the  reaction- time  in  all  the  senses;  but  we  have  no  very 
good  means  of  measuring  stimuli  of  one  sense  in  terms  of  another 
sense.  It  has  been  proposed  (by  Wundt)  to  reduce  them  to  a  com- 
mon standard  by  referring  the  sensations  to  the  point  where  they 
barely  reach  the  "threshold  of  excitation"  (Reizschwelle) -,  that  is, 
where  they  are  just  perceptible  in  consciousness.  In  this  way  the 
mean  result  for  sound  (0.337),  light  (0.331),  and  touch  (0.327)  are 
found  to  be  almost  exactly  the  same.  It  has  further  been  argued 

1  Moldenhauer,  Wundt's  Philosoph.  Studien  (1883),   I,   606;  Buccola,  Arch, 
ital.  de  Biologie,  1884,  V,  279;  Beaunis,  Compt.  rend.  Acad.  de  Sciences  (Paris, 
1883),  XCVI,  387. 

2  See  Kiesow,  Zeitschr.  /.  Psychol  (1903),  XXXIII,  444. 

3  National  Academy  of  Sciences,  1893,  VII,  393;  Psychol.  Rev.,  1894,  I,  159. 


EFFECT  OF  INCREASING  INTENSITY  479 

that  the  speed  of  perception  and  the  duration  of  psycho-physical 
time  are  the  same  for  all  the  senses.  On  the  contrary,  there  seems 
good  reason  to  suppose  that  the  reaction-time  of  sight  is  necessarily 
longer  than  that  of  hearing  or  touch,  on  account  of  the  photo- 
chemical nature  of  its  more  immediate  stimulus.  One  observer 
(von  Wittich)  has  even  gone  so  far  as  to  conjecture  that  the  speed 
of  conduction  in  the  optic  nerve  is  less  than  that  of  the  other 
nerves  of  sense;  it  is  rather  to  be  concluded,  however,  that  the  la- 
tent time  of  the  sensory  end-apparatus,  and  of  the  cerebral  proc- 
esses by  which  sensory  impulses  pass  over  into  motor  impulses, 
is  different.  But  if  equality  of  reaction-times  in  the  different 
senses  is  to  be  taken  as  indicating  equally  intense  stimuli,  then  a  de- 
cidedly bright  light  must  be  regarded  as  having  no  more  intensity 
than  a  very  moderate  noise  or  pressure. 

Another  suggestion1  is  that  the  reaction  time  is  very  largely  con- 
sumed in  the  passage  of  the  nerve-impulse  across  synapses  (see  p. 
110);  and  that  the  path  from  the  rods  and  cones  to  the  brain  has 
more  synapses  than  the  path  from  the  organ  of  Corti.  On  the  whole, 
the  suggestion  which  probably  is  most  generally  entertained,  and 
which  seems  certainly  applicable  to  many  of  the  differences  above 
noted,  is  that  already  adopted  by  us — namely,  that  the  inertia  or 
latent  time  of  different  sense-organs  differs. 

§  9.  The  fact  that  increasing  the  intensity  of  the  stimulus  di- 
minishes the  reaction-time  has  been  incidentally  mentioned  above, 
in  the  cases  of  pain  and  temperature.  The  subject  has  been  quan- 
titatively examined  by  several  authors;  as  by  Exner  in  the  case  of 
light,  by  Wundt  in  the  case  of  sound,  by  Berger  and  Cattell  in  light, 
sound,  and  touch,  and  by  a  few  others.  There  is  nearly  universal 
agreement  that  the  reaction  becomes  quicker  as  the  intensity  of  the 
stimulus  increases,  though  the  change  is  not  great,  when  once  the 
range  of  very  feeble  intensities  is  passed.  The  most  exact  determi- 
nations on  this  point  are  those  of  Froeberg.2  The  stimulus  used  by 
this  experimenter  was  light  reflected  from  white,  gray,  and  " black" 
paper;  its  relative  intensity  is  stated  in  the  upper  line  of  the  follow- 
ing table,  the  lower  line  showing  the  reaction-time  of  one  individual 
in  seconds: 

100          56          25          16          10  7  3  2        0.7 

.191       .194       .197       .202       .208       .210       .215       .220       .226 

A  second  individual  gave  shorter  times,  but  the  changes  in  intensity 
varied  to  about  the  same  extent.  The  result  is  summarized  ap- 

1  Schafer,  Textbook  of  Physiology,  1900,  II,  611. 

2  The  Relation  between  the  Magnitude  of  Stimulus  and  the  Time  of  Reaction 
(New  York,  1907). 


480       TIME-RELATIONS  OF  MENTAL  PHENOMENA 

proximately  by  saying  that  doubling  the  intensity  of  the  stimulus 
decreases  the  reaction-time  by  0.003-0.004  sec.;  or,  in  other  words, 
that  the  reaction-time  decreases  in  arithmetical  progression  as  the 
stimulus  increases  in  geometrical  progression.  The  analogy  of 
this  result  to  Fechner's  law  will  at  once  be  noticed.  Froeberg  ob- 
tained a  similar  result  for  varying  intensities  of  an  acoustic  stimulus; 
and  for  variations  in  the  area  and  (within  the  limits  of  the  "action- 
time")  duration  of  a  photic  stimulus. 

§  10.  The  character  of  the  reacting  movement  influences  the  time 
of  reaction.  Reactions  by  the  hand  or  a  finger  are  usually  quicker 
than  those  of  any  other  member.  A  member  unpracticed  in  quick 
reactions  gives  slower  responses  in  these  experiments  than  a  member 
which  is  much  used  in  similar  performances;  this  difference  de- 
creases with  practice  in  the  unfamiliar  movement.1  When  the  re- 
action consists  in  pressing  down  a  telegraph  key,  against  the  force 
of  a  spring,  the  time  of  reaction  increases  with  the  force  of  the  spring.2 
This  effect  is  probably  purely  muscular,  as  more  time  is  required 
by  the  muscles  to  attain  a  strong  than  a  weak  contraction.  The 
movement  more  customary  in  reaction-time  experiments,  a  move- 
ment in  which  the  key  is  kept  closed  by  the  finger  till  the  stimulus  is 
received,  and  then  released,  does  not  show  a  similar  effect  from  the 
force  of  the  spring;  and  hence  is  a  better  device  for  general  use  in 
these  experiments. 

The  movement  executed  by  the  finger  in  such  reactions  has  been 
subjected  to  analysis  by  several  authors 3  and  found  to  be  by  no  means 
simple  in  all  individuals.  In  the  preliminary  period  of  expectancy, 
an  oscillation  between  greater  and  less  tension  in  the  finger  has  been 
observed  in  a  considerable  minority  of  the  individuals  tested.  At 
the  moment  when  the  stimulus  is  received,  there  is  in  some  persons 
a  preliminary  increase  of  the  downward  pressure  on  the  key,  an- 
tagonistic to  the  release  movement  which  then  follows  immediately; 
but  the  downward  movement  consumes  time  and  slows  the  reaction. 
When  the  required  movement  is  itself  a  downward  pressure  on  the 
key,  the  force  exerted  is,  in  unpracticed  subjects,  quite  in  excess  of 
the  amount  needed  to  close  the  key;  but  with  practice,  this  excess 
diminishes,  and  is  accompanied  by  improvement  in  the  speed  of 
reaction.4  In  general,  unpracticed  subjects  are  apt,  in  these  re- 

1  See  the  summary  of  these  and  many  other  results  by  Jastrow,  in  his  Time- 
Relations  of  Mental  Phenomena  (New  York,  1890). 

2  Breitwieser,  Psychol.  Rev.,  1909,  XVI,  352. 

3  W.  G.  Smith,  Mind,  1903,  N.  S.,  XII,  47;  Judd,  McAllister  and  Steele,  Psy- 
chol. Rev.,  Monograph  Supplement  XXIX  (1905),  p.  141;    Titchener,  Exper. 
Psychol.,  Quantitative,  part  II  (1905),  p.  351. 

4  Breitwieser,  op.  cit. 


INFLUENCE  OF  CENTRAL  CONDITIONS  481 

actions  as  in  many  other  performances  (compare  p.  549),  to  make 
larger  and  more  forceful  movements  than  are  required,  and  improve- 
ment in  speed  of  reaction  is  attended  by  a  more  precise  limitation 
of  the  movement.  The  speed  of  the  reacting  movement  is,  in 
practiced  subjects,  nearly  uniform  and  does  not  follow  the  varia- 
tions of  the  reaction- time;  conditions  which  affect  the  speed  of  the 
reaction  do  not  necessarily  affect  the  speed  of  the  reacting  move- 
ment.1 

§  11.  The  "central"  or  mental  conditions  which  influence  the 
reaction-time  are  naturally  of  special  interest  to  the  student  of  psy- 
chology; but  they  are  also  specially  intricate  and  difficult  of  exact  de- 
termination. No  doubt  central  factors  are  interwoven  with  all 
the  sensory  and  motor  influences  which  have  already  been  men- 
tioned. That  changes  in  the  general  condition  of  the  brain,  de- 
pendent on  disease,  fatigue,  the  action  of  drugs,  etc.,  influence  the 
time  employed  by  all  our  bodily  movements  need  scarcely  be  men- 
tioned. Less  obvious  are  influences  dependent  on  the  particular 
manner  in  which  the  experiment  is  arranged. 

In  the  usual  method  of  conducting  an  experiment  in  reaction- 
times,  the  "  subject,"  or  reagent,  is  first  made  acquainted  with  a 
certain  stimulus  to  which  he  is  to  react  by  a  prescribed  movement 
(usually  a  movement  of  the  forefinger).  There  is  no  ready-formed 
connection — instinctive  or  habitual — between  this  particular  stim- 
ulus and  this  particular  movement;  and  the  movement  would  not, 
in  general,  be  made  in  response  to  the  stimulus  except  for  the  in- 
structions of  the  experimenter.  The  subject  assumes  a  position  of 
readiness — placing  his  finger  on  a  key  by  means  of  which  his  reac- 
tion is  to  be  recorded,  and  perhaps  also  directing  his  eyes  toward 
the  place  where  the  stimulus  (if  visual)  is  to  appear.  He  is  usually 
warned  a  few  seconds  in  advance  to  be  " ready"  for  the  stimulus. 
This  preliminary  signal  is  an  important  part  of  the  procedure.  If 
it  is  omitted,  the  time  of  reaction  is  increased.  Wundt2  found  an 
increase  from  0.076  sec.  to  0.253  sec.  in  reacting  to  a  loud  noise  (a 
ball  falling  25  cm.);  and  an  increase  from  0.175  to  0.266  in  reacting 
to  a  weak  sound  (a  ball  falling  5  cm.); — these  increases  being  the 
result  of  omitting  the  "Ready!"  signal.  If  the  stimulus  follows  the 
signal  at  an  irregular  interval,  the  reaction-time  is  not  so  short  as 
when  the  procedure  is  regular.  If,  indeed,  the  procedure  is  so 
regular  that  the  moment  of  the  stimulus  can  be  exactly  anticipated, 
the  movement  may  be  made  to  coincide  in  time  with  the  stimulus, 
and  the  whole  character  of  the  experiment  be  thus  changed.  This 

1  T.  V.  Moore,  Psychol  Rev.,  Monograph  Supplement,  No.  XXIV  (1904). 

2  Physiol  Psychologic,  3d  edition,  1887,  II,  287.     The  increase  is  not  always 
so  great;  see  Wundt's  5th  edition,  1903,  III,  434. 


482       TIME-RELATIONS  OF  MENTAL  PHENOMENA 


result  is  avoided  by  varying  the  preliminary  interval  within  narrow 
limits.  The  most  favorable  interval  between  the  ready  signal  and 
the  stimulus  is  one  or  two  seconds;  a  shorter  time  does  not  allow  the 
subject  to  prepare  himself  fully  for  the  stimulus,  while  a  longer  period 
than  two  seconds  allows  more  time  than  is  needed  and  so  affords  a 
chance  for  wandering  of  the  attention. 

The  preparation  for  reaction  is  clearly  an  essential  part  of  the  proc- 
ess; without  some  degree  of  preparation  the  reaction  would  not  oc- 
cur at  all,  and  the  more  perfect  the  preparation,  the  more  prompt 
the  reaction.  The  preparation  may  be  said  to  include  an  adjust- 
ment to  the  stimulus,  and  an  adjustment  for  the  movement  to  be  made 
— the  total  adjustment  including  and  connecting  these  two.  The 
more  precisely  the  character  of  the  stimulus  is  foreknown,  the  more 
complete  is  the  adjustment  to  it,  and  the  more  rapid  the  reaction. 
Thus,  if  the  intensity  of  the  stimulus  varies  so  that  it  cannot  be  ex- 
actly anticipated,  the  reaction-time  is  increased.  The  increase  is 
the  greater  if  the  alternation  of  intensities  is  very  irregular.  This 
fact  is  exhibited  in  the  following  table  i1 


I.  Uniform 

change  of  intensity 

II.  Irregular  change  of 

intensity 

Loud  sound 

Sec. 
0.116 

Loud  sound  . 

Sec. 
.      .  0.189 

Feeble  sound 

0.127 

Feeble  sound 

.      .  0.298 

By  suddenly  intercalating  a  feeble  sound  in  a  series  of  loud  noises 
the  reaction-time  may  be  prolonged  to  0.4  or  0.5  sec.  Accordingly, 
the  speed  of  reaction  affords  a  measure  of  the  precision  of  the  pre- 
liminary adjustment. 

§  12.  In  attempting  a  psychological  analysis  of  the  nervous  and 
psychical  factors  involved  in  such  reaction-time  experiments,  it 
is  important,  then,  to  recognize  that  the  whole  process  begins  with 
the  warning  signal  (if  not,  indeed,  still  earlier),  and  that  it  can  prop- 
erly be  divided  into  three  periods2:  (1)  the  period  of  preparation; 
(2)  the  period  of  reaction;  and  (3)  the  period  immediately  following 
the  reaction.  These  are  sometimes  named,  respectively,  the  fore- 
period,  the  main-period,  and  the  after-period. 

The  preparatory  period  is  reckoned  as  beginning  with  the 
"Ready"  signal;  the  reaction  period  with  the  stimulus;  and  the 
after-period  with  the  reacting  movement.  These  three  correspond, 
in  a  way,  to  the  cocking  of  a  gun,  the  discharge,  and  the  smoke. 

1  Wundt,  Physiol.  Psychologic,  5th  edition,  1903,  III,  440. 

2  See  Ach,  Uber  die  Willenstdtigkeit  und  das  Denken  (Gottingen,  1905). 


THE  THREE  PERIODS  483 

During  the  preparatory  period  the  reacting  mechanism  is  adjusted 
or  set  for  a  certain  form  of  action;  then  the  stimulus  comes  and 
causes  the  predetermined  action  to  take  place;  and  then  after-effects, 
such  as  feelings  of  satisfaction  or  of  relief,  may  occur.  When  the 
adjustment  is  complex,  and  more  than  one  response  has  to  be  made 
to  the  stimulus,  some  parts  of  the  response  may  occur  in  the  after- 
period,  and  are  therefore  not  to  be  included  in  the  time  which  is 
measured,  for  only  those  events  which  occur  in  the  main  or  reaction 
period  are  timed  by  the  experiment. 

The  importance  of  the  above  division  into  three  periods  is  made 
clear  by  comparing  it  with  the  most  famous  of  attempts  to  analyze 
the  mental  or  central  process  entering  into  a  reaction.  The  analy- 
sis of  Wundt1  divided  the  central  or  psycho-physical  part  of  the  re- 
action (No.  4  of  Exner's  seven  processes,  p.  472)  into  three  psycho- 
physical  processes:  (1)  entrance  into  the  field  of  consciousness,  or 
simple  perception ;  (2)  entrance  into  the  point  of  clear  consciousness, 
or  apperception  (attentive  or  discerning  perception) ;  (3)  the  excita- 
tion of  the  will,  which  sets  free  in  the  central  organ  the  motor  im- 
pulse to  the  reacting  muscles.  Obviously,  the  mental  processes 
are  here  all  conceived  of  after  the  analogy  of  sight.  Consciousness 
is  regarded  as  a  field  of  vision ;  objects  enter  it  and  are  at  first  only 
obscurely  and  indefinitely  perceived,  as  are  those  visual  objects 
whose  images  enter  the  field  of  the  eye  at  the  sides  of  the  retina. 
Time  is  required  for  the  objects  to  arrive  at  the  spot  of  clear 
vision — the  fovea  centralis  of  consciousness  (Blickpunkt) — where 
discerning  attention  is  bestowed  upon  them  and  they  are  apper- 
ceived.  When  they  are  apperceived,  further  time  is  required  to 
get  up  the  corresponding  molecular  motion  in  the  motor  areas  of 
the  brain.  All  three  foregoing  processes  are  psycho-physical — 
that  is,  they  comprise  physiological  processes  in  the  central  organs 
and  simultaneous  corresponding  changes  of  consciousness  occurring 
in  time-form.  There  is  no  good  reason  to  suppose  that  the  mind 
occupies  time  for  its  own  processes  which  is  separate  from  and — as  it 
were — thrown  in  between  the  physiological  processes.  Indeed,  all 
the  evidence  is  contrary  to  such  an  hypothesis. 

Wundt  has  made  an  elaborate  defence  of  his  positions  with  re- 
gard to  the  nature  of  psycho-physical  time.  He  and  his  pupils 
have  attempted  more  definitely  to  characterize  the  cerebral  changes 
which  correspond  to  each  of  the  mental  elements  of  (1)  perception, 
(2)  apperception,  and  (3)  will;  and  to  determine  what  part  of  the 
total  reaction-time  must  be  assigned  to  each.  His  figure  of  speech, 
which  likens  all  changes  of  conscious  states  to  those  produced  by 
moving  an  image  over  the  retina  to  the  spot  of  clear  vision,  may  be 
\Physiol.  Psychologic,  5th  edition,  1903,  III,  384, 


484       TIME-RELATIONS  OF  MENTAL  PHENOMENA 

accepted  as  helpful  to  the  imagination;  it  must  not  be  forgotten, 
however,  that  it  is  still  a  figure  of  speech.  The  fact  of  which  it 
takes  account  is,  that  all  changes  of  consciousness  require  time  in 
order  to  define  themselves  with  their  maximum  of  clearness  and  in- 
tensity. 

§  13.  Let  us  now  suppose  that  Wundt's  analysis  be  accepted  as 
a  true  account  of  the  mental  process  in  a  voluntary  reaction,  as  dis- 
tinguished from  a  reflex  or  from  an  habitual  and  automatic  response; 
and  let  us  also  grant  that  the  movement  of  the  forefinger  in  response 
to  a  prearranged  stimulus  is  not  a  purely  reflex  or  automatic  re- 
sponse; the  subject's  will,  indeed,  may  be  strongly  excited  to  react 
as  quickly  as  possible.  Still  it  does  not  follow  that  all  of  the 
processes  which  Wundt  recognizes  as  included  in  a  voluntary  re- 
action occur  during  the  reaction  period  of  the  experiment  and  so 
enter  separately  into  the  time  which  is  measured.  Several  of  the 
leading  students  of  reaction-times  (Exner,  Cattell,  Lange,  Ach,  and 
others)  have  observed  that  no  detectable  act  of  will  intervenes  be- 
tween the  stimulus  and  the  reacting  movement,  at  least  in  most 
cases.  The  will  may  be  exerted  in  the  preparatory  period,  in  set- 
ting up  the  adjustment  for  reaction;  but,  when  once  this  adjust- 
ment is  established,  the  stimulus  calls  out  the  response  without  any 
further  act  of  will.  In  such  cases,  accordingly,  "will-time"  does 
not  usually  enter  into  the  time  which  is  measured.  Somewhat  the 
same  thing  is  true  regarding  perception-time.  Since  the  stimulus 
is  anticipated  arid  adjusted  for,  it  does  not  need  to  win  its  way  to 
the  focus  of  attention,  but  its  way  is,  as  it  were,  " cleared"  before  it 
by  the  preparatory  adjustment.  No  time  is  consumed,  during  the 
reaction  period,  in  turning  the  attention  upon  the  stimulus,  for  the 
attention  is  directed  toward  the  stimulus  before  it  arrives.1 

As  to  "apperception-time,"  this  is  partly  covered  by  the  preceding 
remarks  on  perception-time;  however,  the  full  apperception  of  a 
stimulus  involves  a  progressive  apprehension  of  its  exact  character; 
it  may  first  be  apprehended  simply  as  a  change  in  the  situation,  and 
then  as  a  particular  color  or  sound;  and  so  on.  The  full  process  of 
apperception  may  take  considerable  time,  but  it  is  not  ordinarily 
necessary  for  this  process  to  be  complete  before  the  reactive  move- 
ment is  initiated.  Unless  the  reaction  is  required  to  occur  only  when 
the  stimulus  has  a  particular  character  which  must  be  recognized 
before  reacting,  the  movement  may  occur  at  any  stage  in  the  devel- 
opment of  the  apperception.2  Part  of  the  apperceptive  process  thus 
occurs  in  the  after-period,  and  is  not  included  in  the  measured  time. 

1  See  Ach,  op.  cit.,  p.  116. 

2  See  Cattell,  Wundt's  Philos.  Stud.,  1886,  III,  452;  and  National  Acad.  of 
Sciences,  1893,  VII,  393;  Ach,  op.  cit.,  p.  117. 


WUNDT'S  ANALYSIS  EXAMINED  485 

How  much  of  apperception  occurs  within  the  measured  time  can- 
not easily  be  told;  the  amount  is  variable,  and  introspection  can 
scarcely  be  trusted  to  indicate  at  what  stage  in  a  rapidly  developing 
process  a  motor  impulse  leaves  the  brain. 

The  outcome  of  this  discussion  is  that  Wundt's  analysis  of  a 
complete  reaction  cannot  be  regarded  as  applying  accurately  to 
the  process  which  occurs  between  the  application  of  the  stimulus 
and  the  accomplishment  of  the  movement.  Experiment  does  not, 
therefore,  generally  afford  a  measure  of  the  duration  (nor  the  latent 
period)  of  the  processes  of  perception,  apperception,  and  will;  the 
amount  of  conscious  process  included  in  the  measured  time  may  be 
very  small.  What  is  measured  is  rather  the  time  needed  to  set  in 
action  a  previously  prepared  adjustment;  and  the  more  perfectly 
the  adjustment  is  prepared,  the  shorter  will  be  its  latent  time.  If 
we  try  to  think  in  neural  terms,  we  shall  probably  not  be  far  wrong 
in  supposing  that  the  preparatory  adjustment  consists  in  some  sort 
of  making-ready  (perhaps  a  partial  arousal  or  sub-excitation)  of 
the  cortical  areas  which  are  to  receive  the  sensory  impulse  and  send 
out  the  motor  impulse,  and  of  the  connections  between  these  two 
areas;  thus  the  incoming  impulse,  on  reaching  the  cortex,  finds  a 
particular  pathway  wide  open  to  it,  and  makes  a  quick  passage  to 
the  connected  motor  nerves.  Probably  the  time  consumed  in  the 
brain  depends  partly  on  the  relative  directness  or  circuitousness  of 
the  pathway,  and  partly  on  the  degree  to  which  it  is  prepared  or 
sub-excited.  How  much  consciousness,  therefore,  should  attend 
such  a  nervous  impulse  in  its  passage  through  the  brain  cannot  be 
judged  from  the  neurological  conception.  It  seems  certain,  however, 
that  our  experiences  of  apperception  and  of  willing  are  associated 
with  the  activity  of  longer  and  more  complicated  pathways  than  are 
traversed  by  the  nerve-impulse  in  a  prepared  reaction.  When  viewed 
from  the  more  purely  psychological  point  of  view,  the  entire  process 
seems  to  involve  the  same  substitution  of  the  more  speedy  automatic 
elements  for  the  slower  but  more  distinctly  conscious  operations, 
with  which  our  theory  of  the  growth  of  experience  by  learning 
makes  us  abundantly  familiar. 

§  14.  The  speed  of  any  reaction,  as  can  be  judged  from  the  pre- 
ceding considerations,  differs  according  to  the  particular  adjustment 
which  is  prepared ;  that  is  to  say,  according  to  the  particular  require- 
ments of  the  experiment.  Variations  in  the  reaction-time  experi- 
ment are  suited  to  discover  the  different  speed  with  which  differ- 
ent adjustments  can  be  set  in  action.  We  shall  now  consider  two 
classes  of  such  variations  of  the  experiment.  The  first  variation 
falls  under  the  head  of  the  simple  reaction-time,  and  refers  to  the 
distinction  between  "sensorial"  and  " muscular"  (or  " sensory" 


486       TIME-RELATIONS  OF  MENTAL  PHENOMENA 

and  "motor")  reactions;  while  the  second  class  includes  the  many 
varieties  of  "complex  reaction- times." 

The  distinction  of  sensory  and  motor  reactions  was  made  by 
Lange  in  1888,1  and  has  aroused  a  great  amount  of  observation 
and  discussion.  Lange  found  that  when  the  subject  directed  his 
attention  to  the  expected  stimulus,  the  reaction-time  was  longer 
than  when  he  directed  it  toward  the  movement  which  he  was  about 
to  make.  The  difference  in  time  between  these  two  modes  of  re- 
action was  considerable — the  sensorial  type  being  about  0.100  sec. 
slower  than  the  muscular.  Thus,  Lange's  figure  for  the  muscular 
reaction  to  sound  was  0.123,  and  for  sensorial  reaction  to  the  same 
stimulus,  0.227.  Most  subsequent  observers  have  obtained  re- 
sults which  show  a  much  smaller  difference,  varying  all  the  way 
down  to  zero;  and  some  subjects  even  give  quicker  reactions  when 
they  attempt  to  take  the  sensorial  attitude.2  It  seems  probable, 
therefore,  that  some  individuals  more  readily  and  naturally  adopt  the 
sensorial  attitude,  and  others  the  muscular  attitude;  but  it  is  also  prob- 
able that  the  instructions  are  differently  understood  by  different  indi- 
viduals, so  that  what  has  passed  for  the  same  attitude,  by  being  called 
by  the  same  name,  has  in  reality  varied  much  in  the  different  cases. 

From  more  recent  work3  it  appears  that  the  names  " sensorial" 
and  "muscular"  are  not  well  fitted  to  indicate  the  difference  in 
attitude  of  different  subjects;  and  also  that  the  object  to  which  the 
attention  is  directed  is  not  the  determining  factor  in  causing  the 
differences  in  reaction-time.  In  the  sensorial  attitude,  the  subject 
prepares,  not  only  to  react,  but  also  to  observe  the  stimulus;  but  in 
the  muscular  reaction,  he  does  not  prepare  to  observe  the  movement. 
Rather,  he  prepares  to  make  the  movement,  and  to  make  it  as 
promptly  as  possible  on  receiving  the  stimulus.  This  preparation 
for  a  speedy  reaction  is  always  more  or  less  involved  in  reaction- 
time  experiments;  but  concentrating  the  expectant  attention  on 
the  movement  seems  to  emphasize  the  matter  of  speed,  by  excluding 
every  other  consideration  besides  that  of  barely  reacting.  Sensa- 
tions of  tension  or  of  tingling  are  apt  to  be  felt  from  the  muscles 
in  the  preparatory  period  of  a  muscular  reaction;  and  premature 
reactions  frequently  occur,  indicating  that  the  "sub-excitation" 
of  the  motor  apparatus  has  gone  a  little  too  far.  In  terms  of  the 
neurological  conception  sketched  above  we  may  say  that  the  cere- 

1  Philosoph.  Studien,  IV,  479. 

2  Miinsterberg,  Beitrdge  zur  exp.  PsychoL,  I,  74;  Baldwin,  Psychol.  Rev.,  1895, 
II,  259;  Angell  and  Moore,  Psychol.  Rev,.  1896,  III,  245;  Flournoy,  Observations 
sur  quelques  types  de  reaction  simple  (Geneva,  1896). 

3Ach,  op.  cit.;  Breitwieser,  Attention  and  Movement  in  Reaction  Time  (New 
York,  1911). 


"SENSORIAL"  AND  "MUSCULAR"  REACTIONS      487 

bral  pathway,  in  a  muscular  reaction,  is  as  short  and  uncompli- 
cated as  possible,  and  as  highly  prepared  as  is  possible  without  actual 
movement.  In  this  way  the  greater  speed  of  this  type  of  reaction 
would  be  explained. 

Moreover,  a  "pure  muscular"  reaction  is  an  extreme  case,  the 
opposite  extreme  of  which  would  be  an  attitude  so  purely  sensorial 
— so  exclusively  directed  toward  the  apprehension  of  the  stimulus 
— that  the  preparation  to  move  would  not  be  completed  till  the 
stimulus  had  been  fully  "apperceived."  Such  reactions  as  this 
seldom  occur  under  experimental  conditions;  for  the  mere  holding 
of  the  finger  on  the  key  involves  some  degree  of  readiness  to  make 
a  particular  movement.  Reactions  which  seem,  introspectively, 
to  conform  to  the  above  definition  of  the  pure  sensorial  type  oc- 
cupy a  longer  time  than  the  usual  sensorial  reactions  (0.3  to  0.8 
sec.,  according  to  Ach);  but  also  an  exceedingly  variable  time.  The 
usual  " sensorial"  reaction  is,  therefore,  an  intermediate  type,  in 
which  the  adjustment  to  react  is  more  or  less,  but  not  completely, 
suppressed  by  the  simultaneous  preparation  to  observe  the  stimulus. 
The  reacting  movement  usually  occurs  during  the  process  of  obser- 
vation, but  not  always  at  the  same  stage  of  this  process.  According 
to  this  conception,  therefore,  the  longer  reaction-time  with  "sen- 
sorial" preparation  might  be  due  either  to  a  relatively  long  central 
pathway — a  pathway  including  parts  corresponding  to  factors  in 
the  process  of  apperception;  or  it  might  also  be  due  to  a  relatively 
low  degree  of  sub-excitation  of  the  motor  part  of  the  pathway  and 
of  its  connections  with  the  sensory  part.  Both  of  these  causes 
probably  operate  to  produce  great  variations  in  the  so-called  sen- 
sorial reaction. 

§  15.  "Complex  reactions"  are  those  in  which  several  mental 
factors  intervene  between  the  stimulus  and  the  motor  response. 
The  sensorial  type  of  "simple  reaction"  belongs  here,  in  all  cases 
where  the  reaction  is  actually  held  back  till  a  degree  of  appercep- 
tion of  the  stimulus  has  occurred.  Scarcely  different  from  these  are 
reactions  in  which  a  variety  of  stimuli  are  used,  but  only  a  single 
movement  is  required — the  instructions  being  to  react  only  when  the 
stimulus  is  recognized.  Such  instructions  uniformly  lead  to  varia- 
ble reaction- times;  because  they  afford  no  guarantee  against  a  short- 
circuit  in  the  brain,  through  which  the  reaction  may  be  a  direct  re- 
sponse to  the  mere  reception  of  the  stimulus,  and  the  mental  process 
of  recognition  occur  in  the  after-period. 

A  more  rigorous  procedure  requires  different  movements  in  re- 
sponse to  different  stimuli — as,  for  example,  to  react  with  the  right 
hand  to  one  of  two  stimuli,  and  with  the  left  hand  to  the  other;  or 
to  make  no  movement  except  in  response  to  a  specified  one  of  the 


488       TIME-RELATIONS  OF  MENTAL  PHENOMENA 

stimuli;  or  to  give  a  vocal  response  by  naming  the  stimulus.  With 
such  procedure,  the  reaction,  to  be  correct,  must  be  held  back  till 
the  stimulus  is  recognized  and  discriminated  from  other  stimuli. 
It  was  formerly  assumed  that  the  discrimination  of  the  stimulus 
must  be  followed  by  an  act  of  "choice,"  by  which  the  proper  move- 
ment is  selected;  but  this  natural  assumption,  like  many  others 
which  have  been  made  in  regard  to  the  mental  process  in  reactions, 
is  not  borne  out  by  introspection.1  Unpracticed  subjects,  indeed, 
in  their  first  few  reactions,  may  think  of  the  movement  after  recog- 
nizing the  stimulus,  and  before  executing  the  movement;  but  with 
a  little  practice,  even  this  slight  trace  of  choice  disappears,  and  the 
movement  follows  directly  on  the  recognition  of  the  stimulus.  In 
other  words,  each  movement  becomes  so  firmly  associated  with  its 
appropriate  stimulus  as  to  follow  it  immediately.  It  should  be 
noted  that  discriminative,  or,  as  Jastrow2  has  called  them,  "adap- 
tive reactions,"  are  exceedingly  common  in  ordinary  behavior,  as 
in  reading  and  naming  objects,  in  obeying  commands,  and  in  mak- 
ing appropriate  responses  to  the  numerous  stimuli  which  affect  the 
senses.  When  the  appropriate  responses  have  been  frequently  as- 
sociated with  the  stimuli,  as  in  the  case  of  reading,  no  consciousness 
of  a  volitional  character  intervenes  between  the  stimulus  and  the 
response;  volition  has  no  opportunity  to  appear  except  when  the 
reaction  is  hesitating  and  uncertain.  Choice-time  is  therefore  not 
included  in  the  measured  period  of  a  discriminative  reaction;  for 
an  act  of  will  occurs  only  during  the  phase  of  preparation.  Dis- 
crimination certainly  occurs  in  the  practical  sense  that  the  reaction 
differs  according  to  the  stimulus  which  is  presented;  but  we  are  not 
warranted  in  framing  too  formal  a  notion  of  such  discrimination, 
nor  in  thinking  of  it  as  a  well-defined  act  intervening  between  the 
reception  of  the  stimulus  and  the  response.  There  is  usually  little 
trace  of  a  conscious  comparison  of  the  actual  stimulus  with  other 
possible  stimuli;  but,  the  rather,  there  is  a  simple  apprehension  of 
the  actual  stimulus,  followed  by  movement;  and  the  apprehension, 
as  in  the  simple  reaction,  is  prepared  beforehand,  and  also  need 
not  be  completed  till  after  the  movement.  It  must,  however,  be 
so  far  completed  as  to  determine  the  right  movement. 

§16.  The  time  occupied  by  a  "reaction  with  discrimination" 
varies  greatly  according  to  the  conditions.  Speed  is  favored  by 
a  "natural,"  i.  e.,  previously  well-trained,  association  between  each 
stimulus  and  the  movement  which  is  assigned  to  it  as  its  reaction. 
The  speed  of  reaction  increases  with  practice  in  associating  a  given 
movement  with  a  given  stimulus.  But  the  discrimination  reaction 

1  See  especially  Ach,  op.  cit.,  pp.  129,  146. 

8  The  Time  Relations  of  Mental  Phenomena  (New  York,  1890),  p.  26. 


REACTION  WITH  DISCRIMINATION  489 

never  becomes  so  prompt  as  the  muscular  form  of  the  simple  re- 
action, though  it  may  become  as  prompt  as  the  sensorial  reaction. 
This  is  what  we  should  expect;  because  the  preparation  to  move  can- 
not be  so  complete  when  there  is  uncertainty,  in  the  period  of  prep- 
aration, as  to  which  of  two  movements  is  to  be  made;  and  besides, 
the  perceptive  process  must  be  allowed  to  go  further  before  the 
movement  is  initiated,  than  is  the  case  in  the  more  purely  muscular 
reaction.  The  quickest  discriminative  reactions  reported  are  about 
.180  sec.,  when  two  movements  have  to  be  held  in  readiness  and 
each  executed  in  response  to  one  of  two  stimuli;  when  only  one 
movement  is  held  in  readiness,  and  this  is  to  be  made  in  response  to 
only  one  of  two  stimuli,  the  time  may  sink  to  .170  or  even  .160  sec. 
The  preceding  remarks  suggest,  what  has  been  found  to  be  true 
in  fact,  that  the  time  of  a  discriminative  reaction  increases  with  the 
number  of  movements  that  are  held  in  readiness,  to  be  executed,  each 
in  response  to  an  assigned  stimulus.  Of  the  several  authors  who 
have  investigated  this  matter,  the  results  of  Merkel1  may  be  cited. 
He  used  as  stimuli  the  numerals  1,  2,  3,  4,  5,  and  I,  II,  III,  IV,  V, 
by  assigning  them  in  a  natural  way  to  the  ten  fingers.  He  thus  ob- 
tained the  following  reaction-times,  as  the  average  of  ten  individ- 
uals: 

One  movement  (simple  reaction) 188  sec. 

Two  movements,  with  two  stimuli       .     .     .    ..     •    .276 
Three  movements,  three  stimuli      .     .     .     .      .      .    .  330 

Four  movements,  four  stimuli 394 

Five  movements,  five  stimuli 445 

Six  movements,  six  stimuli        .      .      .      .  •    .      .      .    .  489 

Seven  movements,  seven  stimuli 526 

Eight  movements,  eight  stimuli 562    " 

Nine  movements,  nine  stimuli 581    " 

Ten  movements,  ten  stimuli 588    " 

The  discriminative  reaction  is  lengthened  not  only  when,  as  above, 
the  preparation  of  the  movements  becomes  more  complex  and  un- 
certain, but  also  when  the  preliminary  adjustment  for  the  stimulus 
becomes  less  definite.  Thus  Cattell  found2  that  it  required  slightly 
longer  to  react  to  one  only  out  of  ten  possible  colors  than  to  react 
to  one  out  of  two  possible. 

§  17.  It  appears  from  the  preceding  paragraphs  that  discrimina- 
tive reaction  is  in  part,  like  the  simple  reaction,  a  measure  of  the 
perfection  of  the  preparation  to  react  to  a  stimulus.  In  part,  however, 
it  is  also  a  measure  of  the  difficulty  of  the  discrimination  involved. 
The  more  difficult  the  discrimination,  i.  e.,  the  more  nearly  alike 
the  stimuli  which  must  be  held  apart  in  reaction,  the  slower  is  the 

1  Wundt's  Philos.  Studien,  1885,  II,  73. 
"Wundt's  Philos.  Stud.,  1886,  III,  460. 


490       TIME-RELATIONS  OF  MENTAL  PHENOMENA 

reaction.  On  this  point  we  may  cite  the  results  of  Henmon.1  The 
method  employed  by  this  experimenter,  when  the  stimuli  to  be  dis- 
criminated were  visual,  was  the  following:  Both  stimuli  were  pre- 
sented simultaneously  and  side  by  side,  and  both  hands  were  made 
ready  to  react,  each  resting  on  a  telegraph  key.  The  subject  was 
directed  to  react  to  a  prescribed  one  of  the  two  possible  stimuli, 
reacting  with  that  hand  which  was  on  the  side  of  the  prescribed 
stimulus.  For  example,  if  red  and  green  were  the  two  stimuli  em- 
ployed in  a  series  of  reactions,  the  red  was  presented  lying  sometimes 
to  the  right  of  the  green  and  sometimes  to  the  left,  and  the  subject 
reacted  with  the  right  hand  when  the  red  appeared  on  the  right  and 
with  the  left  hand  when  the  red  appeared  on  the  left.  This  form 
of  association  between  stimulus  and  movement  was  found  to  be  es- 
pecially easy,  and  so  to  give  short  and  regular  reaction  times. 

Experimenting  in  this  manner,  Henmon  obtained  the  following 
times  for  discriminative  reactions  between  various  colors: 

White  and  black 197  sec. 

Red  and  green 203  " 

Red  and  blue 212  " 

Red  and  yellow 217  " 

Red  and  orange 246  " 

Red  and  orange  mixed  with  25  per  cent,  red       .      .    .  252  " 

Red  and  orange  mixed  with  50  per  cent,  red       .      .    .  260  " 

Red  and  orange  mixed  with  75  per  cent,  red       .      .    .  271  " 

All  of  these  differences  in  the  stimuli  are  well  above  th,e  threshold 
of  color  discrimination  (compare  p.  360),  and  yet  they  do  not  all 
require  the  same  time  for  discrimination,  but  the  time  increases  as 
the  difference  decreases. 

Similar  results  were  obtained  in  the  times  of  reaction  to  differences 
of  pitch,  as  can  be  seen  in  the  following  table : 

Difference  of  pitch  in  vibrations        ....      16  12  8  4 

Time  of  the  reaction 290         .299         .311         .334 

Again,  when  the  subject  was  required  to  react  to  the  shorter  of 
two  lines,  exposed  side  by  side,  the  following  times  were  obtained:2 

In  discriminating  lines  of  10  and  13      mm.     .      .      .    296  sec. 


H  tl 

(f  tl 


It  II 

It  II 


(t 


10  '  12.5    "  .      .      .      .  298  ' 

10  '  12        "  .      .      .      .  305  " 

10  '  11.5   "   .      .      .      .  313  " 

10  '  11       "...  324  " 

10  '  10.5   "   .                  .  345  " 


1  The  Time  of  Perception  as  a  Measure  of  Differences  in  Sensations  (New  York, 
1906). 

2  The  times  cited  are  those  of  the  quicker  of  two  subjects;  the  relations  of  the 
times  of  the  slower  subject  were  approximately  the  same. 


REACTION  WITH  DISCRIMINATION 


491 


In  general,  then,  diminishing  the  difference  between  two  stimuli 
increases  the  discriminative  reaction-time;  and,  further,  diminishing 
the  objective  difference  successively  by  equal  amounts  causes  a 
greater  and  greater  increase  in  the  reaction-time,  as  the  threshold 
of  discriminable  difference  is  approached.  If  we  interpret  this 
result  in  the  light  of  our  former  discussions,  we  may  probably  con- 
clude that  the  process  of  apprehension  of  the  stimulus  has  to  pro- 
ceed further,  before  the  initiation  of  the  movement,  when  the  differ- 
ence to  be  reacted  to  is  small  than  when  the  difference  is  large;  and 
also  that  the  preliminary  adjustment  of  the  movement  is  less  com- 
plete when  greater  difficulty  of  discrimination  is  anticipated.  In 
other  words,  the  smaller  the  difference  between  the  simuli,  the  more 
the  discriminative  reaction  approaches  the  pnre  type  of  sensorial  or 
apperceptive  reaction. 

§  18.  It  should  now  be  made  clear  that  the  study  of  discrimina- 
tive reactions  is  capable  of  giving  information  regarding  the  com- 
parative ease  or  difficulty  of  different  processes  of  discrimination, 
provided  only  that  all  the  conditions  of  the  experiment  remain  the 
same,  so  that  the  only  factor  influencing  the  reaction-time  is  the 
difference  between  the  stimuli.  In  this  respect,  results  like  those 
quoted  below  from  von  Kries  and  Auerbach1  are  of  interest.  The 
numbers  given  in  this  table  are  obtained  by  subtracting  the  simple 
reaction-time  from  the  time  of  the  discriminative  reaction,  and  need 
to  be  increased  by  .150-.  200  sec.  to  make  them  comparable  with 
the  reaction-times  previously  given.  Such  comparison  cannot, 
however,  be  properly  made,  because  of  individual  differences.  The 
validity  of  the  method  of  "elimination  by  subtraction,"  employed 
by  these  authors  to  determine  the  pure  time  of  discrimination,  will 
be  discussed  later;  meanwhile,  the  differences  which  appear  in  the 
table  may  be  taken  as  indicative  of  differences  in  the  difficulty  of 
discrimination. 


Discernment  of  the  direction  of  light 

Discernment  between  two  colors 

Localization  of  sound  (minimum) 

Discernment  of  tone  when  higher 

Localization  of  sensations  of  touch 

Localization  of  distance  by  sight 

Discernment  between  tone  and  noise 

Judgment  of  intensity  of  sensations  of  touch  (strong)  . 
Discernment  of  tone  when  lower        ...... 

Judgment  of  intensity  of  sensations  of  touch  (weak)    . 
Localization  of  sound  (maximum) 


Auerbach     Von  Kries 


Sec. 
0.011 
0.012 
0.015 
0.019 
0.021 
0.022 
0.022 
0.023 
0.034 
0.053 
0.062 


Sec. 
0.017 
0.034 
0.032 
0.049 
0.036 
0.030 
0.046 
0.061 
0.054 
0.105 
0.077 


Archiv  /.  (Anat.  w.)  Physiol  (1877),  p.  298. 


492       TIME-RELATIONS  OF  MENTAL  PHENOMENA 

§  19.  Various  interesting  discoveries  were  made  during  the  course 
of  the  experiments  which  resulted  in  preparing  the  foregoing  table. 
For  example,  it  was  found  that  the  simple  reaction-time  for  A. 
(Auerbach),  when  stimulus  was  applied  to  the  middle  finger  or 
back  of  the  hand,  was  0.146-0.147  sec.;  and  for  K.  (Kries),  0.117- 
0.119  sec.  But,  as  the  table  shows,  when  discernment  was  re- 
quired of  the  two  observers,  the  reaction-time  of  K.  was  relatively 
so  much  increased  as  to  make  his  discernment  time  greater  than 
that  of  A.  The  result  of  practice  in  discernment  was  found  to  hold 
good  for  other  areas  of  the  skin  than  those  in  experimenting  upon 
which  the  practice  was  gained.  For  discernment  among  three 
places  (middle  finger,  back  of  hand,  and  middle  of  lower  arm),  the 
order  being  unknown  and  only  one  to  be  reacted  on — the  mean  in- 
terval required  was  for  A.  0.028  sec.,  and  for  K.  0.050  sec.;  fur- 
ther practice,  however,  reduced  this  interval  to  about  the  same  as 
that  required  for  two  places. 

Discernment  between  two  intensities  of  the  sensation  of  touch  was 
found  to  be  very  uncertain  and  difficult.  Many  more  false  reactions 
followed  the  attempt  to  tell  whether  the  dorsal  side  of  the  last  of 
the  phalanges  of  the  middle  finger  was  being  hit  with  the  weaker 
or  the  stronger  of  two  stimuli  than  occurred  in  the  attempts  to 
localize  tactile  sensations.  The  discernment  time,  when  reaction 
followed  the  stronger  stimulus,  was  0.016-0.034  sec.  for  A.,  and 
0.05-0.07  for  K.;  when  reaction  followed  the  weaker  stimulus,  the 
discernment  time  was  0.035-0.069  sec.  for  A.,  and  0.089-0.114  for  K. 
The  character  of  our  judgments  of  intensity  is,  perhaps,  dependent 
on  the  steepness,  as  it  were,  with  which  the  curve  rises  in  con- 
sciousness; but,  however  this  may  be,  it  appears  that  we  discern 
how  and  where  we  are  affected  with  a  sensation  more  promptly 
than  about  how  much  we  are  affected. 

When  discernment  between  two  simple  tones  of  different  pitch  is 
required,  the  reaction  follows  the  one  of  higher  pitch  more  promptly. 
Thus  the  discernment  time,  under  such  circumstances,  was  for  A., 
0.015-0.044  sec.,  and  for  K.,  0.043-0.11;  but,  if  reaction  followed 
the  tone  of  lower  pitch,  the  discernment  time  for  A.  was  0.03-0.059 
sec.,  and  for  K.  0.045-0.092.  To  discern  tone  from  noise,  when  re- 
action followed  the  tone,  A.  required  0.015-0.023  sec.,  and  K.  0.036 
-0.055;  when  reaction  followed  the  noise,  A.'s  discernment  time  was 
0.017-0.025  sec.,  and  K.'s,  0.045-0.047.  The  reaction-time  dimin- 
ishes as  the  pitch  rises;  for  very  high  notes  it  nearly  reaches  the 
limit  required  for  hearing  the  noise  of  the  electric  spark. 

The  simple  reaction-time  for  sensations  of  sound  remains  nearly 
the  same  for  all  changes  in  the  angle  by  which  the  locality  of  the 
sound  diverges  from  the  median  plane  between  the  two  ears.  But 


ASSOCIATIVE  REACTION-TIME  493 

the  time  required  for  discerning  the  locality  of  the  sound  varies 
greatly  for  the  different  sizes  of  this  angle.  Thus  the  discernment 
time  for  locality,  as  to  right  or  left,  varied  for  Auerbach  and  Kries 
as  follows: 


Angle  120°-35° 

Angle  35°-26° 

Angle  26°-ll° 

Auerfo&ch 

0  .  020  sec. 

0.033  sec. 

0  .  120  sec. 

Kries        

0.013   " 

0.122    " 

0.153   " 

The  discernment  time  required  for  localizing  the  direction  of  a 
spark  by  direct  vision  varied  for  A.  from  0.005  to  0.025  sec.,  and  for 
K.  from  0.006  to  0.029  sec.;  by  indirect  vision,  for  A.  from  0.008 
to  0.028  sec.,  and  for  K.  from  0.007  to  0.028  sec.  For  localizing 
distance,  A.  required  0.019  to  0.027  sec.  of  discernment  time,  when 
the  object  arose  in  front  of  the  fixation-point,  and  K.  0.027  to  0.035 
sec.;  but  A.  required  0.019  to  0.029  sec.,  and  K.  0.021  to  0.036  sec., 
when  the  object  arose  behind  this  point. 

§  20.  From  the  discriminative  reaction  we  pass  now  to  the  as- 
sociative. It  is  true  that  a  certain  amount  of  association  is  already 
involved  in  the  discriminative  reaction,  since  definite  movements 
are  associated,  for  the  purposes  of  the  experiment,  with  definite 
stimuli.  But  the  stimuli  and  movements  employed,  and  the  asso- 
ciations between  them,  are  few  in  such  cases;  and  the  subject  knows, 
within  a  narrow  range,  what  stimulus  to  expect  and  for  what  move- 
ment to  prepare.  In  the  typical  experiment  in  association  time, 
also,  the  subject  knows,  indeed,  the  general  type  of  stimulus  which 
he  is  to  receive  and  the  general  type  of  movement  which  he 
is  to  make ;  but  neither  the  exact  stimulus  nor  the  exact  reaction  need 
have  been  previously  mentioned  in  the  course  of  the  experiments. 
He  may  be  told,  for  example,  that  he  will  be  shown  some  number, 
and  that  he  is  to  react  by  calling  out  the  number  next  larger;  or  that 
he  is  to  be  shown  a  word,  and  is  to  react  by  calling  out  another 
word,  the  first  that  is  suggested  by  the  presented  word.  The  asso- 
ciation between  stimulus  and  response  is  here  one  of  long  standing, 
instead  of  being,  as  in  the  discriminative  reaction,  specially  formed 
for  the  purposes  of  the  experiment.  The  long-standing  of  the  asso- 
ciation favors  quick  response;  but,  on  the  other  hand,  the  immediate 
preparation  for  the  stimulus  and  for  the  movement  cannot  be  so 
complete  here  as  in  the  discriminative  reaction;  so  that,  on  the  whole, 
the  associative  reaction  takes  the  longer  time,  and  often  a  very  much 
longer  time.  There  is  in  fact  no  upper  limit  to  the  time  which  may 
be  occupied  by  such  a  reaction;  at  its  quickest,  it  occupies  about 
half  a  second  or  a  little  less. 


494       TIME-RELATIONS  OF  MENTAL  PHENOMENA 

In  the  simple  and  discriminative  reactions,  we  recognized  two 
factors  as  determining  the  speed  of  the  performance — namely,  (1) 
the  perfection  of  the  preliminary  adjustment  or  preparation;  and 
(2)  the  degree  to  which  the  perceptual  process  must  be  carried  be- 
fore the  impulse  to  movement  could  be  initiated.  In  the  associative 
reaction,  these  two  factors  are  joined  by  a  third — namely,  the  de- 
gree of  closeness  of  the  previously  formed  association  between  the 
stimulus  and  the  response.  We  may  profitably  examine  the  in- 
fluence of  each  of  these  factors  on  the  association  time,  beginning 
with  the  last  as  the  most  obvious. 

§  21.  That  familiar  associations  operate  more  quickly  than  un- 
familiar is  too  trite  a  fact  to  require  elaboration.  It  is  obvious,  for 
example,  that  one  who  is  familiar  with  the  multiplication  table  will 
call  out  the  product  of  two  presented  numbers  more  promptly  than 
one  who  is  ill  trained  in  multiplication;  or  that  it  will  take  less  time 
to  name  objects  in  one's  native  language  than  in  an  unfamiliar 
language.  What  is  more  worthy  of  note  is  that  the  association  time 
reveals  differences  in  the  firmness  of  associations  which  are  not  evi- 
dent to  less  precise  modes  of  observation.  Thus  one  may  have 
lived  for  some  years  in  a  foreign  country  and  have  learned  to  use 
its  language  fluently,  and  still  require  more  time  to  name  an  object 
in  that  language  than  in  one's  own.1  Cattell  found  that  it  took 
much  less  time  (0.345  and  0.389  sec.,  respectively,  for  two  subjects), 
to  name  the  month  following  a  given  month,  than  it  took  (0.763  and 
0.832  sec.)  to  name  the  month  preceding  a  given  month.  Simi- 
larly, more  time  was  required  to  name  the  letter  preceding,  in  the 
alphabet,  a  given  letter  than  to  name  the  letter  following;  or  to 
respond  to  a  given  number  by  the  number  next  smaller  than  by  the 
number  next  larger.  In  all  such  familiar  series,  association  is 
much  quicker  in  the  direction  of  the  series  than  in  the  reverse  di- 
rection. Less  obvious  is  the  reason  for  the  fact2  that  it  ordinarily 
takes  less  time,  on  the  average,  to  pass  from  a  part  to  the  whole 
(i.  e.,  to  give  the  name  of  the  whole  object  when  the  name  of  a  part 
is  presented  as  the  stimulus),  than  to  pass  from  the  whole  to  a  part; 
and  less  to  pass  from  a  special  class  to  a  more  general  (e.  g.,  "dog — 
animal")  than  from  the  more  general  to  the  more  special.  These 
differences  in  speed  of  reaction  are  probably  to  be  attributed,  in 
part  at  least,  to  differences  in  the  previous  training  of  the  associa- 
tions. Within  the  same  category  of  associations,  some  are  im- 
mensely more  familiar  and  quick-acting  than  others;  and  even 
among  those  which  seem,  all  alike,  thoroughly  familiar,  some  are 

1  Cattell,  Wundt's  PhUos.  Stud.,  1888,  IV,  241;  and  Mind,  XII,  68. 

2  Cattell,  op.  cit.;  Trautscholdt,  Wundt's  Philos.  Studien,  I,  213;  Watt,  Archiv 
/.  d.  ges.  Psychol,  1905,  IV,  289. 


ASSOCIATIVE  REACTION-TIME  495 

shown  by  experiment  to  give  quicker  reactions  than  others.  Thus, 
if  the  requirement  is  to  respond  by  a  word  having  the  opposite 
meaning  to  that  of  the  stimulus  word,  about  twice  as  much  time 
is  consumed  in  reacting  to  "smooth"  or  to  "broad,"  as  in  re- 
acting to  "good"  or  "long."  In  this  way,  the  study  of  associa- 
tion times  reveals  facts  regarding  the  experience  and  training  of 
an  individual  or  of  a  social  group  which  would  otherwise  be  only 
vaguely  suspected. 

§  22.  The  time  of  an  associative  reaction  is  not,  however,  an 
unequivocal  measure  of  the  firmness  with  which  an  association  has 
been  established  by  past  experience;  the  speed  of  reaction  depends 
in  part  on  the  perfection  of  the  momentary  preparation  and  in  part 
on  the  complexity  of  the  mental  process  which  intervenes  between 
the  stimulus  and  the  reaction.  Introspective  studies1  have  revealed 
some  interesting  facts  regarding  both  the  period  of  preparation  and 
the  principal  or  reaction  period.  The  preparation  may  be  more  or 
less  precise,  according  to  the  character  of  the  experiment.  Thus 
it  has  become  customary  to  distinguish  between  "free"  and  "con- 
trolled" or  "constrained"  association:  in  the  former  case,  the  sub- 
ject is  simply  directed  to  respond  by  the  first  word  suggested  by 
the  stimulus  word;  in  the  latter,  he  is  required  to  respond  by  a  word 
which  stands  in  some  assigned  relation  to  the  stimulus  word — as, 
for  example,  a  word  of  opposite  meaning,  or  the  name  of  a  class 
of  objects  within  which  the  object  named  in  the  stimulus  word  is 
included.  The  constraint  may  be  more  or  less  complete;  since  in 
some  cases  there  is  only  one  right  answer,  while  in  others  any  one 
of  a  greater  or  smaller  number  of  answers  is  correct.  Free  associa- 
tion is  intended  to  be  wholly  unconstrained;  but  this  ideal  cannot 
be  reached  in  practice,  since  the  subject  has  at  least  to  make  a  verbal 
response,  and  since,  also,  in  many  cases,  he  involuntarily  sets  him- 
self a  more  definite  task. 

One  seemingly  strange  result  from  these  experiments  is  that  free 
association  often  requires  more  time  than  constrained.  The  reason 
probably  is  that  the  constraint,  by  limiting  beforehand  the  field 
of  operations,  permits  of  a  more  perfect  preparation  to  react.  The 
reality  and  efficiency  of  the  preparation,  in  general,  is  shown  by  the 
promptness  with  which  a  correct  response  is  given  in  constrained 
association;  as  well  as  by  the  fact  that  changing  the  preparation — 
for  example,  from  an  adjustment  to  multiply  to  an  adjustment  to 
add  two  presented  numbers — leads  to  an  entirely  different  set  of 
responses  to  the  same  stimuli.  Watt  found  that,  at  the  commence- 
ment of  a  series  of  associative  reactions  of  the  same  type,  the  task 
or  problem  (naming  a  higher  class,  etc.)  was  at  first  consciously 
See  especially  Watt,  op.  cit. 


496       TIME-RELATIONS  OF  MENTAL  PHENOMENA 

represented  in  the  preparation  for  each  single  reaction;  but  that  this 
consciousness  of  the  task  decreased  as  the  series  of  similar  reac- 
tions progressed,  till  there  might  be  nothing  present  to  indicate 
the  prepared  condition  except  an  unspecialized  feeling  of  readiness. 
The  adjustment  for  the  reaction  remained  in  force,  however;  and 
it  even  improved  as  the  consciousness  of  it  waned. 

On  changing  from  one  task  to  another,  the  subject  is  likely  to 
become  more  specifically  conscious  of  the  new  problem;  he  is  also 
likely  to  be  slower  in  his  reactions,  even  though  the  "new"  problem 
may  have  been  made  familiar  by  previous  experiments.  This  also 
goes  to  indicate  the  reality  of  some  central  adjustment  by  which 
the  pathway  of  each  fresh  stimulus  through  the  brain  is  made  ready 
beforehand. 

§  23.  The  amount  of  conscious  process  which  intervenes  be- 
tween the  stimulus  and  an  associative  reaction  varies  enormously 
in  different  cases.  When  the  preparation  is  perfect,  and  the  asso- 
ciation of  response  to  stimulus  is  familiar,  the  time  of  reaction  is 
brief,  and  there  is  little  introspective  evidence  of  the  process,  which 
seems  to  have  become  automatic.  This  is  especially  apt  to  be  the 
case  when  there  is  only  one  correct  response.  When  two  or  more 
responses  are  correct,  and  about  equally  familiar,  the  time  is  apt 
to  be  longer,1  apparently  because  of  an  interference  between  the 
tendencies  toward  the  two  responses.  Introspectively,  this  inter- 
ference may  be  in  evidence  as  an  experienced  need  of  selection 
between  two  consciously  suggested  responses.  At  other  times, 
interference  results  from  the  occurrence  of  a  conscious  tendency  to 
some  false  reaction;  even  if  such  a  tendency  is  checked  in  time  to 
prevent  an  incorrect  motor  response,  it  slows  the  reaction.  Time 
may  also  be  required  in  searching  for  a  correct  response.  This 
searching,  in  not  too  difficult  eases,  is  introspectively  a  waiting  for 
the  correct  response  to  be  suggested.  Sometimes  the  correct  re- 
sponse seems  to  come  gradually,  so  that  its  coming  can  be  observed; 
at  other  times  it  appears  suddenly,  or  jumps  into  consciousness. 

The  interference  of  one  associative  tendency  with  another  can 
also  be  strengthened  by  requiring  a  response  other  than  the  one 
which  is  likely  to  be  suggested.  Thus  Ach,2  after  first  forming  by 
repetition  strong  associations  between  pairs  of  nonsense  syllables, 
then  presented  these  syllables  with  the  requirement  that  the  response 
to  each  should  be,  contrary  to  the  formed  associations,  a  syllable 
rhyming  with  it.  Under  these  conditions,  the  first  response  suggested 
was  almost  sure  to  be  the  familiar  but  now  incorrect  syllable.  If 
this  wrong  response  was  not  checked  in  time,  the  false  reaction  led 

1  Compare  Cattell,  op.  cit. 

2  Uber  den  Willensakt  und  das  Temperament  (Leipzig,  1910). 


GENERAL  CHARACTER  OF  REACTION-TIME        497 

to  a  strong  determination  to  react  correctly,  and  the  effectiveness 
of  this  strong  determination,  which  formed  part  of  the  preparation 
for  the  succeeding  reactions,  was  often  visible  in  a  successful  check- 
ing of  the  wrong  response  in  the  next  trial.  When  the  wrong  re- 
sponse was  checked  in  time,  the  thought  of  it  was  followed  by  a  con- 
sciousness of  the  task  to  be  accomplished  (such  consciousness  not 
ordinarily  being  present  between  the  stimulus  and  the  response); 
and,  next,  by  a  period  of  searching  for  a  correct  response. 

§  24.  It  is  clear  that  the  amount  of  the  entire  mental  process  in- 
tervening between  the  stimulus  and  the  associative  reaction  differs 
greatly  in  different  cases.  The  time  of  such  a  reaction  is  not  al- 
ways the  time  of  the  same  processes;  because  the  total  associative 
mechanism  which  may  be  called  into  play  is  vast  and  compli- 
cated, and  subject  to  many  varying  influences,  so  that  it  cannot  al- 
ways be  perfectly  controlled.  The  time  of  an  associative  reaction, 
therefore,  is  a  measure  partly  of  the  perfection  and  strength  of 
the  preliminary  adjustment,  partly  of  the  strength  of  the  partic- 
ular associative  tendency  which  leads  to  the  response,  and  partly 
of  the  amount  of  interference  between  different  associative  ten- 
dencies. 

§  25.  The  object  of  the  study  of  reaction-times,  as  it  presented 
itself  to  the  minds  of  the  founders  of  this  study,  was  that  of  deter- 
mining the  duration  of  definite  mental  operations,  such  as  sensation, 
perception,  apperception,  discrimination,  association,  choice.  Cer- 
tain of  these  processes  they  recognized  as  involved  in  the  simple  re- 
action, and  these  they  labored  to  isolate  by  variations  of  the  experi- 
ment. Others  of  these  processes  they  supposed  to  be  added  when 
the  simple  reaction  was  changed  to  the  discriminative  form,  or 
this  to  the  associative.  Accordingly,  they  argued  that  the  time  oc- 
cupied by  these  added  operations  could  be  obtained  by  subtracting 
the  time  of  the  simple  reaction  from  the  discriminative  or  associ- 
ative reaction- time;  and  great  ingenuity  was  displayed  in  so  vary- 
ing the  conditions  and  requirements  of  the  reaction  as  to  permit  of 
this  "  elimination  by  subtraction."  More  recent  studies  have  shown, 
as  described  above,  that  the  total  actual  process  which  is  timed  does 
not  correspond,  introspectively,  with  the  analyses  drawn  up  for  it; 
and  probably,  also,  it  does  not  correspond  physiologically.  These 
supposed  constituent  processes — perception,  apperception,  discrim- 
ination, choice — were  discovered  by  logical  analysis  of  the  result 
or  outcome,  and  not  by  a  direct  study  of  the  process  itself.  The 
process  gone  through  in  a  discriminative  reaction  is  not  that  of  a 
simple  reaction  with  a  process  of  discrimination  interpolated  into 
it;  but  the  whole  performance  is  different,  beginning  with  the  pre- 
liminary  adjustment.  It  follows,  therefore,  that  the  procedure  of 


498       TIME-RELATIONS  OF  MENTAL  PHENOMENA 

elimination  by  subtraction  is  invalid,1  and  that  the  times  occupied 
by  the  supposed  elementary  processes  are  not  revealed  by  the  work 
on  reaction-times.  Yet  the  results  obtained  are  of  value,  and  the 
method  of  reaction-times  promises  to  prove  of  still  further  value — 
when  taken  in  connection  with  introspective  analysis  of  the  process 
and  with  other  indications  of  the  nature  of  the  brain's  action — in 
elucidating  the  dynamics  of  mental  operations. 

§  26.  One  further  remark  should  be  made  regarding  reaction- 
times.  In  these  experiments,  the  aim  is  to  isolate  as  far  as  possi- 
ble a  single  factor,  so  as  to  determine  its  speed  and  the  process  by 
which  it  is  accomplished.  From  the  very  nature  of  the  case,  how- 
ever, it  is  probable  that  this  aim  can  never  be  accomplished  per- 
fectly. In  ordinary  life,  while  reactions  essentially  like  the  simple 
discriminative  and  associative  reactions  of  the  experimenters  are 
common  enough,  they  do  not  occur  in  isolation,  but  as  parts,  usu- 
ally, of  continued  performances.  Hence  the  importance  of  Cat- 
tell's  attempt 2  to  study  the  speed  of  such  reactions  under  condi- 
tions approaching  those  of  ordinary  application.  When  a  whole 
page  of  words  or  letters  was  placed  before  the  subject,  and  he  was 
directed  to  read  or  name  these  consecutively  and  as  rapidly  as  possi- 
ble, the  time  consumed  per  letter  was  .188  sec.  and  per  word  .200 
sec.  In  the  isolated  reactions,  however,  the  time  for  a  word  or 
letter  was  .320  to  .360  sec.  Two  important  facts  appear  in  these 
figures.  The  first  is  that  scarcely  more  time  is  required  to  name  a 
(short)  word  than  to  name  a  single  letter;  it  is  clear,  therefore,  that 
the  reaction  to  a  word  does  not  consist  of  a  sum  of  reactions  to  the 
letters  composing  it.  The  reaction  is  by  larger  units.  The  other  fact 
to  be  noted  is  that  considerably  less  time  is  required  to  react  to  each 
one  of  a  series  of  letters  or  words  when  all  are  visible  at  once 
than  when  it  is  required  to  react  to  a  single  isolated  letter  or  word. 
There  is  evidence  here  of  an  overlapping  of  the  successive  reactions. 
While  one  word  is  being  pronounced,  the  next  is  already  in  process 
of  apprehension.  The  overlapping,  in  fact,  extends  beyond  the 
adjoining  unit;  by  a  special  experiment,  Cattell  determined  that  it 
extended,  in  the  case  of  separate  letters,  over  four  or  five  of  them. 
(We  note  in  passing  that  this  corresponds  to  what  we  should  ex- 
pect to  follow  from  the  facts  already  brought  out  in  the  study  of 
visual  perception.  See  pp.  461  f.)  Both  overlapping  and  reaction 
to  large  units  enter  into  such  skilled  performances  as  those  of  a 
typewriter  writing  from  copy  or  of  a  musician  playing  from  score. 
These  performances  consist  of  a  series  of  discriminative  reactions; 

1  This  procedure  has  been  searchingly  criticised  by  several  of  the  authors  cited 
above,  and  also  by  Aliotta,  La  Misura  in  Psicologia  Sperimentale  (Firenze,  1905). 

2  Wundt's  Philos.  Studien,  1885,  II,  635. 


GENERAL  CHARACTER  OF  REACTION-TIME        499 

and  yet  their  speed  may  be  such  that  only  an  eighth  or  a  tenth  of 
a  second  is  occupied  by  each  movement — much  less  than  the  time  of 
a  discriminative  reaction.  The  high  speed  is  due  partly  to  appre- 
hending and  reacting  to  phrases  rather  than  to  single  notes,  words, 
or  letters;  and  partly  to  carrying  on  processes  of  apprehension  and 
movement  simultaneously.  In  both  these  processes,  "practice 
makes  perfect"  by  the  expedient  of  dropping  out  of  consciousness 
many  of  its  procedures  which,  although  originally  necessary,  have 
now  become,  under  the  laws  of  association  and  habit,  quite  dis- 
pensable without  impairing,  but  rather  with  improving,  the  speed 
and  certainty  with  which  the  desired  result  is  secured.  Nor  can 
there  be  any  reasonable  doubt  that  a  corresponding  "short-circuit- 
ing" takes  place  in  the  nervous  mechanism,  especially  in  the  cere- 
bral processes.  Thus  skill  is  acquired.  To  the  same  question  as 
to  how  such  skill  is  acquired,  we  shall  have  occasion  to  recur  in  a 
later  chapter  on  "learning."  We  close  this  discussion  by  referring 
again  to  the  truth  that  both  the  reactions  of  the  nervous  mechanism 
and  the  correlated  mental  activities  are  immensely  complicated; 
while  at  the  same  time,  the  factors  which  can  be  either  definitely 
isolated  or  reasonably  suspected  are  bound  together,  in  both  cases, 
into  a  marvellous  unity. 


CHAPTER  VII 
FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

§  1.  We  have  already  had  repeated  occasion  to  remark  upon  the 
extreme  difficulty  of  recognizing  either  by  purely  introspective  or 
by  experimental  analysis  the  absolutely  elementary  factors  of  our 
complex  mental  states.  As  a  matter  of  course,  when  such  factors 
are  successfully  analyzed,  they  are  found  to  be  incapable  of  iden- 
tification with  one  another;  or  of  being  substituted  for  one  another, 
without  changing  the  entire  mental  complex  of  which  they  form  a 
part.  All  the  difficulties  connected  with,  or  consequent  upon,  this 
fundamental  fact  of  our  experience,  accompany  in  an  exaggerated 
manner  every  attempt  to  form  a  satisfactory  psychological  doctrine 
of  the  nature  and  conditions  of  the  feelings  and  emotions.  And, 
indeed,  as  to  the  essential  nature  of  feeling,  no  satisfactory  definition 
can  be  given;  since  to  feel  is  as  simple  and  fundamental  an  opera- 
tion of  mind  as  it  is  to  know.  Feeling  can  never  be  stated  in  terms 
of  knowledge.  Inasmuch,  then,  as  all  definition  is  only  the  ex- 
pression of  an  elaborate  and  complex  form  of  knowledge,  the  nature 
of  feeling  is  not  capable  of  being  defined;  it  must  be  felt.  When, 
then,  this  nature  is  defined  as  consisting  in  some  relation  to  physi- 
cal sensation  or  to  mental  images,  it  is  deprived  of  the  very  character- 
istic which  makes  it  to  be  feeling  rather  than  sensation  or  idea. 
Various  theories,  however,  have  succeeded  in  stating  certain  con- 
ditions or  antecedents  of  the  reaction  of  mind  in  the  way  of  feeling. 

§  2.  To  this  indefiniteness  in  the  experience,  the  indefiniteness 
in  the  language  which  attempts  to  describe  the  experience  bears 
witness.  The  term,  "feeling,"  has,  in  popular  speech,  a  wide  and 
varied  usage.  Sometimes  applied  to  the  sense  of  touch,  it  is  also 
used  to  indicate  any  and  every  emotional  experience,  whether  de- 
pendent chiefly  upon  certain  characteristic  sensations,  or  chiefly 
upon  conditions  of  an  ideal  or  intellectual  sort.  An  example  of 
this  last  usage  is  seen  in  the  expression,  "I  feel  that  there  is  some 
mistake  in  the  argument,  though  I  cannot  tell  just  where  the  mis- 
take lies."  Psychologists  have  abandoned  the  first  of  these  usages, 
but  are  not  yet  agreed  as  to  the  limits  to  be  drawn  within  the  broad 
field  indicated  by  the  second  and  third  usages.  Some  would  pre- 
fer to  retain  feeling  as  a  vague  term,  applicable  to  any  condition  of 

500 


VARIABLE   CONCEPTION  OF  FEELING  501 

consciousness  which  is  lacking  in  clear  definition.  Others  would 
narrowly  limit  the  term  to  the  feelings  of  pleasantness  and  unpleas- 
antness, regarding  which  no  doubt  exists  that  they  should  be  thus 
named. 

The  question  is  not  entirely  one  of  scientific  terminology;  for, 
from  the  point  of  view  of  the  analysis  of  consciousness,  many  of  these 
vague  states  must  be  regarded  as  composite;  whereas  pleasantness 
and  unpleasantness  are  apparently  not  susceptible  of  further  analy- 
sis, but  seem  themselves  to  be  of  an  elementary  character.  From 
this  point  of  view,  therefore,  the  question  arises  as  to  what  elements 
of  consciousness,  different  from  sensation,  and  analogous  to  these 
paradigms,  pleasantness  and  unpleasantness,  can  be  discovered. 
Even  on  this  question,  however,  competent  opinion  is  still  widely 
divergent,  since  some  authorities  would  limit  the  elementary  feel- 
ings to  the  two  mentioned,  while  others  would  increase  the  number 
to  four  or  six,  and  still  others  hold  to  a  very  large  and  indefinite 
number.  One  writer,1  for  example,  presents  a  long  list,  including 
feelings  of  interest,  of  reality  and  unreality,  of  belief  and  doubt,  of 
clearness,  confusion,  effort,  ease,  eagerness,  hesitation,  pride,  hu- 
mility, admiration,  scorn,  reasonableness,  etc.  In  the  opinion  of 
many  psychologists,  most  of  these,  as  well  as  the  numerous  other 
emotions  which  might  be  mentioned,  are  certainly  complexes  of 
many  elements. 

The  dual  conception  of  feeling,  which  would  reduce  all  feelings 
to  pleasantness  and  unpleasantness  (in  combination  with  a  variety  of 
intellectual  elements),  is  the  traditional  view,  and  has  usually  been 
in  favor  with  the  analytic  school  of  psychologists.  Recently,  how- 
ever, Wundt 2  and  Royce 3  have  made  the  novel  suggestion  that  there 
are,  in  addition  to  this  one  dimension  of  feeling,  other  dimensions. 
The  latter  proposes  to  regard  feeling  as  a  two-dimensional  con- 
tinuum, like  a  plane;  he  would,  therefore,  add  to  the  traditional 
pleasantness-unpleasantness  dimension  that  of  restlessness-quies- 
cence. This  is  to  say  that  any  state  of  feeling  may  differ  from  any 
other  state  in  being  more  or  less  pleasant  and  also  in  being  more  or 
less  restless.  Wundt  proposes  to  regard  feeling  as  a  three-dimen- 
sional continuum,  and  adds  to  the  dimension  of  pleasantness  and 
unpleasantness  that  of  excitement  and  calm  or  depression  (in  which 
connection  depression  is  to  be  taken  as  a  simple  opposite  of  excitement 
and  is  not  to  include  any  notion  of  unpleasantness),  and  that  of 
tension  and  relief.  Those  who,  in  opposition  to  these  suggestions, 

1  Baldwin:  Handbook  of  Psychology,  Feeling  and  Will,  1894,  p.  242. 

2  Grundriss  d.  Psychologic,  1896,  p.  98;  Grundzuge  d.  physiol.  Psychol.,  6th  ed., 
1910,  II,  294. 

3  Outlines  o/  Psychology,  1903, 


502    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

continue  to  uphold  the  one-dimensional  theory,1  believe  that  the 
excitement,  tension,  etc.,  incident  to  certain  states  of  feeling,  are 
not  elementary,  but  are  compounds  of  bodily  sensations. 

§  3.  In  criticism  of  the  foregoing  views,  we  remark  that  the  real- 
ity of  feelings  of  excitement  and  depression,  of  tension  and  relief, 
cannot  be  doubted,  but  only  their  elementary  character;  and  this 
has  not  yet  been  established.  Among  the  sensations,  some  classes 
are  indeed  characterized  chiefly  with  a  tone  of  either  pleasantness 
or  unpleasantness;  here  belong  tastes,  odors,  and  sensations  of 
temperature.  Visual  and  auditory  stimuli  do  not  usually  arouse 
such  intense  feelings  of  pleasantness  or  unpleasantness;  unless  they 
have  become  associated  with  certain  meanings,  as  in  pictures, 
landscapes,  and  speech.  Visual  sensations,  more  than  auditory, 
are  likely  to  be  relatively  free  from  pleasantness  and  unpleasant- 
ness. There  is  much  greater  unanimity  among  individuals  as  to 
which  is  the  most  exciting  color  than  as  to  which  is  the  most  agreea- 
ble; the  latter  judgment  seems  to  depend  largely  on  the  associa- 
tions of  the  various  colors,  while  the  exciting  or  calming  effect  of 
a  color  seems  to  be  more  intrinsic.  Feelings  of  tension  and  relief 
are  especially  connected  with  auditory  sensations,  perhaps  because 
the  temporal  sequence  of  this  class  of  sansations  is  particularly 
prominent.  In  listening  to  a  series  of  metronome  beats — to  take 
Wundt's  example — one  can  detect  a  feeling  of  tension  in  anticipa- 
tion of  each  beat,  which  gives  way  to  a  temporary  relaxation  or 
relief  on  the  actual  occurrence  of  the  beat.  So,  also,  in  listening 
to,  or  in  reading,  a  sentence  which  has  a  periodic  structure,  a  feeling 
of  tension  can  be  observed  which  gives  way  to  relief  on  the  comple- 
tion of  the  sentence.  Tension  is  thus  closely  associated  with  ex- 
pectancy, and  relief  with  the  fulfilment  of  expectation.  Along 
with  the  expectancy  there  may  be  present,  according  to  the  circum- 
stances, either  pleasantness  or  unpleasantness;  and  the  relief  may 
also  be  accompanied  with  either  of  these  feelings,  according  as  the 
event  is  agreeable  or  the  opposite. 

As  we  have  already  seen,  according  to  the  theory  of  Wundt,  the 
qualities  of  simple  feeling  have  three  dimensions,  and  no  more. 
But  it  is  very  doubtful  whether  this  view  gives  an  adequate  account 
of  our  feeling  experience.  Certainly  there  are  many  feelings  which 
do  not  readily  fit  into  these  grooves,  and  which,  though  we  may  sus- 
pect them  to  be  composite,  do  not  as  yet  admit  of  being  analyzed. 
Among  these  may  be  mentioned  a  class  of  feelings  connected  with 
memory  and  judgment.  When  one  is  trying  to  recall  a  name,  the 
feeling  of  "being  near,"  or  "on  the  right  track,"  often  arises.  If  an- 

1  See  Titchener,  Lectures  on  the  Elementary  Psychology  of  Feeling  and  Atten- 
tion, 1908;  Ebbinghaus,  Grundziige  der  Psychologic,  1905,  p.  567. 


DIFFICULTIES  OF  ANALYSIS  503 

other  than  the  right  name  occurs  to  the  mind,  it  is  usually  attended 
with  a  feeling  of  its  incorrectness,  but  when  the  right  name  occurs, 
it  is  felt  to  be  right.  Such  affective  experiences  are  accepted  as  the 
warrants  of  correct  or  incorrect  recollection;  they  cannot  be  de- 
fended; they  cannot  be  analyzed;  they  are  distinctly  subjective,  and 
not  referable  to  anything  beyond  the  self;  in  all  ways  they  answer 
to  the  conception  of  feeling  rather  than  of  any  form  of  sensation  or 
knowledge.  The  same  can  be  said  of  the  feeling  of  familiarity 
with  a  face  or  an  odor  which  is  not  completely  recognized,  but  is 
felt  to  be  known  as  distinguished  from  "  knowing  it  to  be  known." 
Then,  too,  there  is  the  feeling  of  conviction  which  obtains  when  we 
pass  from  the  former  into  the  latter  state  of  consciousness.  Ex- 
periments on  reaction  time  and  association  time  have  also  brought 
to  light  feelings  of  readiness  or  unreadiness,  of  clearness  or  con- 
fusion, which  have  resisted  analysis.  Most  of  the  feelings  mentioned 
in  this  paragraph  may  be  attended  with  pleasantness  or  unpleas- 
antness, but  they  are  not  simply  pleasant  or  unpleasant.  On  the 
contrary,  they  have,  each  one,  a  specific  character.  This  character 
is  not  identical  with  the  character  of  the  name,  for  example,  which 
is  being  recalled,  for  the  same  feeling  may  attach  to  any  name 
under  the  circumstances  of  its  being  recalled  after  difficulty.  The 
feeling  seems,  the  rather,  to  be  appropriate  to  a  certain  sort  of  mental 
performance.  To  resolve  it  into  a  complex  of  bodily  sensations  is 
at  least  premature;  though  it  might,  conceivably,  be  so  composed, 
it  might  equally  well,  for  all  we  can  now  see,  be  the  specific  accom- 
paniment of  a  special  sort  of  brain  action.  As  the  case  stands  at 
present,  therefore,  psychological  analysis  does  not  permit  us  to 
limit  the  number  of  elementary  feelings  to  two,  or  to  six,  or  to  any 
specified  number. 

§  4.  We  recur,  then,  to  our  previous  statement  with  added  con- 
fidence. About  that  aspect  of  our  experience  which  we  call  our 
feelings,  or  our  emotions — their  nature,  origin,  relation  to  a  physi- 
cal basis  and  to  sensations  and  ideas— we  know  remarkably  little. 
The  reason  for  this  fact  is  not  difficult  to  discover.  By  their  very 
nature,  the  phenomena  are  obscure,  indefinite,  and  yet  extremely 
variable  and  multiform.  They  are  also  connected  with  our  sensa- 
tions and  ideas  in  such  a  way  as  to  make  all  separation  in  fact  quite 
impossible.  The  psychology  of  the  feelings,  as  studied  from  the 
introspective  point  of  view,  has  therefore  always  been  peculiarly  un- 
productive of  assured  results.  The  fact  that  their  physiological  con- 
ditions are  laid  so  largely  in  obscure,  rapid,  and  infinitely  varied 
changes  within  the  central  organs,  such  as  cannot  be  either  directly 
observed  or  indirectly  subjected  to  experimentation,  increases  the 
difficulties  of  the  subject.  What  is  the  nature  of  feeling  ?  How  do 
the  different  feelings  differ,  and  what  elements  have  they  in  com- 


504    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

mon?  Under  what  conditions  do  we  have  sensuous  feelings;  and 
under  what  conditions  are  these  feelings  pleasant  or  unpleasant? 
Is  feeling  ever  perfectly  indifferent?  is  there  a  zero-point  of  feel- 
ing? How  are  the  feelings  related  to  the  quality  and  intensity  of 
physical  stimuli  ?  What  is  the  physiological  basis  (if  any  exist)  of 
the  higher  aesthetic,  moral,  and  religious  feelings?  These  and 
other  similar  questions  may  be  asked  of  psycho-physical  science 
with  little  satisfactory  result.  The  attempt  unduly  to  simplify, 
instead  of  increasing  our  scientific  clearness  in  dealing  with  the  sub- 
ject, adds  to  its  confusion. 

§  5.  The  experimental  study  of  feeling  in  some  of  its  aspects 
has  developed  two  methods  of  investigation,  called  "the  method 
of  impression"  and  "the  method  of  expression."  Both  are  alike 
in  that  they  bring  to  bear  on  the  subject  certain  known  stimuli,  in 
order  to  determine  their  effect  in  exciting  the  feelings.  The  stimuli 
range,  in  different  experiments,  from  simple  colors,  tones,  odors,  or 
tastes,  to  complex  presentations,  such  as  pictures  or  witticisms  or 
musical  and  literary  compositions.  The  two  methods  differ  in  the 
means  by  which  the  feeling  of  the  subject  is  made  known.  In  the 
method  of  impression,  the  person  on  whom  the  stimuli  are  made  to 
act  has  simply  to  take  note  of  the  resulting  changes  in  his  affective 
tone,  and  to  indicate  the  result  of  his  observation  in  words.  The 
method  of  expression  is  based  on  the  fact  that  bodily  reactions  often 
appear  in  connection  with  changes  of  feeling — such  as  reactions  of 
the  organs  of  circulation,  respiration,  secretion,  digestion,  and 
involuntary  movements  and  tensions  of  the  muscles.  The  method 
of  expression,  acting  on  the  assumption  that  these  bodily  reactions 
are  the  signs  of  inner  feeling,  seeks  to  record  these  signs  themselves. 
The  two  methods  are  often  joined,  in  that  the  subject  is  asked  to 
report  his  feelings  in  words,  and  the  correlation  of  the  feelings,  as 
introspectively  observed,  with  the  bodily  expressive  reactions,  is 
made  the  main  purpose  of  the  experiment. 

In  the  use  of  the  method  of  impression,  the  most  approved  pro- 
cedure is  to  present  stimuli  in  pairs,  and  then  to  require  a  judgment 
on  each  pair,  as  to  which  of  the  two  is  the  more  agreeable,  etc.  A 
variation  of  this  method  consists  in  presenting  a  whole  series  of 
stimuli  at  once,  and  requiring  that  they  be  arranged  in  the  order 
of  their  agreeableness  or  other  value.  The  use  of  the  method  of 
impression  has  occurred,  largely,  in  the  field  of  aesthetics,  into  which 
Fechner1  was  the  first  to  introduce  the  experimental  method. 

§  6.  Since  it  is  the  special  purpose  of  this  Treatise  to  investigate 
the  phenomena  of  man's  mental  life  in  their  relations  to  the  func- 
tions of  the  nervous  mechanism,  some  attempt  at  a  physiological 
theory  of  the  feelings  and  emotions  would  seem  to  be  our  appropri- 
1  Vorschule  der  JZsthetik,  1876. 


PHYSIOLOGICAL  THEORIES  OF  FEELING          505 

ate  task.  This  method  of  approach,  and  the  field  of  conjecture  to 
which  it  leads,  has  proved  particularly  attractive  to  most  theorists. 
But,  unfortunately,  far  the  greater  number  of  such  theorists  have 
made  no  effort  to  take  into  consideration,  much  less  to  deal  ade- 
quately with,  the  complex  and  intricate  forms  of  nervous  function- 
ing, concerned  in  any  comprehensive  theory.  It  is  true,  as  Bain l 
declares,  that  "a  very  considerable  number  of  the  facts  may  be 
brought  under  the  following  principle — namely,  that  states  of  pleas-t 
ure  are  connected  with  an  increase,  and  states  of  pain  with  an  abate- 
ment,  of  some,  or  all,  of  the  vital  functions."  But  other  facts  in  no 
small  number  cannot  be  brought  under  this  principle.  It  is  not  a 
difficult  task  for  the  physician  to  abate  all  the  vital  functions  of 
the  patient,  even  down  to  or  beyond  the  line  of  danger,  with  the  im- 
mediate result  of  producing  pleasure  rather  than  pain.  After  ob- 
jecting to  Bain's  statement  as  being  "too  vague,"  etc.,  Grant  Allen2 
declares  the  true  principle  of  connection  to  be  the  following:  "Pleas- 
ure is  the  concomitant  of  the  healthy  action  of  any  or  all  of  the  organs 
or  members  supplied  with  afferent  cerebro-spinal  nerves,  to  an  ex- 
tent not  exceeding  the  ordinary  powers  of  reparation  possessed  by 
the  system."  ^Esthetic  pleasure  he  provisionally  defines  as  "the 
subjective  concomitant  of  the  normal  amount  of  activity,  not  directly 
connected  with  life-serving  function,  in  the  peripheral  end-organs 
of  the  cerebro-spinal  nervous  system."  Now,  that  pleasure  is  the 
reflex  of  healthy  and  unimpeded  activity  is  an  old  psychological 
truism;  and  that  we  are  dependent  upon  impulses  propagated  in 
the  sensory  nerves  of  the  cerebro-spinal  system  for  sensations, 
pleasurable  or  painful,  of  muscular,  organic,  or  more  special  sort, 
scarcely  needs  statement  as  a  newly  discovered  law  of  "physiologi- 
cal aesthetics."  Nothing,  however,  could  well  be  more  "vague" 
than  the  limit  fixed  by  the  words  "to  an  extent  not  exceeding  the 
ordinary  powers  of  reparation  possessed  by  the  system." 

Even  to  undertake  the  study  of  the  bodily  expressions  of  feeling, 
it  is  necessary  to  have  a  knowledge  of  the  physiology  of  circulation, 
respiration,  secretion,  reflex  action,  etc.  The  movements  of  breath- 
ing, for  instance,  are,  indeed,  responsive  to  mental  conditions — 
laughing,  sobbing,  and  sighing  are  striking  examples  of  this  re- 
sponsiveness— but  they  are  also  responsive  to  many  other  influ- 
ences, such  as  the  need  of  the  muscles  for  oxygen.  Any  sensory 
stimulus  is  likely  to  exert  an  influence  on  the  breathing,  and  that 
even  in  a  condition  of  unconsciousness  through  anaesthesia.  The 
rate  and  strength  of  the  heart-beat  is  likewise  subject  to  many  re- 
flex influences,  as  well  as  to  influences  proceeding  downward  from 
the  cerebrum;  and  it  is  further  influenced  by  the  respiration.  The 

1  The  Senses  and  the  Intellect,  pp.  281  f. 

2  Physiological  ^Esthetics,  1877,  pp.  21-34. 


506    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 


circulation  through  any  organ  is  influenced  not  only  by  the  rate  and 
force  of  the  heart-beat,  but  also  by  the  constriction  or  dilatation  of 
the  arteries  leading  to  the  organ,  and  to  other  organs  as  well.  Even 
the  activity  of  the  voluntary  muscles  is  certainly  subjected  to  reflex 
influences,  as  well  as  to  the  influence  of  the  cerebrum.  It  thus  be- 
comes a  difficult  matter  to  observe  the  effects  of  simple  states  of 
feeling,. because  of  the  complication  of  these  with  other  effects. 

The  principal  kinds  of  the  observation  of  bodily  functions  in 
connection  with  the  feelings  have  been  the  following:  The  move- 
ments of  the  chest  and  perhaps  also  of  the  abdomen  in  breathing 
have  been  recorded,  by  aid  of  pneumographs;  and  the  relation  of 
different  feelings  to  changes  in  the  rate  and  depth  of  respiration  has 
been  studied  in  the  graphic  records.  The  pulse  has  been  recorded 
by  sphygmographs  applied  to  various  arteries,  among  which  the 
radial  is  the  most  accessible,  though  the  carotid,  since  it  supplies  the 
brain,  has  appeared  to  many  investigators  as  the  most  important 
for  psychological  purposes.  Changes  in  the  volume  of  the  arm,  or 
of  a  finger — changes  which  are  due  to  the  varying  amount  of  blood 
contained  in  the  member,  and  which  depend  partly  on  the  output  of 
the  heart  and  partly  on  the  vasomotor  condition  of  the  member  and 
of  other  parts — have  been  deemed  specially  important  by  many 
authorities,  and  have  been  recorded  by  means  of  plethysmographs. 
Similar  instruments  can  even  be  applied  to  the  brain  itself,  in  cases 
where  part  of  it  has  been  exposed  through  removal  of  a  portion  of 
the  skull.  One  or  two  other  methods  of  study  will  be  briefly  re- 
ferred to  below. 

§  7.  We  may  begin  by  bringing  forward  the  most  ambitious  at- 
tempt to  reduce  the  results  of  the  method  of  expression  to  the  form 
of  a  law.  Wundt,  in  whose  laboratory  several  experimenters  have 
studied  the  pulse  and  respiration  during  moments  of  pleasant  feel- 
ing, etc.,  summarizes  the  results  in  the  following  scheme:1 


Feeling 

Pulse 

Breathing 

Tension     

Strength 

Speed 

Strength 

Speed 

+ 

+ 
+ 

+ 
+ 

+ 

+ 
+ 

+ 
+ 
+ 

Calm               .           ... 

Unpleasantness    .... 
Pleasantness  
Excitation      
Relief        

1  Physiol  Psychol,  6th  ed.,  1910,  II,  310.  The  arrangement  of  the  scheme  is 
here  somewhat  modified  from  that  given  by  Wundt.  +  indicates  an  increase 
in  strength  or  rapidity;  — ,  a  decrease;  =,  no  change. 


KINDS  OF  FEELING  507 

In  commenting  on  this  scheme  of  results,  it  should  be  noted,  first, 
that  not  all  the  results  fit  well  into  the  scheme.  This  is  even  true 
of  some  of  the  work  done  in  Wundt's  laboratory,  so  that  his  scheme 
has  undergone  certain  modifications  from  its  earlier  form,  in  conse- 
quence of  later  results  which  seemed  more  trustworthy.  In  the 
earlier  scheme,  the  positions  of  excitation  and  relief  are,  approx- 
imately, interchanged;  and  the  same  is  true  of  calm  and-  tension. 
Moreover,  the  results  obtained  by  other  experimenters  are  often 
at  variance  with  those  of  Wundt's  laboratory.  In  analyzing  the 
results  of  the  table  with  a  view  to  determine  what  they  probably 
mean,  the  manner  of  conducting  the  experiments  must  first  be 
understood.  The  subject,  being  taken  in  as  nearly  as  possible  a 
neutral  state  of  feeling,  is  presented  with  certain  sensory  stimuli; 
or  is  required  to  perform  some  brief  mental  operation,  such  as  multi- 
plying together  two  numbers.  The  effects  of  these  interruptions  of 
the  neutral  state  are  noted  in  the  records  of  pulse  and  respiration, 
and  are  then  compared  with  the  subject's  own  testimony  as  to  the 
change  of  feeling  induced.  Interruptions  inducing  feelings  of  ten- 
sion are,  particularly,  those  which  arouse  attentive  and  expectant  ob- 
servation on  the  part  of  the  subject,  or  those  which  require  him  to 
perform  some  brief  mental  operation;  while  relief  is  experienced  at 
the  successful  accomplishment  of  any  such  task.  That,  under  these 
circumstances,  the  breathing  becomes  weaker  and  shallower  is  a  fact 
observed  by  practically  all  experimenters;  it  may  be  taken,  in  con- 
nection with  the  correlative  fact  that  the  opposite  effect  occurs  at 
the  close  of  the  mental  operation,  as  the  best-established  result  of 
this  entire  line  of  study.  At  times,  indeed,  respiration  may  be  en- 
tirely suspended  during  a  brief  attentive  act. 

Other  movements  besides  the  respiratory  show  similar  changes. 
When,  for  example,  an  audience  is  restless,  the  speaker  may  suc- 
ceed, for  a  moment,  in  checking  the  slight  movements  of  numerous 
individuals  which  go  to  make  up  this  restlessness,  by  showing  them 
a  picture,  pointing  to  some  object,  posing  a  question,  or  otherwise 
arousing  the  mental  activity  of  the  audience.  When  the  activity 
so  aroused  has  reached  its  goal,  there  is  likely  to  be  a  general  shift- 
ing of  position  throughout  the  audience,  and  a  return  to  other  un- 
easy movements.  It  seems  clear,  then,  that  brief  and  attentive 
mental  acts  inhibit  bodily  movement,  and  that  respiration  partici- 
pates in  this  inhibition.  The  subsequent  reverse  effect  is  doubtless 
due,  in  the  case  of  breathing,  to  the  need  of  air,  i.  e.,  to  a  slight 
dyspnoea  produced  by  insufficient  respiration  during  the  brief 
period  of  mental  activity.  It  would  perhaps  be  better  to  think  of 
the  inhibited  respiration  during  such  periods  of  mental  activity  as 
a  symptom  of  the  activity  (or  of  "attention")  rather  than  of  the  feel- 


508    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

ing  of  tension  which  goes  with  the  activity.  It  is  claimed  by  those 
who  refuse  to  admit  tension  and  relief  into  the  category  of  elementary 
feelings,  that  the  feeling  of  tension  is  itself  composed  of  sensations 
of  shallow  breathing  and  of  the  immobile  and  often  strained  posi- 
tion of  the  limbs ;  and  that  the  feeling  of  relief  is  composed  of  sensa- 
tions of  deeper  breathing  (sometimes  amounting  to  a  sigh),  and  of 
the  relaxation  of  the  limbs  from  their  fixed  positions. 

It  should  be  noted  that  the  statements  made  above  regarding 
the  effects  of  mental  activity  (or  of  "attention")  on  pulse  and  respira- 
tion hold  good  mainly  of  brief  periods  of  activity.  When  mental 
work  is  long  continued,  the  principal  effect  observed1  is  a  quick- 
ening of  the  heart-beat. 

As  to  excitement  and  its  opposite,  there  is  also  little  doubt  that 
the  above  table  corresponds  pretty  closely  to  the  facts,  and  that  ex- 
citement is  usually  accompanied  by  an  increase  in  breathing,  and 
calm  by  a  decrease.  It  is  difficult  to  attain  the  feeling  of  calm 
immediately  after  vigorous  muscular  exercise,  when  pulse  and  res- 
piration are  strong  and  rapid.  In  explaining  the  fact  that  the 
symptoms  of  tension  so  closely  resemble  those  of  calm,  and  those 
of  relief  so  closely  resemble  those  of  excitement,  it  must  be  remem- 
bered that  tension  corresponds  to  a  repression  of  bodily  activity,  and 
that  relief  brings  a  compensatory  rebound  from  this  repression. 

§  8.  If  we  now  turn  to  the  accompaniments  of  pleasantness  and 
unpleasantness,  we  shall  find  it  hard  to  reach  any  satisfactory  con- 
clusion. For  one  thing,  it  is  almost  impossible,  in  an  experiment, 
to  keep  these  feelings  free  from  tension  and  relief;  that  is,  from  the 
effects  of  observant  attention  and  its  satisfaction.  Some  stimulus 
must  be  used  to  arouse  the  pleasant  or  unpleasant  feeling,  and  this 
stimulus  arouses  also  the  subject's  attention.  Thus  Lehmann,2  one 
of  those  who  have  most  devoted  themselves  to  this  line  of  study,  re- 
ports that  colors  and  tones  are  unsuited  for  testing  the  effects  of 
pleasantness  and  unpleasantness,  since  the  only  effects  obtained 
by  their  use  are  those  of  attention;  only  olfactory  stimuli  seemed  to 
him  suited  for  the  purpose;  .and  even  with  these,  the  changes  in 
the  pulse  were  not  constant,  but  sometimes  consisted  in  a  slowing 
and  sometimes  in  a  hastening.  In  general,  the  circulatory  and  re- 
spiratory symptoms  of  these  feelings  are  inconstant,  and,  according 
to  many  observers,  there  is  no  clear  difference  between  the  accom- 
paniments of  pleasantness  and  of  unpleasantness. 

The  same  thing  is  even  true  of  the  cerebral  circulation,  as  ob- 
served in  cases  where  a  portion  of  the  brain  has  been  exposed  by  the 

1  See  E.  Gley,  Etudes  de  psychologic  physiologique  et  pathologique,  1903,  p.  29; 
Billings  and  Shepard,  Psychol  Rev.,  1910,  XVII,  217. 

8  Die  korperliche  Ausserungen  psychischer  Zustdnde,  1905,  III,  481-482. 


INDUCED  ELECTRICAL  CHANGES  509 

removal  of  part  of  the  skull  in  a  surgical  operation.  The  brain 
can  then  be  observed  to  expand  and  contract  according  to  the  amount 
of  blood  in  it;  and  its  volume  changes  according  to  the  activity  of 
the  brain.  But  whether  it  expands  or  contracts,  specifically,  in 
conditions  of  pleasantness  and  unpleasantness,  is  a  question  to 
which  the  observations  so  far  reported  give  divergent  answers.1 

The  more  intense  and,  as  they  are  considered,  more  complex, 
states  of  feeling  known  as  emotions,  are  also  far  from  uniform  in 
their  respiratory  symptoms.  On  the  whole,  therefore,  the  evidence 
from  this  line  of  study  would  seem  distinctly  opposed  to  analyzing 
the  feelings  into  combinations  of  sensations  with  only  a  varying  feel- 
ing-tone of  pleasantness  and  unpleasantness. 

§  9.  Of  late  years,  a  new  symptom  of  the  condition  of  feeling  has 
come  into  prominence.  The  instrument  used  for  the  observation 
of  this  symptom  is  the  galvanometer,  and  the  bodily  change  noted 
is  electrical.  Fe're'2  and  Tarchanoff,  independently,  discovered 
that  an  electrical  current,  or  a  change  of  potential,  could  be  ob- 
served by  connecting  a  delicate  galvanometer  to  two  points  of  the 
skin,  and  then  subjecting  the  person  under  observation  to  stimuli 
which  influenced  the  state  of  feeling.  Such  a  stimulus  as  tickling 
caused  a  current  to  appear  in  the  galvanometer;  as  did  also  any 
emotion  or  the  thought  of  an  emotion.  Mental  activity  of  a  non- 
affective  sort  produces  comparatively  slight  currents.4  But  mus- 
cular activity  produces  deflections  of  the  galvanometer.  Vera- 
guth5  found  that  if  a  story  were  read  to  the  subject  while  his  skin 
was  connected  with  the  galvanometer,  deflections  occurred  when 
emotional  passages  were  read.  Jung6  turned  the  experiment  to 
practical  account,  by  showing  that  ideas  having  for  an  individual 
an  emotional  import  could  be  detected  by  reading  him  a  list  of 
words,  and  observing  which  words  caused  an  electrical  response. 
The  detection  of  such  ideas,  or  " complexes"  of  ideas,  which  have 
a  strong  emotional  tone  for  an  individual,  is  often  of  value  in  treat- 
ing hysteria  and  other  neuroses. 

The  source  of  the  electricity  which  is  revealed  by  the  galvanom- 
eter in  these  experiments  is  not  yet  certainly  known.  We  know  that 
activity  of  any  organ  is  attended  by  the  production  of  electric  cur- 

1  See  H.  Berger,  Uber  die  korperliche  Ausserungen  psychischer  Zustande,  1904 
and  1907;  E.  Weber,  Der  Einfluss  psychischer  Vorgange  auf  den  Korper,  1910; 
and  also  short  articles  by  these  same  authors  in  Zeitschr.  /.  Psychol.,  1910,  LVI, 
299,  305. 

2  C.  R.  Soc.  de  Biologie,  1888,  p.  217. 

3  Pfliiger's  Arch.  f.  d.  ges.  Physiol,  1890,  XL VI,  46. 

4  Starch,  Psychol.  Rev.,  1910,  XVII,  19. 

5  Arch,  de  Psychol.,  1906,  VI,  162. 

6  See  Peterson  and  Jung,  Brain,  1907,  XXX,  153. 


510    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

rents.  The  sweat-glands  in  the  skin,  it  may  be,  are  excited,  through 
their  nerves,  by  brain  activity,  and  so  give  rise  to  the  current. 
Sidis  and  Nelson,1  experimenting  on  animals,  used  electrodes  which 
penetrated  through  the  skin,  and  still  obtained  currents;  they  there- 
fore concluded  that  it  is  the  muscles  which  produce  the  electric 
currents — the  muscles  being  involuntarily  excited  by  the  activity  of 
the  brain. 

Though  these  electric  phenomena  are  delicate  indicators  of  emo- 
tional states,  they  do  not,  apparently,  differ  with  the  particular 
quality  of  the  emotion.  They  do  not  furnish,  therefore,  a  differ- 
ential symptom  of  pleasantness  and  of  unpleasantness.  It  is  diffi- 
cult, indeed,  to  discover  such  a  symptom  in  the  bodily  accompani- 
ments of  feeling.  To  judge  by  the  method  of  expression,  we  should 
be  brought  to  the  unexpected  conclusion  that  pleasant  and  un- 
pleasant states  of  mind  resemble  each  other  more  than  either  of 
them  resembles  indifference.  In  other  words,  they  are  all  alike 
states  of  excited  feeling. 

§  10.  To  recur  to  a  subject  to  which  reference  has  already  been 
made :  It  is  impossible  to  make  any  final  statement,  or  even  to  offer 
any  weighty  evidence,  regarding  the  brain  activities  which  corre- 
spond to  pleasant  and  unpleasant  feelings.  The  general  biologicalN 
interpretation  of  the  feelings  connects  pleasure  with  beneficial 
stimuli  and  a  good  condition  of  the  organism,  and  unpleasantness 
v^with  injurious  stimuli  and  bad  bodily  conditions — a  rule  to  which, 
however,  there  are  evident  exceptions.  But  this  conception  does 
not  yet  indicate  in  what  manner  the  feelings  are  generated.  Is 
feeling  a  sensation2  originating  in  the  excitation  of  sensory  nerve- 
ends — and,  if  so,  are  there  specific  nerves  and  sense-organs  for  feel- 
ing, or  can  many  or  all  afferent  nerves  convey  to  the  brain  impulses 
which  are  capable  of  arousing  these  forms  of  consciousness; — or, 
on  the  contrary,  do  pleasantness  and  unpleasantness  arise  from 
conditions  of  the  brain  itself  ?  The  latter  class  of  explanations 
has  several  representatives.  Meynert 3  suggested  that  feeling  is  an 
indication  of  the  nutritive  condition  of  the  brain,  especially  as  de- 
pendent on  the  blood  supply. 

Lehmann's  " dynamic  theory"  of  feeling4  regards  pleasantness  as 

1Psychol  Rev.,  1910,  XVII,  98. 

2  Stumpf,  ZeUschr.  /.  Psychol,  1906,  XLIV,  1;  Titchener,  Textbook  of  Psychology, 
1909,  p.  261. 

3  Psychiatric,  p.  171,  cited  after  Ebbinghaus,  Grundzuge  der  Psychologic,  1905, 
p.  577. 

'Die  korperliche  Ausserungen  psychischer  Zustdnde,  1901,  II,  291;  1905,  III, 
403;  other  interesting  conceptions  of  the  physiology  of  feeling  have  been  enter- 
tained by  Marshall  in  his  Pain,  Pleasure  and  ^Esthetics,  1894,  and  by  M.  Meyer, 
Psychol.  Rev.,  1908,  XV,  201. 


OTHER  THEORIES  OF  FEELING  511 

the  index  of  a  proper  balance  between  the  supply  and  the  expendi- 
ture of  energy  by  the  nerve-cells  of  the  brain;  while  unpleasantness 
results  from  an  excess  of  expenditure  over  supply;  and  therefore 
every  very  intense  sensory  stimulus  is  unpleasant,  because  it  over- 
works some  of  the  brain-cells,  causing  their  expenditure  to  exceed 
the  supply  brought  to  them  by  the  blood  stream. 

The  Herbartian  doctrine  (see  just  below)  of  pleasantness  as 
the  measure  of  harmonious  co-operation  of  ideas,  and  of  unpleas- 
antness as  the  measure  of  conflict  between  ideas,  might  be  trans- 
lated into  the  neural  terms  of  facilitation  and  inhibition,  a  facili- 
tated reaction  being  pleasurable  and  an  inhibited  or  obstructed 
response  being  unpleasurable.  Somewhat  similar  is  the  theory  of 
Ziehen,1  that  the  ready  discharge  of  an  excited  portion  of  the  cor- 
tex along  paths  of  projection  or  of  association  is  pleasurable;  where- 
as the  obstruction  of  such  discharge  constitutes  unpleasantness. 
Where  pertinent  facts  are  so  meagre,  an  attempt  at  selecting  the 
best  of  these  hypotheses  is  scarcely  worth  while. 

§  11.  On  account  of  the  nature  of  our  work,  only  a  brief  refer- 
ence is  necessary  to  one  of  the  most  elaborate,  and  in  many  respects 
successful,  of  all  the  theories  which  derive  the  phenomena  of  the 
feelings  and  emotions,  in  general,  from  other  and  different  men- 
tal factors.  We  refer  to  that  best  known  as  connected  with  the 
name  of  the  great  German  psychologist,  Herbart.  This  theory 
makes  feeling  dependent  upon  the  relations  of  the  ideas  as  further- 
ing or  checking  each  other.  It  cannot  be  admitted,  to  begin  with, 
that  feeling  is  a  secondary  or  derived  form  of  consciousness.  No 
form  of  mental  activity  is  more  primitive  and  unanalyzable  than 
feeling;  none  is  earlier  in  the  development  of  mental  life.2  Be- 
fore the  infant  has  localized  the  different  sensations,  and  combined 
them  into  percepts  of  the  different  parts  of  its  own  organism,  the 
consciousness  of  being  affected  in  a  given  way,  either  pleasurable 
or  not,  must  predominate.  Other  forms  of  feeling — of  desire,  un- 
easiness, comfort,  etc. — are  inseparably  connected  with  its  first 
states  of  consciousness;  they  belong  to  its  inherited  impulses  and 
instincts,  and  are  only  later  definitely  related  to  the  appropriate 
ideas.  The  primary  formation  of  self-consciousness  is  quite  as 
truly  connected  with  self-feeling ,  pleasurable  or  painful,  as  with  the 
process  of  ideation  in  constructing  the  concept  of  "me"  and  "not- 
me."  Volkmann  von  Volkmar,  in  his  great  work,3  considers  feeling 

1  Physiologische  Psychologic  der  Gefuhle  und  Affekte,  1902. 

2  The  same  view  of  the  feelings  is  maintained  by  Horwicz,  and  developed  at 
length,  polemically,  by  Lotze  (Horwicz,  Psychologische  Analysen,  i,  pp.  168  f.; 
Lotze,  Medicin.  Psyehologie,  pp.  235  f.). 

3  Lehrbuch  d.  Psychologic,  1884,  II,  pp.  298  ff. 


512    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

as  the  consciousness  of  the  process  of  ideation  itself  as  distinguished 
from  consciousness  of  this  or  that  idea,  and  it  is  conditioned  upon 
some  resistance  being  offered  to  the  process.  Feeling  is,  then,  no 
one  proper  idea,  to  be  placed  in  conjunction  or  classed  with  others. 
It  is  rather  a  becoming  conscious  of  the  degree  of  tension,  as  it  were, 
which  characterizes  the  process  of  ideation  at  each  particular  mo- 
ment. The  condition  of  the  origin  of  a  feeling  is,  then,  the  existence 
of  two  simultaneous  opposed  ideas.  Their  coexistence  occasions 
a  state  of  tension  (" Spannung"),  as  it  were,  and  this  state  gives 
way  as  one  idea  triumphs  over  the  other.  The  type  of  simple  feel- 
ing may  be  illustrated  by  the  condition  in  which  the  mind  finds 
itself  when  listening  to  harmonious  or  discordant  musical  sounds. 

As  has  already  been  said  (p.  511),  this  theory  closely  approaches 
one  of  the  more  prominent  of  the  purely  physiological  theories. 

But,  although  we  are  to  distinguish  sensation  from  feeling,  we 
must  regard  the  feeling  which  inseparably  accompanies  sensation 
as  feeling,  strictly  speaking,  and  not  as  tone  of  sensation;  or,  in  other 
words,  the  tone  of  every  sensation,  as  either  pleasurable  or  pain- 
ful, is  given  to  it  by  the  feeling  which  accompanies  and  blends  with 
it.  The  sensation,  as  having  a  certain  quality,  quantity,  and  lo- 
cality, is  capable  of  being  built  into  a  " Thing"  which  the  mind 
perceives  as  not  itself.  But  the  feeling,  the  pleasurable  or  pain- 
ful tone  of  the  sensation,  is  always  recognized  as  purely  and  simply 
a  way  in  which  the  mind  is  affected.  To  refuse  to  speak  of  sensa- 
tions and  emotions,  with  all  their  complicated  physical  basis,  as 
belonging  at  all  in  the  realm  of  "  feeling,"  is  to  restrict  the  use  of  the 
word  unwarrantably.  The  Herbartian  theory  commits  in  this  mat- 
ter the  mistake  which  it  is  guilty  of  committing  repeatedly;  it  re- 
gards the  "ideas"  as  realities  that  have  in  some  sort  a  substantial 
existence,  and  can  do  something  by  way  of  furthering  or  hindering 
each  other.  But  ideas  are  themselves  nothing  more  than  mental 
products  that  exist  only  when  and  so  long  as  the  mind  acts  with  a 
definite  degree  and  kind  of  energy.  In  determining  the  kind  and 
degree  of  this  ideating  energy,  the  previous  action  and  habit  of  the 
mind  by  way  of  feeling  is  quite  as  influential  upon  the  mode  of 
feeling  as  the  manner  of  its  ideating  energy.  Finally,  this  theory 
wrecks  itself  upon  the  denial  of  all  that  which  the  physiological 
theory  maintains  and  establishes.  The  two  theories,  then,  sup- 
plement and  correct  each  other;  but  even  when  combined  they  only 
tell  us  in  part  what  are  the  physical  and  mental  conditions  under 
which  feeling  arises. 

§  12.  As  a  logical  consequence  of  the  very  nature  of  all  kinds 
and  degrees  of  feeling,  a  strict  classification  of  our  affective  and  emo- 
tional states  is  impossible.  The  difficulty  is  even  greater  than  that 


CLASSIFICATION  OF  FEELINGS  513 

which  was  found  to  be  encountered  by  every  attempt  to  classify 
the  sensations  of  smell,  as  such.  In  both  cases — that  of  certain 
sensations  and  that  of  all  feelings — the  popular  language  is  signifi- 
cant of  this  fact  of  universal  experience.  Sensations  of  color  are 
either  red,  green,  blue,  etc.;  but  agreeable  sensations  of  smell  are 
either  of  a  rose,  or  of  a  violet;  and  disagreeable  odors  are  either  of 
asafcetida,  or  of  mercaptan,  etc.  In  somewhat  the  same  way, 
judgments  are  fitly  classified  as  either  positive  or  negative,  universal 
or  particular  or  distributive,  etc.;  but  all  these  classes  of  judgments 
may,  when  considered  in  their  affective  or  emotional  aspect,  be 
distinguished  as  characterized  by  feelings  of  doubt,  or  of  certainty, 
and  accompanied  by  feelings  of  sadness,  or  of  joy,  or  of  mixed 
pleasure  and  pain. 

It  is  obvious,  then,  that  any  attempt  at  classifying  the  feelings 
is  likely  to  be  most  practically  successful,  even  if  it  must  surrender 
the  attempt  at  scientific  accuracy,  when  it  takes  some  one  of  the 
points  of  view  from  which  the  subject  may  be  best  seen  in  indirect 
vision,  as  it  were.  Among  the  classification  schemes  derived  in 
this  way,  that  is  perhaps  most  convenient  which  emphasizes  those 
other  forms  of  mental  activity  on  which  the  various  kinds  of  feel- 
ing are  chiefly  dependent.  In  this  way,  we  may  recognize  three, 
or  four,  great  classes  of  human  experiences,  when  looked  upon  as 
characterized  by  their  affective,  or  emotional,  aspect.  These  would 
be  (1)  the  sensuous  feelings,  or  those  which  are  dependently  re- 
lated to,  and  in  consciousness  blended  with,  the  different  qualities 
and  intensities  of  the  sensations  of  the  special  senses  or  the  more  gen- 
eral organic  functions;  (2)  the  intellectual  feelings,  or  those  which 
precede,  accompany,  or  follow  the  various  activities  of  discrimina- 
tion, association,  judgment,  reasoning,  and,  indeed,  all  the  forms 
of  functioning  of  the  "mind"  in  the  narrower  meaning  of  the  latter 
word;  (3)  cesthetic  feelings,  or  those  which  belong  to  the  perception 
and  appreciation  of  what  we  call  "the  beautiful"  (in  its  various 
forms),  or  its  opposite;  and  (4)  the  moral  feelings,  or  those  affective 
experiences  which  appertain  to  the  good  and  the  bad,  in  human  con- 
duct. To  these  might  be  added  a  fifth  class,  to  be  called  the  relig- 
ious feelings,  were  it  not  for  the  fact  that  the  latter  may  be  satisfac- 
torily treated  as  special  forms  of  the  combination  of  the  intellectual, 
aesthetic,  and  moral  feelings.1 

In  adopting  this,  or  any  similar  classification,  it  must  be  remem- 
bered, as  a  matter  of  course,  that  senses,  intellect,  and  will  enter  into 
all  the  activities  connected  with  every  form  of  complex  and  highly 
developed  emotion;  and  such  are  all  the  emotional  states  of  our 

1  A  similar  classification  is  that  proposed  by  Horwicz:  Psychologische  Analy- 
sen,  ii,  pp.  82  f. 


514    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

adult  experience.  For  this  reason  also,  no  hard  and  fixed  line  can 
be  drawn  about  the  different  so-called  classes  of  feelings.  The 
aesthetic  feelings  cannot  be  separated  from  the  sensuous;  for  ex- 
ample, the  feeling  which  accompanies  the  sensation  of  a  musical 
chord,  or  of  the  color  purple,  may  be  classed  under  either  head. 
Nor  can  the  intellectual  feelings  be  separated  from  the  aesthetic;  the 
perception  of  harmony  of  colors  and  sound  is  inseparably  connected 
with  aesthetic  and  sensuous  feeling,  and  the  latter  is  intensified  or 
otherwise  modified  under  the  intellectual  laws  of  contrast,  change, 
habit,  and  higher  association.  Even  the  feelings  which  we  call 
"moral"  on  account  of  their  connection  with  will  and  desire,  often 
have  an  indefinite  part  of  them  so  combined  with  feelings  located  in  the 
bodily  organism,  or  so  dependent  on  its  functions  for  their  quantity 
and  quality,  that  a  strict  separation  becomes  impossible.  Love  is  sel- 
dom or  never  so  purely  ideal  as  not  plainly  to  involve  in  itself  feel- 
ing of  sensuous  and  aesthetic  sort;  hate  not  mixed  with  anger,  and 
so  supported  on  some  elements  of  that  physical  basis  which  under- 
lies the  latter,  is  hard  to  discover  in  real  life. 

§  13.  All  feelings  are  characterized  by  tone,  strength,  rhythm, 
and  content.  Their  content  is  determined  by  the  ideating  activity 
with  which  they  are  directly  connected,  or  to  which  they  are  relat- 
ed; and  this  content  may  be  comparatively  simple,  as  is  the  case 
with  the  feeling  connected  with  the  presentation  of  a  colored  surface 
(for  example,  purple  or  green),  or  obviously  complex,  as  is  the  case 
with  the  sentiments  of  patriotism,  loyalty,  and  religious  devotion. 

Feelings,  like  all  other  mental  phenomena,  occur  under  time- 
form;  they  are,  in  general,  rhythmic  in  character,  and  change  in  re- 
spect to  content,  tone,  and  intensity,  with  a  movement  marked  more 
or  less  distinctly  by  the  quality  of  periodicity.  Their  rhythm, 
with  respect  to  content,  is,  of  course,  determined  by  the  recur- 
rence of  changes  in  the  ideating  activity  as  dependent  especially 
upon  attention  and  the  laws  of  association.  Feelings  of  sadness  or 
joy,  comfort  or  discomfort,  may  come  around  again  in  conscious- 
ness, as  it  were,  according  to  the  rhythmic  movement  of  the  sensa- 
tions which  occasion  them.  Sometimes  an  alternation  of  tone  takes 
place,  which  carries  the  mind  back  and  forth  by  the  point  of  indif- 
ference (or  hypothetical  zero-point  of  feeling)  between  agreeable 
and  disagreeable  sensations,  or  ideas  of  the  same  kind.  Thus  we 
are  sometimes  forced  to  say  that  we  do  not  know  whether  a  certain 
combination  of  colors,  or  quality  of  taste  or  smell,  is  pleasing  to  us 
or  not;  in  such  a  case  feeling  seems  to  move  rhythmically  back  and 
forth  between  a  slightly  pronounced  tone  of  pleasure  and  a  slightly 
pronounced  tone  of  pain. 

The  intensity,  too,  of  feelings  rises  and  falls  alternately  in  de- 


CHARACTERISTICS  OF  ALL  FEELING  515 

pendence  upon  the  rhythmic  movement  of  the  nervous  processes 
and  of  the  train  of  ideas.  No  feeling  is  kept  at  a  long  continuous 
level  with  respect  to  its  vigor  and  pitch  of  strength.  The  law  of 
quickly  alternating  exhaustion  and  repair  of  the  nervous  elements 
underlies,  to  a  large  extent,  this  rhythmic  movement  of  the  inten- 
sity of  the  feelings.  This  is  one  of  many  proofs  which  go  to  show 
that,  the  conditions  of  the  end-organs  and  of  the  central  organs  are 
determinative  of  the  tone  and  strength  of  feeling.  Even  when  we 
are  strictly  attending  to  our  painful  feeling,  the  toothache  is  not  a 
perfectly  uniform  and  steady  strain;  even  when  we  are  doing  our 
best  to  abstract  attention  from  the  pain,  we  succeed  only  intermit- 
tently. But  the  course  of  the  ideas  must  also  be  taken  into  account 
as  influencing  the  rhythm  of  feeling.  As  our  sensations  or  mental 
images  become  more  clear  and  vivid,  the  feelings  attached  to  them 
gather  strength;  as  the  former  become  more  obscure  and  feeble, 
the  feelings  also  die  away  in  consciousness. 

§  14.  It  has  been  said  (p.  501)  that  some  psychologists  would 
divide  all  our  affective  states  into  two  classes  of  feelings — viz., 
pleasantness  and  unpleasantness.  But  we  have  seen  that  this  classi- 
fication is  entirely  unsatisfactory;  primarily  because  it  fails  to  take 
account  of  the  almost  infinite  variety  of,  not  only  our  complex  feel- 
ings, emotions,  and  sentiments,  but  also  of  those  affective  experi- 
ences whose  analysis  has  hitherto  resisted  all  our  most  ingenious 
methods,  both  introspective  and  experimental.  We  have,  therefore, 
preferred  to  regard  pleasantness  and  unpleasantness  as  the  "tone" 
of  feeling.  This  is  to  recognize  the  fact  that,  not  only  can  we  speak 
appropriately  of  the  feelings  of  pleasantness  and  of  unpleasantness, 
but  that,  with  even  greater  propriety,  we  speak  of  nearly,  or  quite, 
all  our  feelings,  of  every  class,  as  being  themselves  either  pleasant 
or  unpleasant.  And,  indeed,  the  feeling  of  pleasure  and  pain  is 
probably  the  most  general,  most  simple,  and  earliest  psychical 
process.  That  almost  all  feelings  are  characterized  by  some  posi- 
tive tone — or,  in  other  words,  are  not  absolutely  indifferent  to  us — 
there  can  be  no  question.  Is  it  agreeable  or  disagreeable,  at  least 
in  some  slight  degree  and  in  some  more  or  less  indefinite  manner  ? 
is  an  inquiry  which  we  can  pretty  readily  answer  with  respect  to 
nearly  all  our  sensations  and  ideas.  The  question  has  for  a  long 
time  been  debated,  however,  whether  this  is  necessarily  true  of  all 
our  feelings.  Is  there  any  such  thing  as  completely  "neutral"  feel- 
ing, or  feeling  that  is  in  no  respect  or  degree  either  agreeable  or 
disagreeable  to  us  ?  Neutral  or  indifferent  feelings  were  recognized 
by  Reid,  but  disputed  by  Hamilton.1  Bain2  asserted  it  as  un- 

1  Hamilton's  Works  of  Thomas  Reid,  p.  311  (Edinburgh,  1854). 
3  The  Emotions  and  the  Will,  3d  ed.,  p.  13. 


516     FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

doubted  that  "we  may  feel,  and  yet  be  neither  pleased  nor  pained," 
and  that  "  almost  every  pleasurable  and  painful  sensation  and  emo- 
tion passes  through  a  stage  or  moment  of  indifference."  Wundt1 
argued,  on  theoretical  grounds,  that  pleasure  and  pain,  as  tones  of 
feeling  having  a  variable  intensity,  are  conditions  which  may  be 
regarded  as  on  different  sides  of  a  zero-point,  or  point  of  indif- 
ference lying  between  them.  It  does  not  follow,  however,  that, 
because  the  mind  passes  in  time  from  feeling  of  one  positive  tone 
(pleasure)  to  feeling  of  the  opposite  tone  (pain),  it  must,  therefore, 
at  some  instant  be  in  a  state  of  feeling  that  has  no  tone  and  lies 
between  the  two.  The  curve  plotted  to  represent  the  rise  and  fall 
of  feeling  is  a  material  line;  it  cannot  be  at  one  time  below,  and  at 
another  above,  the  abscissa-line,  without  at  some  single  point  (the 
zero-point)  coinciding  with  it.  But  it  does  not  follow  that,  because 
such  a  curve  is  a  picture  of  the  phenomena  of  feeling  in  one  respect, 
it  is  so  in  all  other  respects.  The  question  whether  there  is  any 
zero-point  to  the  tone  of  feeling  can  only  be  answered  by  an  appeal 
to  consciousness;  and  this  answer,  like  all  others  given  to  similar 
appeals,  is  likely  to  contain  dubious  and  conflicting  elements.  It 
is  quite  certain  that  one  can  pass  from  a  high  state  of  pleasure  to 
one  of  intense  pain  without  any  consciously  interpolated  neutral 
feeling.  For  example,  if  while  one  is  viewing  a  beautiful  landscape 
one  is  stung  by  hornets,  the  condition  of  quiet  massive  pleasure  may 
be  converted  into  one  of  great  physical  suffering  without  any  inter- 
vening feeling  of  indifference. 

§  15.  As  to  the  nervous  apparatus  and  physiological  processes 
concerned  in  the  imparting  of  pleasant  or  unpleasant  tone  to  the 
various  kinds  of  feelings,  two  principal  views  have  been  current 
hitherto.  As  bearing  upon  the  pleasure-pain  tone  of  the  sensations, 
one  view  holds  that  the  same  apparatus  of  end-organs,  conducting 
nerve-tracts,  and  central  areas,  which  on  moderate  excitement  pro- 
duces the  simple  sensations  of  pressure  or  of  temperature,  or  the 
more  complex  sensations  of  tickling,  shuddering,  etc.,  produces  the 
feeling  of  pain  when  irritated  with  increased  intensity.  Such  a 
view  would  apparently  have  also  to  hold  that  muscular  sensations 
have  the  same  physical  apparatus  as  do  feelings  of  muscular  weari- 
ness or  exhaustion;  and,  perhaps,  that  cardialgia  and  hunger  are 
due  to  modifications  of  the  action  of  the  same  nerves  of  the  stomach. 
But  from  the  introspective  point  of  view  it  is  as  certain  that  sensations 
of  pressure  or  mere  temperature  are  unlike  the  feeling  of  pleasure 
produced  by  gentle  rubbing  or  by  comfortable  warmth,  or  the  pain 
that  comes  from  heavy  pressure  or  burning,  as  it  is  that  sensations 
of  light  are  unlike  those  of  musical  tone. 

1  Physiolog.  Psychologic,  2d  ed.,  I,  pp.  465  f. 


PLEASURE-PAIN  TONE  OF  ALL  FEELING  517 

It  is  now  known,  however,  that  there  are,  for  the  skin  at  least, 
end-organs  at  the  "pain  points"  (see  p.  345.)  It  becomes  a 
reasonable  conjecture  that  the  pain  which  follows  the  excessive 
stimulation  of  the  other  end-organs  of  sense,  such  as  the  eye  or 
ear,  is  due  to  the  excitement  of  specific  "pain  nerves"  connected 
with  those  organs.  There  are  other  physiological  reasons  for  doubt- 
ing the  complete  identity  of  the  nervous  apparatus  of  pleasurable 
and  painful  feeling  with  that  of  the  sensations  with  which  the  feel- 
ing is  allied.  The  facts  upon  which  Schiff  and  others  supported 
the  view  that  nervous  impulses  resulting  in  pain  travel  by  more  or 
less  distinct  paths  along  the  spinal  cord  have  already  been  stated. 
More  recent  experiments  seem  to  show  that  the  end-organs  of  tem- 
perature, pressure,  and  pain  are  locally  separable  in  the  different 
minute  areas  of  the  skin.  Pathological  results  indicating  the  same 
separation  of  the  nervous  elements  of  feeling  also  deserve  a  brief 
mention.  In  certain  cases  the  sensibility  of  the  skin  to  pain  is  lost, 
while  its  sensibility  to  touch  is  not  weakened  or  is  even  increased. 
The  reverse  condition  also  sometimes  occurs.  "Analgia,"  as  occa- 
sioned by  pathological  states  of  the  spinal  cord  due  to  lead-poison- 
ing, was  noticed  in  many  cases  by  Beau.  This  loss  of  sensibility  to 
pain  can  hardly  be  explained  by  any  change  in  the  activity  of  cer- 
tain end-organs  common  both  to  touch  and  to  painful  feeling.  What 
impairment  of  function  could  possibly  result  in  destroying  the  sen- 
sitiveness to  strong  mechanical  and  thermic  excitations,  such  as 
ordinarily  occasion  great  pain,  while  the  response  by  way  of  sensa- 
tions of  touch  to  much  feebler  excitations  remains  undiminished  ? 

The  same  argument  would  appear  decisive  against  identifying, 
locally,  the  central  nervous  processes  which  result  in  sensation  with 
those  which  result  in  feeling.  In  certain  stages  of  narcosis,  pro- 
duced by  ether  or  chloroform,  the  patient  is  able  to  perceive  the 
slightest  contact  with  the  skin,  but  feels  no  pain  even  when  the  same 
area  is  treated  severely.  Moreover,  in  some  cases  of  tabes  dorsalis, 
a  constant  difference  seems  to  exist  in  the  time  at  which  the  sensa- 
tions of  pressure  and  the  feelings  of  pain,  simultaneously  excited  at 
the  end-organ,  arise  in  the  mind.  If  the  patient  is  pricked  with  a 
needle,  he  will  instantly  feel  the  contact,  and  the  pain  only  one  to 
two  seconds  later.1  The  case  of  the  eye,  which  responds  with  sensa- 
tions of  light  and  color  when  the  optic  nerve  is  moderately  excited, 
and  with  the  painful  feeling  of  being  blinded  when  the  stimulus  is 
increased,  is  not  perfectly  clear;  for  cases  of  amaurosis  are  on 
record  where  the  painful  feeling  persisted  after  the  eye  had  lost  all 

1  See  Funke,  in  Hermann's  Handb.  d.  Physiol,  III,  ii,  pp.  297  f.;  such  phenomena 
have  been  especially  discussed  by  Osthoff,  Die  Verlangsamung  d.  Schmerzemp- 
findung  bei  Tabes  dorsalis,  1874. 


518    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

power  to  distinguish  light.  It  may  well  be,  therefore,  that  while 
the  specific  sensations  of  light  and  color  are  due  to  the  irritation  of 
the  optic  nerve,  the  excitement  of  feeling  indicates  a  simultaneous 
irritation  of  part  of  the  trigeminus. 

We  are  compelled,  then,  to  confess  that  the  localizing  of  the 
nervous  apparatus,  and  the  nature  of  the  physiological  processes 
which  give  the  tone  of  painful  and  pleasurable  feeling  to  our  sensa- 
tions, require  further  investigation.  The  tendency  of  the  evidence, 
however,  is  toward  a  theory  which  assigns  to  feeling  a  more  or  less 
separate  mechanism  of  end-organs,  conducting  nerve-tracts,  and 
central  areas  (or  at  least  of  nervous  elements  in  the  central  areas). 

§  16.  Certain  mixtures  of  vaguely  localized  sensations,  with  feel- 
ings of  a  more  or  less  pleasant  or  unpleasant  tone,  have  acquired 
the  name  of  sensus  communis,  or  "common  feeling."  Such  feel- 
ing may  have  more  or  less  of  content  of  one  kind  or  another,  ac- 
cording to  the  state  of  perception  and  ideation  with  which  it  is 
combined.  Nervous  impulses  of  indefinite  variety  and  the  most 
manifold  peripheral  origin  are  constantly  pouring  in,  as  it  were, 
upon  the  cerebral  centres — each  one  contributing  some  element  to 
the  characteristic  tone  of  consciousness.  The  resulting  feelings  are 
modes  of  our  being  affected  which  are  not  converted  into  definite 
presentations  of  sense,  or  referred  to  a  particular  part  of  our  own 
bodies.  The  effect  of  changes  in  the  minute  blood-vessels  and 
other  capillaries  about  the  nerve-endings,  the  presence  of  impuri- 
ties in  the  blood,  the  condition  of  the  lower  cerebral  centres,  the 
action  of  the  heart  and  lungs  and  other  internal  organs,  and  the 
connection  of  the  sympathetic  with  the  cerebro-spinal  nervous  sys- 
tem, are  all  felt  in  this  way.  Moreover,  inasmuch  as  few  (if  any) 
sensations  are  without  some  tone  of  feeling,  while  many  sensations 
are  exceedingly  heterogeneous  in  their  elements,  and  not  clearly 
referred  to  the  place  of  their  origin,  a  melange,  as  it  were,  of  ob- 
scure bodily  affections  is  readily  formed. 

Sensations  in  themselves  heterogeneous  may  also  be  brought 
into  a  temporary  relation  by  the  partial  identity  of  their  source  of 
excitation,  and  of  the  nervous  connections  in  the  central  organs. 
It  is  also  always  a  very  important  question,  how  the  more  obscure 
and  mixed  bodily  feelings  stand  related  to  the  mind's  course  of 
ideation,  to  attention,  association,  etc.  This  relation  often  de- 
termines whether  such  obscure  impressions  shall  be  definitely  ob- 
jectified or  not;  whether  they  shall  not  rather  run  together  in  the 
dark  stream  of  common  feeling.  Let  any  one  suspend  for  an  in- 
stant a  train  of  interesting  thought,  which  has  up  to  the  moment 
been  interrupted  only  by  certain  obscure  feelings  of  uneasiness,  and 
such  a  one  will  be  able  instantly  to  select  and  localize  in  the  cramped 


FORMS  OF  COMMON  FEELING  519 

chest,  or  oppressed  limbs,  or  tired  organs  of  special  sense,  most  of 
the  sensations  whose  painful  tone  has  thus  colored  the  stream  of 
common  feeling.  Separation  from  localized  sensations  is,  then,  the 
chief  negative  characteristic  of  common  feeling.  Under  its  dif- 
ferent principal  forms  we  may  distinguish  different  total  results,  ac- 
cording to  the  general  relation  in  which  the  being  aware  merely 
that  we  are  affected  in  an  agreeable  or  disagreeable  manner  stands 
to  the  being  aware  of  what  affects  us  in  this  manner.  Thus  we 
sometimes  feel  well  or  ill,  elevated  or  depressed,  without  ability  to 
assign  these  feelings  at  all  definitely  to  the  physical  organism,  either 
as  perceived  or  imaged,  or  to  any  reason  in  the  train  of  ideas.  At 
other  times  the  general  impression  of  being  in  the  body,  for  some 
greater  or  less  amount  of  either  weal  or  woe,  is  emphatic;  we  feel 
ill  all  over,  or  seem  to  enjoy  the  caursing  of  the  blood  through  every 
artery  and  vein,  as  though  mentally  present  in  the  extended  tissues. 

This  melange  of  sensations  and  feelings,  the  so-called  sensus 
communis,  is  connected  in  an  important  way  with  the  higher  forms 
of  self-consciousness,  and  with  our  entire  sense  of  personality. 
Temporary  and  relatively  unimportant  disturbances  of  its  more 
essential  characteristics  may  result  in  our  "  feeling  queer,"  or  "  feel- 
ing not  exactly  like  ourselves."  More  important  and  permanent 
disturbances  take  the  form  of  those  illusions  which  are  character- 
istic of  certain  forms  of  insanity,  such  as  that  some  part  of  the  body 
is  made  of  glass,  or  that  wheels  are  whirling  inside  the  head,  etc. 
Upon  the  introspective  basis  of  such  experiences  of  "common 
feeling"  a  whole  system  of  perverted  judgments  may  be  based, 
and  essential  changes  in  the  conduct  of  life  brought  about.  This 
is  what  would  be  expected  as  a  consequence  of  the  fundamental 
psychological  truth  that  our  feelings  are  the  factors,  and  the  aspects, 
of  our  total  experience  which  we  attribute  most  directly  and  essen- 
tially to  our  very  "own  self,"  as  distinguished  from  other  selves. 
Functional  disturbances  of  the  nervous  mechanism,  both  peripheral 
and  central,  chiefly  concerned  with  the  feeling  aspect  of  our  experi- 
ence, alter  in  the  most  important  way  our  conception  of  the  Self. 

§  17.  The  question  whether  every  sensation  has  some  feeling 
must  be  distinguished  from  the  question  whether  every  feeling  is 
of  either  painful  or  pleasurable  tone.  The  tone  of  the  feeling  of 
sensations  is  the  agreeable  or  disagreeable  affection  of  conscious- 
ness which  they  often  carry,  as  inseparably  connected  with  them. 
The  particular  tone  belonging  to  any  sensation  is,  to  a  large  ex- 
tent, dependent  on  its  intensity.  Sensations  of  moderate  inten- 
sity— that  is,  of  intensity  below  the  point  at  which  the  minimum  of 
painful  feeling  begins — are  usually  pleasurable.  The  feeling  of 
pain  rises  in  intensity,  from  the  point  where  it  begins,  as  the  in- 


520    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

tensity  of  the  stimulus  increases ;  but  the  curves  which  represent  the 
increase  of  feeling  and  the  increase  of  sensation  by  no  means  com- 
pletely correspond.  The  amount  of  pleasurable  feeling  is  also  de- 
pendent on  the  element  of  time.  It  has  been  thought  to  reach  a 
maximum  at  about  the  point  where  the  strength  of  sensation  is 
the  most  favorable  for  accurate  discernment  of  the  objective 
stimulus. 

As  to  the  dependence  of  the  tone  of  feeling  belonging  to  a  sen- 
sation upon  the  quality  of  the  latter,  it  has  been  held  that  no  sen- 
sation is  absolutely  pleasant  or  unpleasant  irrespective  of  its  in- 
tensity. Even  then,  however,  it  would  have  to  be  admitted  that 
qualitatively  different  sensations  differ  greatly  in  the  amount  which 
is  consistent  with  an  agreeable  tone  of  feeling.  It  is,  of  course, 
with  regard  to  the  organic  sensations,  and  the  special  sensations  of 
touch,  smell,  and  taste,  that  the  relation  between  tone  of  feeling 
and  the  quality  of  sensation  is  most  apparent.  Doubtless  large  al- 
lowance must  be  made  in  all  cases  for  individual  peculiarities  of 
organism,  association,  etc.  The  disagreeable  tone  of  feeling  which 
almost  universally  attaches  itself  to  certain  qualities  of  sensation, 
however  moderate  or  unobtrusive  their  intensity,  may  be  largely  ex- 
plicable on  the  principle  of  heredity.  But,  taking  matters  as  they 
stand  in  present  experience,  it  is  impossible  to  maintain  that  the 
tone  of  feeling  is  not,  in  certain  cases,  directly  dependent  on  the 
quality  of  sensation.  This  is  a  question  upon  which  only  conscious- 
ness can  pronounce.  All  degrees  of  some  tastes  and  smells  are  disa- 
greeable to  most  persons.  Bitter  is  a  distinctive  species  of  the  qual- 
ity of  gustatory  sensations;  but  the  pleasure  which  some  persons 
have  in  greater  or  less  degrees  of  it  is,  as  a  rule,  acquired.  It  is 
true  that  some  substances,  whose  odor  in  large  quantity  is  disagreea- 
ble, become  tolerable,  or  even  pleasant,  when  the  smell  from  them 
is  faint.  But  this  faint  smell  is  not  the  same,  but  a  distinctly  dif- 
ferent quality;  oftentimes  it  could  not  be  immediately  recognized  as 
coming  from  the  same  substance  as  that  which  emitted  the  strong 
odor.  Discordant  sounds  are,  in  all  degrees  of  intensity,  naturally 
unpleasant;  and  so  most  witnesses  would  pronounce  certain  com- 
plex sensations  of  the  skin  (as  of  creeping,  prickling,  etc.). 

§  18.  The  character  of  the  disagreeable  or  painful  feeling  belong- 
ing to  different  classes  of  sensations  also  differs  with  respect  to  the 
nature  of  its  attachment  to  a  recognized  physical  basis.  Inhar- 
monious colors  produce  in  us  a  feeling  of  mild  dissatisfaction,  which 
appears  as  almost  wholly  of  a  spiritual  kind.  Discordant  tones 
cause  more  of  physical  suffering;  and  disagreeable  smells  or  tastes 
create  a  wide-spread  sense  of  organic  discomfort.  Pains  in  the 
skin  and  interior  organs,  however,  may  take  a  character  of  intense 


MIXTURES  OF  FEELING  521 

bodily  anguish,  which  is  distinctive  of  no  other  qualities  of  sensation, 
and  which  is  capable  of  submerging  all  sensation,  as  such,  in  a  flood 
of  painful  feeling. 

The  tone  of  sensuous  feeling  is  also  dependent  upon  the  total 
condition  of  consciousness  as  determined  by  attention,  mental  habit, 
association  of  the  feelings  among  themselves  and  with  the  ideas, 
control  of  the  will,  etc.  Such  feeling  is,  therefore,  largely  a  second- 
ary element  of  experience,  which  arises  through  certain  acquired 
effects  of  the  sensations  as  connected  with  previous  activities  of 
the  mind.  But  concerning  the  physical  basis  of  the  feelings,  in 
this  aspect  of  them,  we  have  scanty  scientific  knowledge;  and  the 
subject  is  not  as  yet  one  with  which  physiological  psychology  can 
very  successfully  deal. 

§  19.  Characteristic  mixtures  of  feeling — some  of  them  scarcely 
describable — seem  to  be  attached  inseparably  to  different  kinds  of 
sensations.  This  is  obvious  when  we  consider  the  marked  differ- 
ence in  the  way  we  are  affected  by  major  and  minor  chords,  by  suc- 
cessive tones  having  different  musical  intervals  (for  example,  the 
diminished  third,  etc.),  and  by  the  characteristic  clangs  of  different 
musical  instruments.  Writers  upon  this  part  of  musical  theory 
may  disagree  as  to  the  precise  significance  of  the  violin,  clarinet, 
cornet,  or  hautboy,  with  respect  to  the  tone  of  feeling  belonging  to 
each;  but  they  can  scarcely  deny  the  fact  of  a  marked  difference. 
Goethe1  called  attention  to  the  change  in  spiritual  tone,  as  it  were, 
which  harmonizes  with  what  the  eye  sees  when  looking  upon  the 
world  through  different-colored  glasses.  Here,  again,  the  precise 
equivalent,  or  value,  in  terms  of  feeling,  which  the  different  color- 
tones  possess,  may  be  a  matter  of  dispute;  but  the  fact  that  the 
tones  of  feeling  change  with  the  color-tones  is  beyond  dispute. 
That  feelings  of  soberness  or  gloom  go  with  black,  of  excitement 
with  red,  of  cheerfulness  with  light  green,  of  cool  quiet  with  dark 
blue,  of  intense  sensuous  pleasure  with  saturated  purple,  would 
probably  be  admitted  by  most  persons.  Fewer  would  agree  to  de- 
scribing the  tone  of  feeling  belonging  to  dark  yellow  or  spectral 
orange  as  one  of  "suppressed  excitement,"  or  to  brown  as  one  of 
"perfectly  neutral  mood." 

§  20.  Nearly,  if  not  quite,  all  the  various  forms  of  feeling,  when 
they  become  stronger  and  the  disturbances  of  their  organic  basis, 
so  to  say,  more  powerfully  show  themselves  in  consciousness,  par- 
take of  certain  common  characteristics  which  lead  to  their  being 
classified  as  "emotions."  But,  again,  they  are  subdivided  in  an  in- 
definite way,  according  to  the  relation  they  sustain  to  their  objects, 
or  to  the  actions  which  they  tend  to  induce  or  to  influence.  In 

1  Farbenlehre,  §  763. 


522    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

this  way,  such  different  words  as  "affection,"  "passion,"  "impulse," 
and  "desire"  (the  former  two  emphasizing  rather  the  passive,  and 
the  latter  two,  the  active  aspect  of  the  affective  condition)  may  be 
applied  to  essentially  the  same  class  of  feelings.  A  sufficient  in- 
crease in  their  intensity,  and  a  resulting  increase  of  the  "somatic 
resonance,"  may  even  convert  the  higher,  and  usually  milder,  in- 
tellectual, aesthetic,  and  moral  feelings  into  an  emotional  experi- 
ence. In  truth,  essentially  the  same  mental  state,  so  far  as  dis- 
tinctions of  affective  quality  are  concerned,  may  be  called  simply 
a  feeling,  or  an  emotion,  or  a  passion,  or  a  sentiment.  For  example, 
love,  whether  sexual  or  so-called  Platonic,  or  that  distinctive  of  any 
of  the  various  social  relations,  may  on  different  occasions  merit  the 
distinctions  involved  in  any  of  these  terms.  Owing  to  this  general 
fact,  the  physiology  of  the  various  more  complex  forms  of  emotion 
may  be  studied  from  the  same  points  of  view. 

§  21.  All  emotional  forms  of  feeling  are  accompanied  by  abrupt 
and  marked  changes  in  the  character  and  time-course  of  the  mental 
train.  Such  changes  may  be  regarded  as  standing  in  the  relation 
both  of  cause  and  of  effect  to  these  feelings.  Some  impression 
with  which  strong  feeling  has  become  associated  is  made  upon  the 
mind;  the  result  is  a  transitory  interruption  of  the  mental  equi- 
poise. This  constitutes  in  part  the  justification  for  the  saying  that 
from  mere  feeling  to  affection  is  a  "leap." 1  As  a  rule,  the  effect  of 
any  sudden  and  surprising  impression — perception  of  some  object 
of  sense,  or  remembered  image — is  to  start  the  flow  of  emotion. 
Thus  anger,  fear,  desire,  avarice,  take  men  "off  their  guard";  the 
feelings  of  such  kind  that  are  started  by  a  given  mental  impression 
themselves  produce  a  confusion  of  the  mental  train.  But,  on  the 
other  hand,  this  very  disturbance  of  the  mental  train  is  itself  pro- 
ductive of  a  new  phase  of  feeling,  such  as  is  associated  with  the 
particular  ideas  that  in  confused  and  hurried  throngs  rush  into 
consciousness,  as  well  as  with  the  general  state  of  consciousness 
considered  as  one  of  haste  and  confusion.  The  physical  basis  of 
this  state  is  laid  in  the  extraordinary  condition  of  excitation  that 
exists  within  the  central  organs — the  ideo-  and  sensory-motor  cen- 
tres of  the  cerebral  hemispheres. 

A  not  unreasonable  conjecture  as  to  the  central  conditions  on 
which  the  excitement  of  the  more  complex  forms  of  emotion  are  de- 
pendent, would  state  the  case  in  somewhat  the  following  way: 
It  is  a  well-known  fact  that  different  individuals  differ  more  widely 
and  incalculably  as  to  the  particular  feelings  evoked,  on  different 
particular  occasions,  than  as  to  the  sensations  and  ideas  oc- 
casioned by  changes  in  the  amounts,  kinds,  and  time-rates  of  the 
1  Compare  Nahlowsky,  Das  Gefuhlsleben,  etc.,  Einleitung. 


NATURE  OF  THE  EMOTIONS  523 

stimuli  which  act  upon  the  peripheral  nervous  system.  This  fact 
suggests  that  our  feelings  are  determined  by  the  changeable  relations 
of  the  neural  processes  to  the  constitution,  previous  habits  and 
temporary  mood  of  the  nervous  system,  and  by  the  relations  of 
each  neural  process  to  all  the  others  within  the  central  system,  in 
a  more  irregular  way  than  are  our  sensations  and  our  knowledge. 
Those  conditions  of  the  nervous  processes  which  depend  immediately 
upon  the  quality,  intensity,  and  time-rate  of  the  stimuli  that  act 
upon  the  end-organs  of  sense,  are  in  general  conformable  to  law; 
they  are  regular  and — as  it  were — to  be  depended  upon.  In  cor- 
respondence with  them  are  the  regularity  and  the  dependableness 
of  our  sensations  and  of  our  knowledge  by  the  senses.  But  over 
and  above  the  more  uniformly  recurrent  similar  elements  in  all  the 
peripherally  originated  nervous  processes,  there  is  more  or  less  of 
a  " semi-chaotic  surplus"  of  nervous  action  occasioned  in  the  brain 
centres.  In  this  semi-chaotic  surplus — the  general  character  of 
which  depends  upon  what  the  whole  nervous  system  was,  and  is, 
and  has  recently  been  doing,  and  upon  how  the  various  new  stimula- 
tions, running  in  to  the  brain  centres,  fit  in  with  all  this  and  with 
one  another — may  we  find  the  physiological  conditions  of  the  emo- 
tions. No  wonder,  then,  that  these  conditions  are  so  indeterminate 
for  different  individuals,  and  so  changeable  in  the  same  individual. 
At  any  particular  moment  the  kind  and  amount  of  feeling  experi- 
enced has  for  its  physiological  condition  the  total  complex  relation 
in  which  all  the  subordinate  neural  processes,  set  up  by  the  stimuli 
of  that  moment,  stand  to  one  another  and  to  the  set,  or  direction, 
of  pre-existing  related  neural  processes.1 

§  22.  Besides  the  physiological  changes  of  central  origin,  which 
accompany  or  follow  certain  perceptions  and  trains  of  ideas,  the 
wonderful,  characteristic  effect  which  these  forms  of  feeling  pro- 
duce upon  certain  of  the  vital  organs  is  the  most  noteworthy  pe- 
culiarity of  all  affections,  emotions,  and  passions.  Upon  this  point 
science  has  far  less  than  we  could  wish  of  information  reaching 
beyond  the  observations  of  ordinary  experience.  Of  such  informa- 
tion, perhaps  the  most  important  concerns  the  influence  exerted 
through  many  groups  of  muscles,  from  the  central  organs,  upon  the 
vaso-motor  system.  The  effect  of  shame,  fear,  or  anger,  for  ex- 
ample, upon  the  circulation  of  the  blood  is  matter  of  common  re- 
mark. But  some  grow  pale  and  others  red,  when  angry.  As  long 
ago  as  1854,  R.  Wagner  investigated  the  effect  of  fear  upon  the  heart 
of  a  rabbit.  A  blow  on  the  table  near  the  animal  was  found  to 
cause  its  heart  to  stand  still  a  short  time,  and  then  resume  beating 

1  See  Ladd,  Psychology,  Descriptive  and  Explanatory,  1894,  p.  173;  and  on  the 
whole  subject  of  feeling,  ibid.,  chapters  IX,  X,  XXIII,  XXIV,  and  XXV. 


524    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

with  accelerated  frequency  of  stroke.  Subsequent  investigations 
have  made  obvious  the  general  effect  of  emotion  upon  the  curve  in- 
dicating the  blood  pressure.  The  effect  produced  upon  the  pulse 
of  a  dog  by  hearing  the  sudden  cry  of  another  dog  depends  for  its 
character  upon  whether  the  vagus  nerves  are  cut  or  not;  but  even 
after  their  severance  a  marked  effect  of  this  kind  is  still  manifest.1 
The  great  influence  of  these  forms  of  feeling  upon  all  the  action  of 
the  capillary  vessels,  upon  the  secretions,  etc.,  and  upon  the  respira- 
tion to  retard,  or  accelerate,  or  make  it  irregular,  is  of  the  same 
order.  That  care  and  anxiety  disturb  nutrition,  that  pain  and  sor- 
row cause  the  tears  to  flow,  that  fear  and  love  and  anger  act  upon 
the  abdominal  organs,  is  generally  recognized.  The  effect  is  some- 
times seen  in  suddenly  innervating,  and  sometimes  in  depressing, 
one  or  more  of  the  bodily  organs;  or  in  both  innervating  and  then 
depressing  them,  in  certain  well-recognized  cases.  On  the  basis  of 
such  facts,  Kant  suggested  a  division  of  the  affections  into  "sthenic" 
and  "asthenic."  But  many  forms  of  feeling,  as  they  run  their 
course,  become  by  turns  sthenic  and  asthenic.  Strong  emotions  or 
passions  of  all  kinds  tend  to  destroy  the  nervous  mechanism;  "the 
sthenic  kill  by  apoplexy,  the  asthenic  by  laming  the  heart."  Un- 
usual tension  or  relaxation  of  certain  groups  of  muscles  character- 
izes all  these  forms  of  feeling. 

§  23.  The  marked  effect  which  certain  feelings  have  upon  par- 
ticular organs  of  the  body  is  complemented  by  the  fact  that  such  or- 
ganic effect  has  in  turn  a  marked  effect  upon  the  feelings.  The  or- 
ganic disturbances  advance  step  by  step  to  form  the  physical  basis 
of  a  rising  tide  of  emotion,  and  then  fall  off  with  equal  pace  as  the 
tide  of  emotion  subsides.  The  organic  changes  are  not  merely  an 
expression  of  the  mental;  they  are  its  material  cause  and  support. 
Hence,  from  the  physiological  point  of  view  every  strong  emotion 
must  be  regarded  as  involving  certain  factors  of  a  peripheral  origin; 
and  from  the  introspective  point  of  view,  as  including  factors  which 
should  be  analyzed  into  vaguely  (or  not  at  all)  localized  sensations 
having  a  somewhat  marked  tone  of  pleasant  or  unpleasant  feeling. 
This  general  truth  is  well  expressed  by  Sherrington2  as  follows : 
"Of  points  where  physiology  and  psychology  touch,  the  place  of 
one  lies  at  'emotion/  Built  upon  sense-feeling  much  as  cognition 
is  built  upon  sense-perception,  emotion  may  be  regarded  almost  as 
a  feeling — a  feeling  excited,  not  by  a  simple  little-elaborated  sensa- 
sation,  but  by  a  group  or  train  of  ideas.  To  such  compound  ideas 

lfThis  subject  has  been  investigated  by  Conty  and  Charpentier,  by  Cyon, 
Heidenhain,  and  others;  compare  Exner,  in  Hermann's  Handb.  d.  Physiol.,  II, 
ii,  pp.  289  f. 

2  The  Integrative  Action  of  the  Nervous  System,  1906,  p.  256. 


PERIPHERAL  THEORY  OF  EMOTIONS  525 

it  holds  relation  much  as  does  feeling  to  certain  species  of  simple 
sense-perceptions.  It  has  a  special  physiological  interest  in  that 
certain  visceral  reactions  are  peculiarly  colligate  with  it.  Heart, 
blood-vessels,  respiratory  muscles,  and  secretory  glands  take  special 
and  characteristic  part  in  the  various  emotions.  These  viscera, 
though  otherwise  remote  from  the  general  play  of  psychical  process, 
are  affected  vividly  by  the  emotional."  Some  of  these  movements  and 
secretions  are  visible  from  the  outside;  others,  however,  are  hidden 
from  view.  Under  the  latter  class  may  be  mentioned  the  inhibition 
of  the  wave-like  movements  of  the  stomach  in  anger,  as  observed  by 
Cannon,1  by  means  of  the  Rontgen  rays,  in  a  cat.  It  is  this  "so- 
matic reaction,"  or  "bodily  resonance,"  which  largely  gives  to  all 
the  various  emotions  their  peculiar  tinge,  or  characteristic  coloring, 
as  they  arise  in  consciousness.2 

The  claim,  however,  that  the  emotions  are  simply  the  expressions, 
or  outcome,  in  consciousness  of  peripherally  initiated  sensations, 
or  organic  reactions  of  a  visceral  or  vaso-motor  character,  accom- 
panied by  the  reduction  of  the  cerebral  and  psychical  processes  to 
a  secondary  r61e,  as  this  claim  has  been  put  forth,  with  minor  dif- 
ferences, by  Lange,  James,  and  Sergi,  contradicts  all  the  evidence 
which  we  can  at  present  bring  to  bear  upon  the  case.  It  is  op- 
posed to  the  order  of  dependent  sequence,  so  far  as  it  can  be  deter- 
mined by  introspective  analysis  or  followed  by  experiment;  to  the  gen- 
eral theory  of  brain  action  in  its  relations  to  mental  life;  and  it  seems 
to  have  been  distinctly  discredited  by  experimentation  upon  the 
lower  animals.3  The  latter  kind  of  evidence  leads  Sherrington  to 
pronounce  "untenable"  the  vaso-motor  theory  of  the  production 
of  emotion,  and  as  well  the  view  that  visceral  sensations  or  presenta- 
tions are  necessary  to  emotion. 

On  the  other  hand,  there  is  evidence  that  some  of  the  expressive 
movements  are  as  independent  of  the  cerebrum  on  the  one  side  as 
they  are  of  the  viscera  on  the  other;  for,  as  already  mentioned  (p. 
158),  movements  "expressive"  of  pain,  fear  and  anger  can  be  elic- 
ited from  a  decerebrate  animal,  and  belong  therefore  to  the  class  of 

1  Amer.  Journ.  of  Medical  Sci.,  1909,  CXXXVII,  480. 

2  For  an  analysis  of  the  more  prominent  of  the  emotions,  both  on  the  physi- 
ological and  on  the  introspective  side,  and  some  more  specific  account  of  the 
kinds  of  this  "  bodily  resonance,"  see  Ladd,  Psychology,  Descriptive  and  Explana- 
tory, pp.  538  ff. 

3  Sherrington,  by  "appropriate  spinal  and  vagal  transection,"  removed  from 
dogs,  completely,  all  the  sensations  of  the  viscera  and  of  the  skin  and  muscles 
behind  the  shoulder.     In  the  case  of  an  animal,  "  selected  because  of  markedly 
emotional  temperament,"  the  almost  complete  reduction  of  the  field  of  sensa- 
tion "produced  no  obvious  diminution  of  her  emotional   character."     Ibid., 
pp.  260  f.;  Proceedings  of  the  Royal  Society  of  London,  1900,  LXVI,  390. 


526    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

sub-cortical  reflexes.  Expressions  of  pleasure,  such  as  purring  and 
wagging  the  tail,  are  not  observed  in  a  decerebrate  animal.  A  few 
cases  of  visceral  anaesthesia,  of  which  the  best  is  that  of  d'Allonnes,1 
show,  as  we  should  expect,  a  loss  of  the  affective  tone  of  certain 
emotions,  in  which,  in  the  normal  conditions,  the  visceral  sensations 
play  a  prominent  part.  The  woman  whose  case  is  reported  by 
d'Allonnes,  though  previously  affectionate  and  emotional,  given 
to  worry,  etc.,  became  indifferent  to  her  beloved  family,  and  sought 
admission  to  the  hospital  to  be  cured  of  this  lack  of  power  to  feel 
emotions.  She  still  retained  her  former  habits  and  principles,  and 
even  preferences,  but  these,  she  said,  no  longer  had  any  feeling 
connected  with  them.  Yet  her  external  expressive  movements 
were  retained;  in  particular,  she  wept,  but  without  inner  grief, 
and  she  showed  the  signs  of  anger,  shame,  disgust,  etc.,  while  pro- 
testing that  she  felt  no  emotion.  Similar  indefinite  results  have, 
however,  been  reported  as  sequent  upon  injuries  to  various  portions 
of  the  cerebrum,  where  the  peripheral  organic  functions  remained 
practically  normal. 

Among  the  many  and  often  conflicting  investigations  in  this  really 
difficult  field  may  be  mentioned  a  very  careful  piece  of  work  by 
Shepard,2  in  which  the  previous  results  are  reviewed.  Shepard 
made  a  comparative  study  of  circulatory  changes  in  the  brain  (in 
a  patient  whose  brain  was  exposed)  and  in  the  arm,  and  found  that, 
"in  general,  all  agreeable  or  disagreeable  stimuli,  all  sensory  atten- 
tion or  attention  to  arithmetical  problems,  all  agreeably  exciting 
light  or  music,"  gave  a  decrease  in  the  volume  of  the  hand 
and  an  increase  in  the  volume  of  the  brain.  He  concludes  from 
these  results,  and  from  the  studies  of  circulation  and  breathing 
during  affective  conditions  in  other  cases,  that  a  classification  of 
the  feelings  cannot  be  made  on  the  basis  of  their  bodily  accompani- 
ments; that  there  is  no  opposition  between  the  accompaniments  of 
pleasantness  and  unpleasantness,  nor  any  evidence  for  Wundt's 
three  dimensions  of  feeling.  "In  short,  all  moderate  nervous  ac- 
tivity tends  to  constrict  the  peripheral  vessels  and  to  increase  the 
volume  and  size  of  pulse  in  the  brain.  All  moderate  nervous  ac- 
tivity likewise  increases  the  heart-rate."  Strong  stimuli,  such  as 
arouse  fear,  etc.,  induce  a  more  complex  circulatory  reaction,  be- 
ginning, in  the  brain,  with  an  increase  in  volume,  passing  thence 
to  a  decrease,  and  finally  rising  to  a  large  increase,  which  gradually 
passes  away.  The  mechanism  of  these  changes  in  the  blood  sup- 
plied to  the  brain  is  probably  to  be  sought,  largely,  in  the  vaso-motor 
control  of  the  great  abdominal  veins.  These  investigations,  there- 

1  Revue  philos.,  1905,  LX,  592. 

•  Amer.  Journ.  of  PsychoL,  1906,  XVII,  522. 


THE  INTELLECTUAL  FEELINGS  527 

fore,  strongly  support  the  central  theory  of  feeling  which  we  have 
been  advocating. 

§  24.  The  teleological  value  of  many  of  the  emotions,  especially 
such  as  man  shares  with  all  the  higher  animals,  needs  scarcely  more 
than  mention  in  order  to  be  recognized.  It  is  amply  illustrated  by 
the  observations,  both  of  biology  and  of  psychology,  when  studied 
from  the  comparative  and  evolutionary  points  of  view.  In  general 
these  two  principles  apply  to  a  large  class  of  the  emotions:  (1)  The 
motor  reactions  called  forth  as  a  part  of  the  bodily  resonance  are 
adapted  for  the  defence  and  preservation  of  the  individual;  and  (2) 
by  the  application  of  the  principles  of  imitation  and  sympathy, 
these  same  or  other  reactions  operate  for  the  defence  and  preser- 
vation of  the  species.  Evolutionary  biology  is,  therefore,  justified 
in  considering  "the  bodily  expressions  of  emotion  as  instinctive 
actions  reminiscent  of  ancestral  ways  of  life." 

§  25.  That  there  is  a  considerable  class  of  feelings  which  may  be 
properly  classified  as  "  intellectual,"  no  adequate  analysis  of  the 
extremely  complex  form  of  mental  activity  which  we  call  knowledge, 
or  cognition,  allows  us  for  a  moment  to  doubt.  Indeed,  it  might 
also  be  said  that  the  influence  of  feeling  is  as  obvious,  and  almost  if 
not  quite  as  great,  in  determining  all  our  acts  of  knowledge,  whether 
so-called  presentations  of  sense  or  self-consciousness,  as  are  the  ac- 
tivities of  discrimination,  comparison,  association,  etc.  The  so- 
called  faculties  of  intellect  and  feeling  blend  in  all  cognition,  and  the 
complex  result — the  very  object  of  knowledge — is  determined  by 
both.1 

Without  attempting  the  difficult,  if  not  impossible,  task  of  enumer- 
ating all  the  intellectual  feelings,  we  may  remind  ourselves  of.  the 
changing  affective  tone  of  consciousness  when,  on  comparing  two 
or  more  objects,  as  wholes,  or  qualities  of  objects,  we  pass  from  a  con- 
dition of  doubt  and  uncertainty,  through  varying  shades  of  the  rec- 
ognition of  similarities  and  differences,  into  the  feeling  of  certainty 
and  conviction  which  is  both  the  accompaniment,  and  the  test  in  a 
measure,  of  our  arriving  at  a  completed  act  of  cognition.  Indeed, 
this  conviction  itself  involves  a  sort  of  "belief  in  reality,"  for  which 
we  seem  compelled  to  find  a  place  among  our  most  fundamental 
forms  of  feeling. 

What  can  be  claimed,  or  credibly  conjectured,  as  true  with  regard 
to  the  forms  of  the  functioning  of  the  nervous  mechanism  on  which 
many  of  these  so-called  intellectual  feelings  depend,  or  with  which 
they  are  connected,  has  in  large  measure  been  said  while  treating 

1  For  an  extended  exposition  and  defence  of  this  doctrine  see  Ladd,  Psychology, 
Descriptive  and  Explanatory,  chapter  XXII;  Philosophy  of  Mind,  chapter  III; 
and  Philosophy  of  Knowledge  (passim). 


528    FEELING,  EMOTION,  A^D  EXPRESSIVE  MOVEMENTS 

of  the  sensations,  their  combinations  into  presentations  of  sense, 
and  the  physiological  basis  of  the  simpler  forms  of  feeling.  Some- 
thing will  be  added  in  the  following  chapters  which  deal  with 
the  physiological  psychology  of  attention,  association,  memory,  and 
judgment.  In  all  this  field,  experimental  psychology  has  not  much 
of  clearly  ascertained  scientific  truth,  which  can  be  employed  to 
reveal  the  concomitant,  or  otherwise  connected,  processes  of  the 
nervous  system — especially  of  the  cerebral  areas.  But  it  is  not 
by  any  means  a  wholly  unfounded  conjecture  that  hesitancy,  in- 
hibition, confusion,  opposition,  facilitation,  with  varying  degrees 
of  speed  and  smoothness,  fixed  habit  in  " dynamical  associations" 
within  the  brain,  and  a  number  of  similar  terms,  describe  with  rea- 
sonable appropriateness  those  central  nervous  processes  which  are 
correlated  with  the  mental  processes  that  we  are  accustomed  to 
describe  in  similar  terms. 

One  other  thing  is  worthy  of  notice  in  this  connection.  Some 
of  the  intellectual  feelings,  especially  when  they  take  on  the  emotional 
character,  quite  naturally  lead  to  expressive  motions,  or  tendencies 
to  move,  or  to  hold  in  place,  the  muscular  system,  in  whole  or  in 
part.  All  our  language — such  as  " standing  firm,"  or  "pat,"  "hold- 
ing on"  to  one's  belief,  or  opinion,  etc.,  and  the  whole  practice  of 
gesticulating  and  expressive  posturing — is  significant  of  the  close 
relations  existing  between  certain  intellectual  feelings  and  the  mus- 
cular apparatus.  Here,  too,  we  must  look  for  important  "somatic 
reactions." 

§  26.  The  (Esthetic  feelings  arise  and  develop  chiefly  in  connec- 
tion with  presentations  of  sense,  or  with  the  remembered  or  created 
mental  images  that  represent  objects  of  sense.  In  their  elementary 
form,  therefore,  they  plainly  have  a  physiological  side  which  admits 
of  scientific  treatment — although  they  have  received  such  treatment 
far  less  than  could  be  wished.  Many  interesting  facts  and  certain 
partial  generalizations — having  most  application  to  the  lower 
classes  of  pleasurable  feelings  through  the  organs  of  smell,  taste, 
and  the  skin,  when  viewed  in  the  light  of  the  hypothesis  of  evolu- 
tion— have  been  alleged  by  various  observers.  But  even  the  most 
elementary  aesthetic  feelings  cannot  be  considered  as  on  a  par  with 
the  sensuous  feelings,  or  as  mere  aggregates  of  such  feelings.  The 
tone  of  feeling  which  characterizes  the  sensations  furnishes  a  material, 
as  it  were,  for  genuinely  aesthetic  feeling;  but  the  latter  always  im- 
plies also  the  working  of  certain  intellectual  laws,  and  a  union  of 
the  simple  feelings  of  sensation  under  time-form  and  space-form. 
^Esthetic  feelings,  then,  may  be  said  to  spring,  in  a  measure,  from 
the  manner  of  the  combination  of  sensuous  feelings;  time  and  space 
furnish  the  framework  in  which  they  are  arranged.  Hearing  is 


THE  AESTHETIC  FEELINGS  529 

the  principal  sense  for  combining  sensuous  feelings  so  as  to  produce 
aesthetic  feelings  under  time-form,  and  sight  under  space-form. 
The  development  of  even  the  elementary  but  genuine  (Esthetic  feel- 
ings by  other  senses  than  the  eye  and  ear  is  extremely  limited. 
The  agreeable  and  disagreeable  feelings  which  come  through  sensa- 
tions of  smell,  taste,  and  touch  are  for  the  most  part  sensuous, 
rather  than  strictly  aesthetic. 

Hearing,  as  pre-eminently  the  time-sense,  has  two  forms  of  aes- 
thetic feeling — harmony  and  rhythm.  The  nature  of  the  complex 
sensations  which  produce  the  feeling  of  consonance  and  dissonance 
has  already  been  discussed.  Harmony  is  determined  by  the  co- 
incidence of  certain  partial  tones  belonging  to  different  clangs  si- 
multaneously sounded.  The  feeling  of  harmony  is  colored  by  the 
peculiar  way  in  which  the  combination  of  the  clangs  occurs.  The 
principal  difference  of  this  sort  is  that  which  obtains  between  major 
chords  and  minor  chords;  in  the  former  the  different  clangs  are 
perceived  as  firmly  held  together  by  the  fundamental  clang,  while 
in  the  latter  the  coincident  overtone  performs  the  same  office  less 
obviously.  The  one  is  productive  of  agreeable  aesthetic  feeling 
satisfied;  the  other  of  such  feeling  left  unsatisfied — a  feeling  of 
longing.  When,  then,  the  one  form  of  feeling  becomes  very  in- 
tense, it  may  involve  the  pain  of  over-excitement;  the  other,  when 
intensified,  stirs  a  kind  of  agreeable  pain  of  unrest.  In  musical 
time  it  is  the  periodic  nature  of  the  excitation,  with  a  change  in  the 
individual  presentations  of  sense,  which  produces  the  pleasurable 
aesthetic  feeling. 

Two  or  three  regularly  recurring  impressions,  having  the  same 
or  a  different  content  of  musical  sound,  are  combined  into  a  series; 
certain  members  among  the  whole  number  are  then  accentuated, 
in  order  to  form  the  different  series  that  constitute  the  various  kinds 
of  musical  time.  All  musical  time,  fundamentally  considered  as 
respects  its  rhythm,  is  either  two-time  or  three-time.  The  differ- 
ence in  the  feelings  which  respond  to  these  two  classes  of  musical 
rhythm  is  obvious  in  a  pronounced  form,  in  the  funeral  march,  on 
the  one  hand,  and  the  waltz,  on  the  other.  In  general,  it  is  the 
harmony  of  music  which  gives  direction  to  its  feeling,  and  the 
rhythm  which  determines  the  rise  and  fall  of  feeling.  Thus  waves 
of  different  kinds  of  feeling  are  made  by  music  to  pass  over  the 
soul. 

§  27.  Even  less  than  is  the  case  with  the  intellectual  feelings, 
does  experimental  psychology  serve  to  reveal  the  exact  nature  of 
those  physiological  processes  which  are  connected  with  the  more 
complex  aesthetic  feelings  that  greet  the  art  of  music.  Where  these 
feelings  rise  to  an  emotional  character,  the  effect  of  the  "somatic 


530    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

reactions,"  and  the  influence  of  the  associated  ideas  and  tenden- 
cies to  action  (as  to  dance,  to  fight,  to  embrace,  to  march)  become 
more  clearly  distinctive  and  powerful.  The  feelings  with  which  we 
appreciate — whether  favorably  or  unfavorably — those  peculiar  suc- 
cessions of  tones  which  are  required  by  melodies  written  in  the  various 
keys,  or  the  tone-color  of  the  various  instruments  in  an  orchestra, 
are  more  subtle  and  as  yet  quite  indeterminate  as  to  their  essen- 
tial content,  not  to  say  their  physiological  correlates.  Those  feel- 
ings of  right  "relationship"  which  have  come  to  set  the  laws  of  the 
succession  of  tones  in  the  affectively  best  melodies  of  modern  music 
are  of  special  interest  to  the  investigator.  But  beyond  the  mere  fact 
that  some  successions  excite  feelings  of  pleasurable  satisfaction, 
and  "finality,"  or  feelings  of  recognition  of  the  series  as  a  unity, 
while  others  do  not,  little  is  known  at  present  about  the  psychol- 
ogy of  musical  melody.  The  cause  of  this  feeling  of  finality  has 
been  described  as  a  "balanced  muscular  resolution."1 

The  elementary  aesthetic  feelings  which  come  through  sight  lead 
to  the  consideration  of  the  aesthetic  effect  of  visual  form.  Such 
effect  can  be  considered  only  very  imperfectly  from  the  physiologi- 
cal point  of  view.  In  one  important  particular,  however,  pleasura- 
ble aesthetic  feeling  is  directly  dependent  upon  the  combination  of 
the  sensations,  with  their  accompanying  tone  of  feeling,  under  the 
laws  of  the  mechanism  of  vision  with  both  eyes  in  motion.  Beau- 
tiful form  is  determined  by  the  course  of  the  limiting  lines;  and  limit- 
ing lines,  in  order  to  have  the  effect  of  arousing  agreeable  sesthetic 
feeling,  must  accommodate  themselves  to  the  physiological  and  psy- 
cho-physical necessities  of  the  eye  when  in  motion.  These  neces- 
sities thus  determine  both  the  direction  and  the  extent  of  the  limit- 
ing lines.  Lines  of  slight  curvature,  not  too  far  continued  in  one 
direction,  best  comply  with  such  necessities.  Lines  of  very  sharp 
curvature,  or  lines  continued  too  long  in  one  direction,  do  not  pro- 
duce a  pleasing  sesthetic  effect.  So  also  must  the  main  lines  of  a 
building  lie  in  horizontal  or  vertical  directions,  preferably  in  the 
former  direction.  But  long  oblique  lines — for  example,  from  a 
lower  right-hand  to  an  upper  left-hand  corner  of  a  building — are 
scarcely  tolerable.  The  ease  with  which  the  eye  moves,  by  jerks 
(see  p.  459),  along  the  lines,  in  order  to  make  that  synthesis  of  suc- 
cessive similar  presentations  of  sense  in  which  every  perception  of 
a  line  consists,  is  plainly  a  determining  factor  in  all  these  cases. 

The  aesthetic  effect  of  visual  form  is  also  determined  by  the  way 
in  which  the  form  is  constructed,  through  repeating  similar  or  un- 
like simple  shapes  and  combining  them  into  a  totality.  By  this 

1  Studies  in  Melody,  by  W.  Van  Dyke  Bingham.  Monograph  Supplements  of 
the  Psych.  Rev.,  1910,  vol.  XII,  No.  3. 


NATURE  OF  THE  SENTIMENTS  531 

means  a  feeling  of  pleasure  akin  to  the  feeling  of  musical  rhythm 
is  excited  by  the  successive  impressions  which  occur  periodically  as 
the  eye,  with  a  nearly  uniform  movement,  sweeps  the  entire  field. 
In  horizontal  directions,  the  law  for  the  arrangement  of  the  parts  is 
that  of  symmetry  of  the  simple  parts;  in  vertical,  rather  the  law 
of  asymmetry.  Certain  proportions  between  the  connected  parts, 
and  between  the  whole  and  the  parts,  are  favorable  to  the  develop- 
ment of  aesthetic  feeling.  Ease  of  the  mental  apprehension  with 
which  the  relations  in  proportion  of  the  different  parts  are  presented 
is  favorable  to  agreeable  aesthetic  feeling. 

§  28.  It  appears,  then,  that  the  varied  aesthetic  feelings,  pleas- 
ant and  unpleasant,  which  are  dependent  upon  visual  sensations 
and  perceptions,  are  of  all  others  most  closely  connected,  in  respect 
of  their  facts  and  laws,  with  the  so-called  "intellectual"  side  of 
our  objective  experience.  Conditions  which  determine  our  ap- 
perception of  space-relations,  as  well  as  the  harmony  and  contrast 
of  colors,  varied  and  often  obscure  associations,  and  even  heredi- 
tary factors,  are  prominent  in  the  causation  of  the  various  shades 
of  aesthetic  feeling  of  visual  objects. 

In  conclusion  of  this  subject  it  is  worth  while  to  notice  that  the 
emotions  aroused  by  the  markedly  different  classes  of  beautiful 
objects  differ  in  a  marked  way  among  themselves,  in  respect  to 
their  " somatic  reactions"  and  to  the  resulting  affective  tone  of 
consciousness.  Thus,  for  example,  our  appreciation  of  what  we 
consider  sublime  is  distinctly  unlike,  and  in  some  respects  the 
opposite  of,  our  appreciation  of  what  is  beautifully  delicate,  the 
handsome,  or  pretty,  so  called.  So,  too,  the  mingling  of  sensations 
and  feelings  and  ideas  with  which  we  greet  objects  that  have  the 
beauty  of  order  and  proportion  (a  Greek  temple,  for  example)  is 
very  different  from  that  with  which  we  appreciate  the  beauty  of 
luxuriance  and  wildness  (a  tropical  forest,  for  example).  Intro- 
spective analysis  of  the  elements  of  these  complex  aesthetic  emotions 
is  relatively  easy  as  compared  with  the  experimental  analysis  of  the 
simpler  forms  of  feeling,  under  whatever  class  we  may  be  inclined 
to  place  them.1 

§  29.  Not  only  do  the  emotions  involve  elements  derived  from 
the  changes  initiated  in  the  musculature,  but  they  all  tend  to  ex- 
press themselves  in  the  muscles  of  the  limbs  and  trunk,  head,  eyes, 
and  vocal  apparatus.  We  are  probably  safe  in  saying  that  strained 
attention  is  associated  with  a  tense  and  rigid  condition  of  the  mus- 
cles, and  that  relief  brings  relaxation  and  freer  movements.  Excite- 
ment seems  to  be  expressed  by  much  muscular  activity,  and  calm 
and  depression  by  muscular  inactivity. 

1  Compare  Ladd,  Knowledge,  Life,  and  Reality,  1909,  chaps.  XVII-XIX. 


532    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

In  regard  to  pleasantness  and  unpleasantness,  considerable  in- 
terest has  been  aroused  over  the  assertion1  that  pleasantness  is 
associated  with  expansive  movements  of  the  organism,  and  un- 
pleasantness with  contractive  movements.  The  probability  is  that 
there  is  no  such  clean-cut  distinction  between  the  expressive  value 
of  these  two  classes  of  muscles,  in  so  highly  organized  and  so  special- 
ized a  motor  apparatus  as  that  of  man.  Both  flexors  and  exten- 
sors co-operate  in  the  production  of  movements  which  may  be,  as 
wholes,  reactions  to  beneficial  or  harmful  stimuli,  and  so  expressive 
of  pleasant  or  unpleasant  feelings.  Although,  for  example,  the 
primary  reaction  to  a  painful  stimulus  is  the  " flexion  reflex"  (com- 
pare p.  153),  this  changes,  when  the  stimulus  is  intense  or  prolonged, 
to  a  movement  of  flight,  in  which  the  extensors  take  the  leading  part. 
The  rapid  alternation,  in  most  active  movements,  of  extensions  and 
flexions,  also  makes  it  difficult  to  believe  that  either  is  specifically 
related  to  a  certain  tone  of  feeling.  On  the  whole,  then,  there  is 
little  evidence  of  a  specific  bodily  expression  for  pleasantness  or 
unpleasantness — one,  that  is,  which  is  expressive  alike  of  all  con- 
ditions into  which  these  feelings  enter. 

§  30.  There  is,  however,  good  evidence  of  an  exciting  or  depress- 
ing effect  on  the  bodily  activities  of  many  stimuli  and  of  many  states 
of  mind.  These  effects  have  sometimes  been  called  dynamogenic. 
They  are  related  to  the  reinforcement  and  inhibition  which  are  seen 
in  reflex  action.  A  good  index  of  the  excited  or  depressed  condition 
of  the  reflex  mechanisms  of  the  nerve-centres,  and  especially  of  the 
spinal  cord,  is  afforded  by  the  knee  jerk,  or  smart  kick  of  the  foot 
produced  by  the  extensor  muscle  of  the  thigh,  when  its  tendon, 
passing  over  the  knee,  is  struck.  Though  this  movement  is  en- 
tirely involuntary,  it  is  by  no  means  removed  from  the  influence  of 
mental  conditions.  For  example,  anxious  attention  to  the  knee  and 
foot  inhibits  the  reflex,  so  that  the  physician,  to  whom  the  knee  jerk 
is  often  a  symptom  of  importance,  must  needs  distract  the  attention 
of  his  patient  from  the  knee.  This  he  usually  does  by  requiring  the 
patient  to  grip  or  pull  vigorously  with  the  hands  at  the  moment  when 
the  blow  on  the  tendon  is  to  be  struck;  under  these  circumstances  the 
knee  jerk  is  especially  strong.  The  explanation  of  this  result  in 
terms  of  distraction — as  if  the  only  reaction  of  mental  activity  to 
the  reflex  were  one  of  inhibition,  which  must  be  put  aside  by  dis- 
traction— is  not  complete.  Lombard 2  found  that  reinforcement  or 
inhibition  of  the  knee  jerk  could  result  from  many  stimuli  and  mental 
influences,  even  though  the  subject,  from  long-continued  familiar- 
ity with  the  experiment,  required  no  distraction  to  direct  his  mind 

1  See  Munsterberg,  Beitrdge  zur  exp.  PsychoL,  1892,  IV,  216. 
1  Amer.  Journ.  of  PsychoL,  1887, 1, 1. 


DYNAMOGENIC  EFFECTS  OF  THE  EMOTIONS    533 

away  from  his  knee.  Martial  music  increased  the  reflex,  whereas 
quiet  though  interesting  music  had  the  opposite  effect.  An  excit- 
ing noise,  such  as  the  cry  of  an  infant,  increased  it,  whereas  common- 
place and  insignificant  noises,  such  as  the  rattle  of  vehicles  in  the 
street,  were  without  effect.  In  these  last  examples  it  is  evidently 
not  the  mere  sensory  effect  of  the  stimulus,  but  rather  its  meaning 
for  the  individual,  which  exercises  the  exciting  or  depressing  effect 
on  the  spinal  cord. 

One  salient  fact  regarding  the  relation  of  consciousness  to  move- 
ment is,  accordingly,  that  some  conscious  processes  exalt,  while 
others  depress,  the  activity  of  the  reflex  centres  and  through  them 
of  the  muscles. 

Similar  effects  were  observed  by  Fere1  in  the  case  of  voluntary 
movements.  He  required  his  subjects  to  exert  their  utmost  force 
in  squeezing  a  dynamometer  in  the  hand,  and  found  that  this  "  ut- 
most "  could  be  increased  by  sensory  stimuli,  and  that  different 
stimuli  possessed  different  degrees  of  this  dynamogenic  influence. 
Light  reinforced  the  action  of  the  hand  muscles,  darkness  depressed 
it.  The  colors  were  still  more  powerful  and  all  dynamogenic;  but 
red  had  the  strongest  effect.  Of  tastes,  bitter  had  a  strong  dyna- 
mogenic effect,  and  sweet  a  weak  effect  in  the  same  direction;  of 
odors,  the  sharp  and  penetrating  reinforced  the  pressure  of  the  hand, 
and  the  heavy  odors  had  the  opposite  effect — and  this  without  much 
regard  to  the  pleasantness  or  unpleasantness  of  the  odor.  It  should 
be  said  that  these  effects  are  not  strongly  marked  except  in  suggesti- 
ble subjects;  the  average,  "normal"  individual  is  scarcely  influenced 
at  all  by  colors,  for  example.  But  there  are  certain  more  complex 
and,  as  one  may  say,  more  mental  influences  which  do  have  a  strong 
influence  on  the  motor  power  of  the  normal  individual.  Chief 
among  such  influences  is  perhaps  that  of  competition  between  in- 
dividuals who  alternately  squeeze  the  dynamometer,  or  who  are 
running  a  race.2 

In  fact,  the  supposedly  maximal  voluntary  effort  is,  to  judge  from 
its  muscular  effect,  a  variable  quantity,  subject  to  many  involuntary 
influences;  and  the  experiments  quoted  illustrate  the  complexity  of 
the  mental  influences  which  exalt  or  depress  muscular  activity. 

§  31.  The  relation  of  consciousness  to  movement  cannot  be 
fully  appraised  without  considering  another  class  of  facts.  These 
relate  to  the  so-called  automatic  movements,  or  movements  which, 
in  contrast  to  expressive  and  voluntary  movements,  go  on  without 

1  Sensation  et  mouvement  (Paris,  1887;  2d  ed.,  1900);   Annee  psychologique, 
1900,  VII,  69,  82,  143. 

2  Triplett,  Amer,  Journ,  of  PsychoL,  1898,  IX,  507;  Wright,  Psychol.  Rev.,  1906, 
XIII,  23. 


534    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

consciousness,  or  at  least  without  clear  consciousness.  A  good  ex- 
ample is  afforded  by  any  much-practised  movement,  which  con- 
tinues smoothly  while  the  attention  is  on  something  else.  Another 
example  is  found  in  instinctive  movements,  such  as  breathing,  which 
pursue  their  way,  on  the  whole,  undirected  by  consciousness.  In 
studying  reflex  action  (p.  173),  we  noted  the  solidarity  of  the  nervous 
system,  and  the  mutual  influence  which  was  likely  to  be  exerted 
by  the  activity  of  one  part  on  the  activities  of  any  other  part;  but 
we  also  noticed  that  the  activity  of  one  part  might,  on  occasion,  be 
relatively  independent  of  other  parts,  so  that  two  or  more  non-inter- 
fering activities  might  be  going  on  at  the  same  time  in  different  parts 
of  the  nerve-centres.  Automatic  movement  is  an  illustration  of  this 
relative  independence  of  different  parts  of  the  system.  The  regular 
movements  of  respiration  go  on  without  much  reference  to  what 
is  occurring  in  the  conscious  centres  of  the  cerebrum,  except  in 
cases  of  excitement,  strained  attention,  etc.  In  the  same  way,  the 
activity  of  some  well-trained  part  of  the  cerebrum,  which  presides 
over  a  certain  familiar  performance,  may  go  on  without  interfering 
with  the  activity  of  some  other  part  with  which  the  attentive  con- 
sciousness of  the  moment  is  connected,  and  without  being  interfered 
with  by  it.  This  independence  of  different  parts  is,  however,  only 
relative;  interference  is  likely  to  occur,  and  when  it  does  occur,  we 
have  an  expressive  as  opposed  to  an  automatic  movement.  Those 
parts  of  the  motor  apparatus  which  are,  at  any  moment,  much  in- 
fluenced by  conscious  processes  give  rise  to  expressive  movements; 
those  parts  which  are  little  influenced  give  rise  to  automatic  move- 
ments. Expressive  movements,  in  a  word,  illustrate  the  solidarity  of 
the  nervous  system,  while  automatic  movements  reveal  a  certain 
degree  of  dissociation  within  it. 

In  a  negative  way,  automatic  movements  may  themselves  be  ex- 
pressive; for  they  usually  cease  or  are  somehow  interfered  with  as 
soon  as  the  conscious  process  becomes  very  intense.  Breathing, 
as  has  been  said,  is  likely  to  be  inhibited  during  a  brief  period  of 
intense  mental  activity.  A  man  who  is  automatically  walking 
while  immersed  in  thought  may  sometimes  be  observed  to  stand 
stock-still  when  some  specially  interesting  idea  strikes  him.  Lindley1 
found  two  classes  of  involuntary  movements,  which  might  be  ob- 
served, for  example  in  a  school-room:  one  class  of  movements  ac- 
companied intense  mental  effort,  and  consisted  in  strained  positions 
of  the  members;  the  other  class  appeared  when  the  mind  wandered, 
and  consisted  in  rhythmical  movements.  The  latter,  automatic 
class,  gives  way  to  the  former,  expressive  class,  on  passing  from  mind- 
wandering  to  mental  effort.  There  are,  however,  considerable 
1  Amer.  Journ.  of  PsychoL,  1895,  VII,  491. 


AUTOMATIC  AND  IDEOMOTOR  MOVEMENTS      535 

individual  differences  in  regard  to  automatisms.  Binet1  observed 
such  differences  among  children,  and  Stein2  in  young  adults.  The 
last  author  found  some  individuals  in  whom  intense  mental  appli- 
cation favored  instead  of  inhibited  automatic  movements.  In  that 
peculiarly  unstable  condition  of  the  nervous  system  known  as  hys- 
teria, the  tendency  to  automatisms  and  dissociations  is  at  its  maxi- 
mum, and  some  such  individuals  may  carry  on  complex  acts,  such 
as  writing  a  letter,  while  the  attention  is  absorbed  in  something  quite 
different.3  The  relation  of  consciousness  to  bodily  movement  is 
therefore  partly  expressed  by  saying  that  conscious  processes  exert 
general  dynamogenic  and  depressive  effects  on  the  motor  apparatus, 
and  partly  by  calling  attention  to  the  limitations  of  these  influences. 

But  besides  these  general  influences,  specific  conscious  processes 
have  specific  motor  effects.  Certain  ideas  lead  to  certain  definite 
movements,  with  which  they  have  become  associated  by  past  ex- 
perience. They  may  do  so  either  with  or  without  the  full  consent 
of  the  subject.  When  an  idea  leads  to  its  appropriate  movement 
with  the  full  consent  of  the  subject,  we  call  it  voluntary  movement; 
but  when  the  idea  leads  to  movement,  as  it  always  tends  to,  while  the 
subject's  attention  and  intention  are  elsewhere  directed,  the  move- 
ment is  often  named  ideomotor.  Examples  of  the  last  are  seen  in 
involuntary  whispering  of  what  one  reads  or  thinks,  in  involuntary 
gestures,  and  often,  in  rather  an  amusing  way,  in  the  movements  of 
spectators  at  an  athletic  game  or  an  acrobatic  show,  when  they  are 
much  absorbed  in  the  movements  about  to  be  executed  by  the  per- 
formers, and  unwittingly  execute  such  movements  themselves. 

This  specific  relation  between  particular  ideas  and  particular 
movements  has  often  been  so  conceived  as  to  connect  directly  only 
ideas  of  the  movements  themselves  with  those  movements.  In 
other  words,  it  has  been  assumed  that  any  other  sort  of  idea,  to  issue 
in  movement,  must  first  arouse  an  idea  of  the  movement,  which  in 
turn  would  arouse  the  movement.  Introspective  examination  shows4 
that  in  adults,  at  least,  this  conception  is  wide  of  the  mark,  and  that 
the  most  direct  conscious  antecedent  is  likely  to  be  an  idea  of  any 
sort;  it  is  specially  likely  to  be  an  idea  of  some  end  which  is  to 
be  attained  by  the  movement.  There  is  no  reason  why  the  idea  of 
the  end  should  not  become  directly  associated  with  the  making 
of  a  movement  leading  to  that  end;  and  there  is  good  reason  for 
thinking  that  in  children,  as  well  as  adults,  attention  is  attracted 

1  La  suggestibility  (Paris,  1900),  p.  360. 
tpsychol.  Rev.,  1898,  V,  295. 

a  See  especially  Janet,  L'Automatisme  psychologique  (Paris,  1889),  p.  223. 
*  Woodworth,  "The  Cause  of  a  Voluntary  Movement,"  in  Studies  in  Philoso- 
phy and  Psychology,  pp.  351  f.  (Boston,  1906). 


536    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

mostly  to  the  end,  and  that  ideas  of  the  movement,  in  a  strict  sense, 
have  always  but  a  small  place  in  consciousness.  Indeed,  of  our 
habitual  movements  it  is,  in  general,  difficult  or  impossible  for  us  to 
form  any  clear  idea,  strictly  so  called. 

§  32.  It  remains  to  notice  a  certain  mixture  of  sensations  and 
feelings,  usually  attended  by  a  rather  strong  tone  of  unpleasantness, 
which  may  be  grouped  under  the  term,  "Fatigue."  In  ordinary 
usage,  the  word  fatigue  has  a  somewhat  ill-defined  meaning,  inas- 
much as  it  refers  partly  to  a  certain  feeling  or  mass  of  sensations, 
and  partly  to  a  condition  of  actual  inability  to  perform  a  certain  act, 
as  well  before  as  after  the  onset  of  the  feeling  of  fatigue.  It  is  in 
the  latter  sense  that  the  term  is  employed  by  physiologists,  who  be- 


FIG.  146. — Fatigue  Curve  of   a    Frog's  Muscle.     Each  vertical  line  records  a  contraction, 
aroused  by  an  electrical  stimulus,  and  lifting  a  weight. 

gan  their  study  of  fatigue  with  an  isolated  frog's  muscle.  If  this 
muscle  was  excited  by  an  electric  shock  once  every  two  seconds, 
while  being  protected  from  drying,  the  first  effect  of  the  repeated 
stimulation  was  found  to  be  a  slight  increase  in  the  force  of  muscular 
contraction — that  is,  a  slight  increase  in  the  height  to  which  the 
muscle  raised  the  weight  with  which  it  was  loaded.  This  "  stair- 
case effect,"  or  period  of  "warming-up,"  soon  gave  way,  with 
further  stimulation,  to  a  gradual  decline  in  the  force  of  contraction; 
and  this  decline  might  go  on  to  the  zero-point.  The  course  of 
fatigue,  as  indicated  in  such  an  experiment,  is  called  the  "fatigue 
curve."  If  the  muscle,  instead  of  being  entirely  isolated,  is  left 
with  its  circulation  intact,  the  period  of  gradual  increase  of  force 
is  more  prolonged,  and  the  subsequent  decline  is  less  rapid  (Fig. 
146).  If  the  muscle,  instead  of  being  excited  by  shocks  applied 
directly  to  its  substance,  is  aroused  by  exciting  its  motor  nerve,  es- 
sentially the  same  fatigue  curve  is  obtained.  And  the  result  is 
essentially  the  same  if  the  muscle  is  excited  reflexly,  by  stimuli 
applied  to  some  suitable  sensory  nerve. 

Mosso1  devised  the  ergograph  for  obtaining  a  record  of  the  fa- 
tigue of  voluntary  movement  in  man;  and  many  later  investigators 
have  labored  to  add  improvements  to  the  technique  of  this  experi- 
ment. In  general,  the  curve  of  fatigue  obtained  from  human  muscle 

1  Arch.  f.  (Anat.  and}  PhysioL,  1890,  p.  89;  Arch,  italiennes  de  biol.,  1890, 
XIII,  123. 


PHENOMENA  OF  FATIGUE  537 

under  excitation  by  the  will  is  similar  to  that  above  described  in 
case  of  the  isolated  frog's  muscle.  The  period  of  warming-up  is 
usually  in  evidence,  and  then  there  is  a  gradual  decline.  There  is, 
however,  much  more  variability  between  the  separate  contractions 
than  appears  in  the  isolated  muscles ;  for  at  times  the  attention  given 
to  the  muscular  work  wanes,  and  the  force  of  the  contraction  slightly 
declines,  while  at  other  times  there  is  renewed  effort  resulting  in 
a  " spurt"  or  rise  in  the  curve.  Besides  this,  there  are  differences  be- 
tween individuals,  some  showing  a  very  gradual  onset  of  fatigue, 
whereas  others  maintain  nearly  the  original  strength  for  a  consider- 
able time,  and  then  weaken  suddenly.  Under  the  best  conditions, 
a  well-trained  muscle  will  show  but  a  gradual  decline  in  force,  and 
may  be  able,  after  an  hour  or  two  of  such  work,  lifting  the  weight 
as  high  as  possible  once  every  two  seconds,  to  reach  still  60  to  80 
per  cent,  of  its  original  performance.1  In  fact,  after  an  initial  period 
of  decline,  the  performance  approximates  to  a  level,  known  as  the 
"level  of  fatigue." 

Sensory  fatigue,  apart  from  "  adaptation,"  of  which  more  will  be 
said  later,  is  known  principally  in  the  case  of  the  eye;  it  consists 
partly  of  sensations  of  soreness  and  partly  of  fatigue  of  the  muscles 
of  the  eye.  An  actual  loss  of  the  functional  power  of  the  eye  may 
result  from  continued  work.2 

§  33.  Mental  fatigue,  pure  and  simple,  is  difficult  of  observation, 
because  almost  any  mental  task  which  can  be  measured  and  used 
as  the  basis  of  a  test  requires  the  use  of  the  eyes,  or  of  the  muscles, 
and  so  is  likely  to  involve  muscular  and  sensory  fatigue.  In  case, 
however,  the  demand  on  the  eyes  and  muscles  is  as  moderate  as 
possible,  the  signs  of  fatigue — i.  e.,  of  real  inability  to  perform  work 
— are  slow  in  appearing.  Reaction-time  work  has  been  continued3 
for  15  to  20  hours,  and  memorizing4  for  four  or  five  hours,  with 
little  loss  of  efficiency.  On  the  other  hand,  continued  adding  of 
columns  of  figures  has  shown,  in  some  individuals  at  least,  a  more 
pronounced  loss.  No  doubt  common  experience  would  make  us 
incline  to  believe  in  the  reality  of  mental  fatigue,  and  even  in  a 
rather  rapid  rate  in  its  progress;  but  there  is  this  to  be  said,  that 
mental  work  is  not  ordinarily  done  under  test  conditions,  which 

Breves,  Arch,  ital  de  biol,  1898,. XXIX,  157,  and  XXX,  1;  also  in  Pfliiger's 
Arch.  f.  d.  ges.  PhysioL,  1899,  LXXVIII,  163;  Schenck,  Pfliiger's  Archiv.,  1900, 
LXXXII,  390;  Hough,  Amer.  Journ.  of  PhysioL,  1901,  V,  240;  Woodworth,  Le 
Mouvement  (Paris,  1903),  p.  371,  where  a  re*sume"  of  work  on  motor  fatigue  is 
given. 

2  Scripture  and  von  Tobel,  Studies  from  the  Yale  Psychological  Laboratory, 
1896,  IV,  15;   Moore,  ibid.,  1895,  III,  87. 

3  Cattell,  Wundt's  Philos.  Studien,  1886,  III,  489. 
*  Thorndike,  Psychol.  Rev.,  1900,  VII,  571. 


538    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

would  keep  up  the  incentive  to  do  one's  utmost.  In  ordinary  con- 
ditions, we  yield  more  readily  than  in  an  experiment  to  feelings  of 
ennui  and  weariness  which  do  not  necessarily  indicate  actual  de- 
crease of  power.1 

§  34.  In  considering  the  causes  of  fatigue,  we  may  take  our  start 
again  with  the  muscle.  A  priori,  three  causes  might  be  possible :  the 
structure  of  the  muscle  might  be  partially  broken  down ;  or  the  fuel 
from  the  oxidation  of  which  the  muscle  derives  its  energy  might  be 
limited;  or,  finally,  the  waste  products  of  this  oxidation  might  poison 
the  muscle.  There  is  little  sign  that  the  structure  of  the  muscle  is 
impaired  in  normal  fatigue;  there  is  considerable  probability  that 
the  supply  of  fuel  may  run  low;  or  at  least,  as  suggested  by  Treves, 
that  a  store  of  fuel  laid  up  in  the  muscle  during  its  previous  resting 
condition  may  become  quickly  used  up,  so  that  the  muscle  becomes 
dependent  on  the  supply  brought  to  it  by  the  blood,  and  therefore 
does  not  obtain  its  fuel  as  rapidly  as  at  first.  That  the  muscle  pro- 
duces in  its  activity  "fatigue  substances"  which  partially  poison  or 
depress  it,  is  an  established  fact.  The  muscle,  by  its  activity,  evolves 
carbon  dioxide,  lactic  acid,  and  acid  potassium  phosphate.  Lee  has 
shown2  that  two  at  least  of  these  three  substances,  when  injected 
into  a  fresh  muscle,  cause  in  it  marked  signs  of  fatigue  almost  from 
the  beginning  of  its  activity.  He  has  further  shown3  that  the  first 
effect  of  a  moderate  dose  of  the  same  substances  is  to  produce  the 
staircase  effect;  he  therefore  suggests  that  this  effect  may,  in  the  ac- 
tivity of  a  muscle,  be  the  result  of  the  beginning  accumulation  of 
these  substances. 

§  35.  The  fatigue  of  voluntary  muscular  activity,  depending  as 
this  activity  does  on  the  brain  as  well  as  on  the  muscles,  was  as- 
cribed by  some  of  the  early  experimenters,  on  what  seemed  good 
grounds,  to  the  brain  in  larger  measure  than  to  the  muscle.  More 
recently,  this  view  has  been  subjected  to  severe  criticism.  By  a 
method  similar  to  that  which  was  previously  used  (compare  p. 
136)  in  examining  the  fatigue  of  nerve-fibres,  the  reflex  mechanisms 
of  the  spinal  cord  have  beerrthown4  to  be  highly  resistant  to  fatigue. 
When  a  block  is  interposed  between  the  spinal  cord  and  a  muscle, 
and  a  stimulus  is  applied  to  a  sensory  nerve  which  would,  in  the 
absence  of  the  block,  arouse  a  reflex  contraction  in  the  muscle 

1  Kraepelin  and  his  pupils  have  devoted  much  study  to  the  fatigue  curve  of 
mental  work,  or  to  the  "work  curve,"  and  have  attempted  to  analyze  out  the 
various  factors  which  contribute  to  change  the  working  efficiency  from  moment 
to  moment.  See  the  successive  issues  of  Kraepelin's  Psychologische  Arbeiten. 

*Amer.  Journ.  of  PhysioL,  1907,  XX,  170. 

3  Ibid.,  1907,  XVIII,  267, 

4  Joteyko,  Conies  rendues  de  la  Soc.  de  Biol,  1899,  p.  484. 


THE  KINDS  OF  FATIGUE  539 

every  second  or  two,  the  muscle,  here  serving  as  an  indicator  of  the 
condition  of  the  cord,  responds  well  after  the  removal  of  the  block — 
thus  showing  that  the  cord  was  not  much  fatigued  by  a  long  suc- 
cession of  stimuli  which  had  reached  it  and  caused  it  to  discharge 
down  a  motor  nerve. 

Tests1  of  the  excitability  of  a  muscle  which  had  been  employed 
in  voluntary  contractions  to  the  point  of  fatigue  showed  that  the 
muscle  had  lost  some  of  its  responsiveness,  and  was  therefore  actu- 
ally fatigued.  Further,  the  fact  seems  important  that  fatigue  is 
little  if  any  more  speedy  in  its  onset  in  voluntary  muscular  work 
than  in  the  work  of  a  muscle  excited  directly  or  through  its  nerve. 
If  the  nerve-centres  fatigued  rapidly,  activity  involving  these  centres 
should  show  more  rapid  fatigue  than  similar  activity  with  the  cen- 
tres left  out.  The  slow  rate  of  fatigue  in  purely  mental  work 
should  also  be  recalled  in  this  connection. 

§  36.  The  nature  of  brain  fatigue,  in  so  far  as  it  occurs,  is  still 
unexplained.  In  considering  the  matter,  we  are  invited  to  dis- 
tinguish several  sorts  of  " fatigue"  which  may  be  concerned.  There 
is  first  the  metabolic  fatigue,  such  as  has  been  demonstrated  in  the 
case  of  the  muscle.  Some  indications  of  a  metabolic  change  in 
the  brain  after  very  prolonged  and  usually  excessive  activity  have 
been  obtained  by  histologists,2  but  this  evidence  would,  on  the  whole, 
tend  to  show  that  the  metabolism  of  the  brain  is  slight  in  amount. 

A  second  variety  of  fatigue  may  be  named  the  "  transferred 
metabolic."  This  results  from  the  carrying  of  fatigue  products  by 
the  blood  from  the  muscle  or  other  organ  in  which  they  are  produced 
to  some  other  organ  which  has  been  resting.  The  effect  is  the 
same  as  that  shown  by  the  experiment  in  transfusing  fatigue  prod- 
ucts, mentioned  above.  There  is  little  doubt  of  the  reality  of  this 
form  of  brain  fatigue,  as  is  indicated  by  the  drowsiness  and  inability 
to  perform  mental  work  which  immediately  follows  severe  muscu- 
lar exercise. 

A  third  variety  consists  of  feelings  and  sensations  of  fatigue.  This 
is  to  be  distinguished  from  true  fatigue,  as  urged  above;  but  at 
the  same  time  it  must  be  accounted  a  genuine  inhibiting  influence 
acting  to  cause  the  cessation  of  work,  especially  of  the  particular 
work  which  has  been  going  on.  When  a  small  group  of  muscles 
is  repeatedly  innervated,  pain  may  develop  in  the  muscles  and  their 
tendons,  and  the  natural  reaction  to  this  sensation  is,  either  to  stop 
the  work,  or  at  least  to  substitute  the  action  of  some  other  group  of 

1  Storey,  Amer.  Journ.  of  PhysioL,  1903,  IX,  52. 

2  Hodge,  Amer.  Journ.  of  PsychoL,  1889,  II,  376;  Goldscheider  and  Flateau, 
Fortschritte  d.  Med.,  1897,  XV,  241;  Dejerine,  Soc.  de  Biol,  1897,  p.  728;  Ewing, 
Arch,  of  Neurol.  and  Psychopathol,  1898,  I,  157. 


540    FEELING,  EMOTION,  AND  EXPRESSIVE  MOVEMENTS 

muscles.  This  tendency  to  change  may  be  wellnigh  irresistible; 
its  urgency  may  be  compared  with  that  of  the  impulse  to  breathe 
after  holding  the  breath  for  some  time.  In  mental  work,  there  may 
occur  local  fatigue  pains,  because  of  the  cramped  position  in  which 
the  neck  or  other  parts  are  sometimes  held  during  strained  atten- 
tion; but  besides  these,  there  are  more  diffuse  feelings  of  restlessness 
and  ennui,  the  analysis  of  which  is  difficult,  but  the  tendency  of 
which  is  clearly  toward  a  cessation  of  the  activity  in  progress. 

A  fourth  effect  which  is  sometimes  classed  under  the  head  of 
fatigue  is  sensory  adaptation.  In  the  case  of  the  eye,  fatigue  was 
the  explanation  advanced  by  Helmholtz  for  after-images;  but  since 
there  are  histological  changes  in  the  retina  on  exposure  to  the  dark 
or  to  light,  which  are  certainly  not  akin  to  the  forms  of  fatigue  so  far 
mentioned,  but  constitute,  rather,  a  positively  adaptive  phenomenon, 
it  is  now  customary  to  substitute  "adaptation"  for  " fatigue"  in 
speaking  of  such  changes.  Adaptation  occurs  in  the  other  senses, 
but  its  mechanism  is  unknown;  its  effect  is  to  cause  a  stimulus  to  be 
less  fully  sensed  after  it  has  been  steadily  acting  for  a  time  than  at 
first;  so  far,  then,  the  effect  resembles  that  of  fatigue.  It  is  probable 
that  adaptation  occurs  in  the  central  organs  as  well  as  in  the  sense- 
organs;  growing  so  accustomed  to  a  stimulus  as  not  to  notice  it 
(e.  g.,  the  ticking  of  a  clock)  is  often  not  a  purely  peripheral  phe- 
nomenon, since  the  stimulus  can  be  perceived  at  any  moment  if 
occasion  arises  for  noticing  it. 

A  fifth  variety  of  impairment  of  efficiency  which  bears  a  super- 
ficial resemblance  to  fatigue  is  interference,  and  is  best  known  in 
the  case  of  memory.  When  a  series  of  dissimilar  acts  must  be  per- 
formed in  quick  succession,  one  of  these  is  likely  to  interfere  with 
another.  Inasmuch  as  a  short  series  is  relatively  free  from  inner 
interferences,  whereas  a  long  series  involves  many  such  chances, 
a  short  series  is  apt  to  be  performed  with  better  success  than  a  long 
series.  The  appearance  then  is  that  fatigue  has  influenced  the 
longer  performance.  But  since  even  this  longer  performance  need 
be  only  a  few  seconds  in  length,  and  since  recovery  from  the  condi- 
tion is  prompt,  such  fatigue  can  scarcely  be  identified  with  the  pro- 
gressive and  metabolic  type. 

§  37.  The  "refractory  period"  (compare  p.  131)  is  also  a  phe- 
nomenon related,  at  least  superficially,  to  fatigue,  though  in  dura- 
tion it  lies  at  the  other  extreme  from  the  progressive  type.  It 
seems  to  be  of  the  nature  of  a  back-swing  from  the  condition  of 
activity.  There  is,  apparently,  a  radical  difference  between  meta- 
bolic fatigue,  which  really  lowers  the  power  of  an  organ,  and  the 
inhibitions  which  result  from  the  action  of  certain  stimuli  to  the 
organ.  It  is  probable  that  inhibitions  or  interferences  give  the  key 


THE  REFRACTORY  PERIOD  541 

to  most  appearances  of  intellectual  fatigue.  Yet  the  question  cannot 
be  said  to  be  wholly  cleared  up.  In  certain  forms  of  mental  work, 
such  as  performing  calculations  in  the  head,  or  certain  complex 
forms  of  memory  work,1  there  is  sometimes  a  sudden  drop  from  effi- 
ciency to  complete  inability  to  continue;  in  such  cases,  the  key  to 
the  situation  is  lost.  Introspectively,  this  condition  appears  as  a 
diffusion  of  attention,  or  a  tendency  to  relapse  into  sleep;  but  the 
exact  cause  is  not  made  out. 

Finally,  there  can  be  little  doubt  that  such  complex  intellectual 
and  aesthetic  or  moral  feelings  as  loss  of  interest,  ennui,  the  feeling  of 
being  bored,  desire  to  do  something  else,  distate,  whether  of  a  mild 
order  or  amounting  to  disgust,  and  moral  disapprobation,  have 
much  to  do  with  the  onset  of  fatigue,  in  the  psychological  meaning 
of  the  word.  That  there  is,  in  such  cases,  some  corresponding 
"slowing-up"  or  positive  inhibition  of  the  central  nervous  processes 
required  for  doing  the  work  in  hand,  without  the  feeling  of  fatigue, 
scarcely  admits  of  doubt.  There  would  seem,  then,  to  be  no 
reasonable  objection  to  speaking  of  this  correlated  lowering  of 
energy  in  the  cerebral  centres  as  a  true  case  of  brain  fatigue. 
Work  done  under  such  mental  friction  or  "loading"  of  the  cerebral 
activities  wastes  the  cerebral  stores  of  energy. 

1  Compare  C.  S.  Yoakum,  An  Experimental  Study  of  Fatigue,  Psychological 
Monographs,  No.  XLVI,  1909. 


CHAPTER  VIII 
MEMORY  AND  THE  PROCESS  OF  LEARNING 

§  1.  One  of  the  most  distinguishing  characteristics  of  the  higher 
animals,  and  especially  of  man,  isThe  capacity  of  learning  from  ex- 
perience, or  of  acquiring  modes  of  reaction  adapted  to  the  peculiar 
circumstances  of  individual  life.  It  is  chiefly  this  which  produces 
the  individual  modifications  of  behavior,  as  distinguished  from  the 
so-called  "instinctive"  behavior,  or  behavior  common  to  the  species. 
Each  individual  contracts  habits  ofjiis  own,  acquires  skill,  learns  to 
know  things  and  tojreact  to  them  as  known.  In  this  way,  above  all 
other  animals  does  man  come  to  have  the  course  of  his  thoughts  as 
well  as  of  his  actions  modified  by  previously  formed  associations. 
Physiological  psychology,  however,  can  throw  no  light  upon  the 
philosophical  significance  of  recognitive  memory;1  it  must  content 
itself  with  studying  the  more  obvious  modifications  of  behavior  and 
of  thought  by  experience,  and  with  basing  upon  this  study  certain 
conjectures  concerning  the  part  played  in  the  process  of  learning 
by  the  nervous  mechanism.  The  method  required  by  this  investi- 
gation will  lead  us  to  recite  certain  facts  regarding  both  animal  and 
human  learning  (the  latter,  in  cases  of  acts  of  skill  as  well  as  in 
cases  of  memory  of  facts);  and  then  to  seek  to  discover  any  general 
tendencies,  or  laws  of  association,  which  may  emerge  from  our  wide 
though  somewhat  hasty  survey. 

§  2.  All  memory  involves  two  events,  and  a  condition  persisting 
between  them.  The  first  event  is  the  impression,  or  formation  of  an 
association;  the  persisting  condition  is  the  preservation  of  the  modi- 
fication, or  the  retention  of  the  association;  the  second  event  is  the 
changed  reaction  upon  the  recurrence  of  the  original  situation  or 
of  something  resembling  it.  In  place  of  the  single  first  event,  there 
may  be  a  number  of  events,  all  tending  to  impress  the  same  modi- 
fication. In  a  broad  sense,  we  speak  of  the  series  of  events  which 
impresses  the  modification  as  experience.  We  speak  of  the  same 
series  as  the  act  of  memorizing,  when  there  is  a  definite  intention  of 
recalling  later  what  has  been  learned;  and  as  practice,  or  training, 

1  For  discussions  of  the  philosophical  aspects  of  memory,  see  Ladd,  Psychology, 
Descriptive  and  Explanatory,  pp.  397-407;  and  Philosophy  of  Knowledge,  pp. 
122  f.;  262  f.;  386  f. 

542 


PROCESSES  INVOLVED  IN  MEMORY  543 

when  the  object  in  view  is  the  acquisition  of  some  sort  of  skill.  The 
preservation  of  the  results  of  experience,  or  training,  through  an 
interval  of  time,  may  be  called  retention.  What  is  retained,  how- 
ever, is  not  the  newly  learned  fact  or  action;  for  the  fact  is  not  con- 
sciously present  during  the  interval,  nor  is  the  act  continuously 
performed.  If  we  wish  to  avoid,  for  the  time  being,  any  physio- 
logical hypothesis  as  to  the  nature  of  retention,  we  may,  with  Stout,1 
speak  of  a  disposition  left  behind  by  the  modifying  experience, 
ready  to  give  rise,  on  suitable  stimulation,  to  a  reaction  which  shall 
show  the  influence  of  that  experience. 

The  second  event  referred  to  above,  or  the  reaction  showing  the 
effect  of  past  experience,  is  perhaps  better  covered  by  the  name 
reproduction  than  by  any  other  single  word.  But  its  character  may 
be  so  varied,  as  well  as  so  complex,  that  no  one  word  is  really  ade- 
quate to  designate  it.  In  the  sphere  of  conscious  memory,  it  is 
common  to  distinguish  two  component  parts  of  this  event,  as  the 
recall  and  the  recognition — the  recovery  of  something  from  past  ex- 
perience, and  the  conscious  reference  of  it  to  past  experience.  These 
two  are  often  indistinguishably  blended  in  the  remembering  con- 
sciousness; but  the  distinction  between  them  is  justified  by  frequent 
instances  in  which  either  may  occur  without  the  other.  Thus, 
recognition  without  recall  occurs  when,  for  example,  a  name  re- 
fuses to  come  at  our  bidding,  and  yet  is  recognized  at  once  if 
spoken  by  some  one  else.  Recall  without  recognition  occurs  in 
the  habitual  use  of  familiar  words,  and  in  the  practice  of  familiar 
acts  (for  these  are  not  attended  by  a  conscious  reference  to  the 
past),  as  well  as  in  the  interesting  cases  of  "unconscious  plagiarism," 
where  the  person  believes  himself  to  be  inventing  or  composing 
something  newin  literature  or  art,  though  he  is  really  reproducing 
something  previously  read  or  seen. 

§  3.  Under  the  four  heads  of  impression,  retention,  recall,  and 
recognition  may  be  grouped  the  problems  whicfr*memory  p>re§ents 
for  our  consideration,  and  also  the  many  investigations  which  have 
been  conducted  in  the  hope  of  solving  these  problems.  Wecjnay 
stuclyLthe  procesg  of  jorming  an  association  or  learning  an  act  of 
skill,  and  the  conditions  which  help  or  hinder  this  process;  or  we 
ffiay  study  the  permanence  of  acquired  dispositions,  and  the  causes 
which  lead  to  forgetting;  and,  similarly,  the  processes  of  recall  and 
recognition  may  be  chosen  as  the  subjects  for  examination.  The 
results  of  these  four  studies  cannot  always  be  so  easily  disentangled; 
for  neither  the  formation  nor  the  retention  of  associations  can  be  ex- 
amined except  through  their  effects  as  seen  in  recall  and  recognition; 
and  recall  and  recognition  cannot  properly  be  studied  without  tak- 
1  Analytic  Psychology,  1896,  I,  21  ff. 


544       MEMORY  AND  THE  PROCESS  OF  LEARNING 

ing  account  of  the  conditions  under  which  the  associations  were 
originally  formed  and  have  been  retained.  Neither  is  it  always 
possible  to  analyze  the  results  of  investigation,  so  as  to  assign  its 
part  to  each  of  the  processes  involved  in  the  memory  cycle. 

A  sort  of  justification  of  the  above  four-fold  division  of  memory 
is,  however,  seen  in  pathological  conditions.  Among  the  diseases 
of  memory  are  some  which  affect  the  formation  of  associations, 
and  others  which  affect  each  of  the  other  components.  In  senility, 
for  example,  there  is  a  loss  of  modifiability  or  plasticity,  with  the 
result  that  new  associations  cannot  be  formed.  A  similar  result  ap- 
pears in  the  "polyneuritic  psychosis" — often  a,secjuel  of  alcoholism 
— in  which  recent  events  are  promptly  forgotten,  because  of  their 
having,  apparently,  made  no  impression  on  memory.  As  losses  of 
retention  may  be  mentioned  the  aphasias  and  similar  defects 
(compare  pp.  252  ff.)  when  these  are  due  to  the  destruction  of  some 
part  of  the  brain  substance  which  was  modified  in  the  learning  of 
language,  etc.  Under  the  head  of  loss  of  recall  may  properly  be 
counted  the  amnesias  of  hysteria;  since  here  what  seems  to  be  for- 
gotten is  not  wholly  lost,  but  may  later  be  recalled  under  hypnosis 
or  other  conditions.  Some  of  the  losses  of  memory  resulting  from 
shock  also  belong  here;  since  as  the  shock  passes  away,  memory 
may  return.  Finally,  under  the  head  of  recognition  may  be  classed 
pathological  conditions  of  two  kinds:  in  one,  there  is  a  feeling  of 
strangeness,  which  deprives  things  that  are  really  familiar,  and  that 
can  be  dealt  with  as  such,  of  their  air  of  familiarity;  whereas  in 
the  condition  of  "  f alsejrecognition,"  events  which  are  really  new 
are  felt  to  have  occurred  in  exactly  the  same  way  before. 

§  4.  Without  attempting  to  trace  the  phylogenetic  development 
of  the  power  of  learning  through  the  ascending  scale  of  animals, 
we  shall  first  examine  modifications  of  behavior  as  they  occur  in 
the  lowest  animals;  then  in  the  higher  animals,  and,  finally,  in  man.1 

Among  unicellular  animals,  Amoe6a^~lParameciumf  "and""  Stentor 
are  the  forms  which  have  been  best  studied  in  regard  to  their  be- 
havior.2 Of  these,  the  amoeba  is  already  familiar  to  us  (see  p. 
14);  the  other  two  are  somewhat  more  highly  organized  cells,  be- 
ing provided  with  special  motor  organs  in  the  form  of  hairs  or  cilia 
for  swimming.  Only  slight  and  uncertain  evidence  of  modifia- 
bility has  been  attained  by  observation  of  Amoeba,  but  clearer  signs 
appear  in  the  behavior  of  Stentor.  This  little  animal  is  of  a  trumpet 

1  For  a  presentation  of  the  development  of  modifiability  in  the  animal  series, 
see  M.  F.  Washburn,  The  Animal  Mind  (New  York,  1908);    E.  A.  Kirkpatrick, 
Genetic  Psychology  (New  York,  1909);    H.  P.ie"ron,  L'Evolution  de  la  Memoire 
(Paris,  1910).     The  first  of  these  works  contains  a  valuable  bibliography. 

2  See  especially  Jennings,  Behavior  of  the  Lower  Organisms  (New  York,  1906). 


LEARNING  AMONG  INVERTEBRATES  545 

shape,  and  is  usually  to  be  found  attached  by  its  stalk  to  some  solid 
object  in  the  water.  If,  while  thus  attached,  it  is  affected  by  a  harm- 
less stimulus,  such  as  a  mild  jet  of  water,  it  reacts  at  first  by  bending 
away  from  the  jet;  but  if  the  same  stimulus  is  repeated,  the  reaction 
is  discontinued.  The  animal  has  become  "adapted"  to  the  harm- 
less stimulus.  If,  however,  the  stimulus  is  somewhat  harmful,  as 
a  jet  of  some  weak  chemical  solution,  the  first  reaction  being  the 
same  as  before,  repetition  of  the  stimulus  leads  to  a  more  powerful 
reaction — namely,  the  contraction  of  the  whole  animal;  and  if  the 
stimulus  is  still  repeated,  the  stentor  pulls  loose  from  its  attach- 
ment and  moves  away.  This  is  an  instance  of  the  principle  of 
varied  reaction,  which  Jennings  rightly  emphasizes  as  of  funda- 
menfaTimportange  in  animal  behavior.  The  various  possible  reac- 
tions to  the  same  stimulus  are  not  blended  and  confused,  but  are 
tried  in  turn  until  they  are  proved  to  be  not  efficacious. 

Similar  behavior  has  been  noticed1  in  Paramecium.  When  one 
of  these  boat-shaped  creatures  was  sucked  up  into  a  capillary  tube, 
of  a  diameter  less  than  the  length  of  the  animal,  it  would,  as  usual, 
swim  till  it  encountered  some  obstacle.  This  it  found,  under  these 
conditions,  in  the  surface  between  the  water  and  the  outside  air, 
at  either  end  of  the  capillary  tube.  On  encountering  this  obstacle, 
it  at  first  performed  its  customary  reaction,  by  backing  away,  mak- 
ing a  slight  turn,  and  then  going  forward  again.  This  manoeuvre 
was  repeated  many  times,  but  finally  gave  way  to  a  quite  different 
reaction,  the  creature  doubling  on  itself  in  the  tube,  and  swimming 
off  toward  the  other  end.  Here  the  same  obstacle  was  encountered, 
and  the  same  sequence  of  reactions  made.  After  several  such  ex- 
periences, however,  the  doubling  reaction  occurred  more  promptly, 
till  finally,  in  some  individuals,  it  became  the  first  reaction  to  each 
fresh  encounter  with  the  surface  of  air.  Such  a  changeofjaehav- 
ior  can,  perhaps,  be  explained  by  a  heightened  excitaollity  of  the. 
animal  duetto  the  cons tantlv_recurr ing  obstacles  to  locomotion. 
HeightenSTexcitability  might  favor"the  more  extreme  formlTof  re- 
action; just  as,  in  much  higher  animals,  a  state  of  high  excitability 
may  lead  to  violent  reactions  to  feeble  and  insignificant  stimuli. 
If  this  is  the  explanation,  the  modification  of  behavior  would  not  be 
expected  to  hold  over  a  period  of  rest;  and,  in  fact,  there  is  little 
evidence  of  the  retention  by  protozoa  of  such  modifications  ^as  have 
been  detected  in  their  behavior. 

In  .higher  animals,  however,  a  true  retention,  lasting  from  one 
day  to  an^jiieii^jiasjr^qiieiitly  been  demonstrated.  A  spider,  on 
which  experiments  were  made  by  the  Peckhams,2  made  its  usual 

1  Stevenson  Smith,  Journ,  of  Comp.  Neurol  and  Psychol.,  1908,  XVIII,  505. 

2  Journal  of  Morphology,  1887,  I,  383. 


546      MEMORY  AND  THE  PROCESS  OF  LEARNING 

defensive  reaction  of  dropping  from  its  web  to  the  ground,  when  a 
tuning-fork  was  sounded  near  it,  and  after  it  had  regained  its  web, 
it  repeated  the  same  reaction  on  the  repetition  of  the  same  stimulus; 
but  after  several  repetitions  ceased  to  do  so.  On  the  next  day,  it 
reacted  as  at  first;  but  after  many  days,  it  no  longer  made  this  reac- 
tion to  this  stimulus.  It  had  become  permanently  accustomed  or 
" adapted"  to  the  sound. 

Other  forms  of  modification  may  similarly  be  retained  for  long 
periods  by  the  higher  animals.  Yet^  together  with  Detention,  or  rather, 
as  acting  against  it,  can  be  seen  ^tendency  tojecoyer  from  a  modifi- 
cation,  if  the  stimulus  which  calls  it  forth  is  not  frequently  repeated. 
Iri~~the  protozoa,  recovery  is  prompt  and  complete,  and  there  are 
no  after-effects;  whereas,  in  many  metazoa,  after-effects  remain 
when  the  temporary  effects  of  adaptation,  fatigue,  and  heightened 
excitability  have  passed  away.  But  even  these  after-effects  be- 
come weakened  by  time,  provided  the  stimulus  which  produced 
them  is  not  repeated.  This  recovery  and  forgetting  are  clearly 
not  without  utility;  for  the  original,  unmodified  behavior  may  be, 
in  general,  a  better  starting-point  for  meeting  any  novel  situation. 

Still  another  form  of  moclification,  sometimes  dignified  by  the 
name  of  "associative  memory,"  has  been  observe!  in  some  in- 
vertebrates. It  Consists"  in  connecting  a  particular  reaction  with 
a  form  of  stimulus  which  would  not  in  the  first  instance  tend  to 
evoke  it.  Suppose,  to  illustrate,  that  a  stimulus  A  naturally  evokes 
a  certain  reaction,  and  that  another  stimulus  B,  to  which  the  animal 
would  not  react  at  all,  or  to  which,  at  least,  it  would  not  respond 
by  the  reaction  in  question,  is  presented  time  after  time  along  with 
A:  the  result  may  be  that  B  comes  to  evoke  the  reaction  which  was 
originally  appropriate  to  A.  Thus  Spaulding1  found  that  the  hermit 
crab,  which  commonly  avoids  the  darker  parts  of  the  aquarium, 
could  be  attracted  thither  by  the  chemical  stimulus  of  food.  If  a 
wire  screen  was  placed  in  the  wTay,  the  crab  learned  to  go  around 
and  behind  it,  when  attracted  by  this  chemical  stimulus;  and  after 
many  such  experiences,  the  crab  would  go  behind  the  screen  when- 
ever it  was  placed  in  the  aquarium,  even  though  the  stimulus  of 
food  was  absent.  The  sight  of  the  screen,  from  its  frequent  asso- 
ciation with  the  chemical  stimulus,  had  come  to  evoke  the  same  re- 
action. 

§  5.  All  orders  of  vertebrates,  but  especially  birols  and  mammals, 
give  abundant  evidence  of  learning_bvexperience.  The  sfudy  of 
tKTr subject  has  recently,  and  especially  since  the  work  of  Thorn- 
dike,2  taken  on  a  more  precise  and  scientific  character  than  formerly. 

1  Journ.  of  Comp.  Neurol.  and  PsychoL,  1904,  XIV,  49. 

'"Animal  Intelligence,"  PsychoL  Rev.,  Monogr.  Suppl.  No.  VIII,  1898. 


LEARNING  AMONG  VERTEBRATES  547 

Anecdotes  and  incidental  observations  have  given  way  to  experi- 
ments, in  which  the  learning  process  is  under  observation  from  the 
beginning.  A  comparison  of  vertebrates  with  invertebrates,  and 
of  higher  with  lower  vertebrates,  shows  little  that  is  essentially  new 
in  kind  in  the  learning  of  the  higher  animals;  but  it  does  reveal  a 
great  increase  in  fertility  and  readiness  of  modification.  Reptiles, 
as  represented  by  the  turtle,1  learn  more  quickly  than  amphibia, 
as  represented  by  the  frog,2  or  than  fishes.3  Birds4  and  mammals, 
on  the  whole,  learn  better  thanjthe  lowerclajses of  animals.  Among 
mammals,  monkeys1*  certainly  learn  more,  and  more  rapidly,  than 
do  rats 6  and  mice,7  or  dogs  and  cats,8  while  the  raccoon 9  seems  to 
stand,  in  this  respect,  intermediate  between  the  cat  and  the  monkey. 
The  various  orders,  families,  and  genera  differ  in  the  time  required 
for  the  formation  of  a  habit,  in  the  permanence  of  the  habits  ac- 
quired, in  ttee^omplexity  of  performance,  andjn  the_ variety  and 
number  of  the  performances  which  they  are  capable  of  acquiring. 

One  typical  form  of  experiment,  equally  applicable,  in  principle, 
to  a!T"sorts  of  animals,  requires  that  a  path  to  a  desirable  object 
(such  as  food,  the  nest,  mates,  etc.)  shall  be learned! Only  one 
path  is  left  open,  and  sometimes  this  is  complicated  into  a  maze. 
On  the  first  trial,  the  animal  goes  at  random,  but  finally  reaches  the 
goal;  and  on  repeated  trials,  if  the  maze  is  not  too  complicated,  the 
number  of  wrong  turns  decreases,  till  finally  a  fixed  and  successful 
path  is  followed.  Fishes,  frogs,  and  turtles  have  in  this  way  learned 
to  thread  very  simple  mazes,  while  quite  intricate  ones  have  been 
mastered  by  rats,  birds,  and  monkeys.  The  learning  process  may 
be  described,  externally,  by  saying  that  aJdapl  of  natural  selection 
takes  place  among_the ;  djfferent ^  jeactipns,  the  successful  ones  being 
keptlfficl  the  unsuccessful  eliminated. 

A  second  form  of  experiment,  instead  of  providing  rewards  for 
well-doing,  provides  only  punishment  for  ill-doing.  The  training 


1  Yerkes,  Pop.  Science  Monthly,  1901,  LVIII,  519. 

2  Yerkes,  Harvard  Psychological  Studies,  1903,  I,  579. 


3Thorndike,  Amer.  Naturalist,  1899,  XXXIII,  923;  Triplett,  Amer.  Journ. 
of  Psychol.,  1901,  XII,  354. 

4  Thorndike,  Psychol.  Rev.,  1899,  VI,  282;  Porter,  Amer.  Journ.  of  Psychol., 
1904,  XV,  313  and  1906,  XVII,  248. 

6  Thorndike,  Psychol.  Rev.,  Monogr.  Suppl.  No.  XV,  1901;  Kinnaman,  Amer. 
Journ.  of  Psychol.,  1902,  XIII,  98,  173;  Haggerty,  Journ.  of  Comp.  Neurol.  and 
Psychol,  1909,  XIX,  337. 

6  Small,  Amer.  Journ.  of  Psychol.,  1899,  XI,  133,  and  1900,  XII,  206;  Watson, 
Animal   Education  (Chicago,   1903);    and   Psychol.  Rev.,  Monogr.  Suppl.  No. 
XXXIII,  1907. 

7  Yerkes,  The  Dancing  Mouse  (New  York,  1907). 

8  Thorndike,  Psychol.  Rev.,  Monogr.  Suppl.  No.  VIII,  1898. 

9  Cole,  Journ.  of  Comp.  Neurol.  and  Psychol.,  1907,  XVII,  211. 


548       MEMORY  AND  THE  PROCESS  OF  LEARNING 

of  domestic  animals  furnishes  many  rough  experiments  of  this  sort, 
and  the  results  seem  to  show  that  punishment  is  an  effective  means 
of  selection.  More  precise  experiments  have  yielded  the  same  re- 
sult. The  young  chigk,  after  once  pecking  at  a  bee,  avoids  bees  in 
the  future;1  mice  quickly  learn  to  avoid  a  spot  where  they  receive 
an  electric  shock,2  and  a  pike3  or  perch,4  which  instinctively  snaps 
at  minnows,  learns  not  to  do  so  if  he  is  separated  from  them  by  an 


FIG.  147. — Curve  of  Learning  (Thorndike).  The  gradual  descent  of  the  line  represents  the 
decrease  in  time  occupied  by  a  cat  to  escape  from  a  cage.  The  association  here  learned 
(escaping  by  turning  a  button)  was  comparatively  difficult  for  the  cat. 

invisible  glass  partition  against  which  he  bumps  his  nose  at  every 
attempt;  after  some  weeks  of  this  experience,  the  large  fish  can  safely 
be  allowed  to  swim  freely  among  the  little  ones. 

The  thir^Jorm  of  experiment  which  has  been  frequently  em- 
ployed is  like  the  maze  test  in  offering  rewards,  but  differs  from  it 
in  requiring  not  simply  locomotion  on  the  part  of  the  animal,  but 
the  operation  of  some  simple  mechanical  device.  In  Thornolike's 
experiments,  a  cat  was~placed  in  a  cage^from  which  escape  was  pos- 
sible by  pulling  a  string,  or  turning  a  button,  or  pressing  a  lever,  etc. 
To  ensure  an  energetic  attempt  to  escape,  the  cat  was  taken  when 
hungry,  and  food  was  placed  outside  the  cage  and  in  plain  sight  of 
the  animal.  Under  these  conditions,  the  animal  attacked  the  sides 
of  the  cage  vigorously,  trying  to  squeeze  between  the  bars,  biting 
at  them,  clawing  at  anything  loose,  and,  in  the  course  of  these  in- 
stinctive acts,  was  pretty  sure  to  turn  the  button  or  pull  the  string, 

1  Lloyd  Morgan,  Habit  and  Instinct,  p.  53  (London,  1896). 
8  Yerkes,  The  Dancing  Mouse. 

3  K.  Mobius,  Die  Bewegungen  der  Tiere  und  ihre  psychischer  Horizont,  1873. 

4  Triplett,  op.  cit. 


LEARNING  AMONG  VERTEBRATES 


549 


which  opened  the  door  and  gave  access  to  the  food.  On  frequent 
repetition  of  this  experiment,  the  useless  movements  gradually  de- 
creased in  number,  till  finally  all  were  eliminated,  and  only  the  one 
successful  movement  was  continued. 

In  the  experiment  just  narrated,  the  time  taken  by  the  animal 
to  escape  from  the  cage  was  noted,  on  each  trial,  and  the  decrease 
in  this  time  indicated  the  progress  of  the  animal  in  learning.  An 
association  is  established  in 
such  cases  between  the  situa- 
tion of  being  in  a  cage  while 
hungry  and  with  food  in  sight 
outside,  and  a  particular  re- 
sponse which  brings  success. 
This  successful  resgojise,  to 
use  Thorndike's  terms,  he- 
comes  gradjially  "  stamped 
in,"  while  the  unsuccessful 
responses  are  "staiApSTout." 
The  process  of  "  stamping-in  " 
and  "stamping-out"  is  in 
some  cases  rapid,  in  others 
very  gradual;  while  in  still 
others,  little  or  no  progress 
is  apparent  in  a  long  series  of 
trials.  The  "curve  of  learn- 
ing," obtained  by  plotting  the 
times  of  the  successive  trials, 
gives  a  graphic  view  of  the 
progress  in  formation  of  the 
association.  The 


FIG.  148.—  Curve  of  Learning  (Thorndike).  The 
same  cat  in  a  performance  easier  to  learn. 
Here  a  loop  attached  to  a  string  and  hang- 
ing in  the  cage  on  the  side  toward  the  food 
was  to  be  pulled.  The  act  of  pulling  the 
string  was,  apparently,  more  a  definite  unit 
which  would  impress  itself  in  connection  with 
the  resulting  opening  of  the  door;  whereas  the 
turning  of  the  button  was  apt  to  occur  in 
the  midst  of  random  clawing,  and  so  to  have 
little  individuality. 


this  progress  differs  with  the  kind  of  animal,  with  the  difficulty  of 
the  performance,  and  with  the  previous  experien£e  of  the  in3ivio!ual 
tested.  A  performance  which  is  learned  by  a  cat  only  in  the  course 
of  many  trials  may  be  learned  by  a  monkey  in  few  trials;  while  a 
performance  which  requires  the  monkey  many  trials  may  be  prac- 
tically beyond  the  powers  of  the  cat.  The  single  door-button  is 
slowly  learned  by  a  cat,  but  quickly  by  a  monkey.  A  combina- 
tion lock,  first  used  by  Kinnaman,  in  which  four  fastenings  have  to 
be  undone  in  a  certain  order,  was  found  too  hard  for  cats,  but  was 
mastered  by  monkeys  after  many  trials;  an  adult  man  usually  mas- 
ters it  in  from  one  to  three  trials.  The  influence  of  the  previous 
experience  of  the  individual  animal  is  brought  out  by  requiring  the 
animal  to  master  one  puzzle  after  another.  When  the  same  sort 
of  fastening  is  employed  in  two  cages,  but  in  different  parts  of  the 


550       MEMORY  AND  THE  PROCESS  OF  LEARNING 

two,  learning  to  operate  it  in  the  first  cage  often  hastens  the  progress 
of  learning  to  operate  it  in  the  second.  This  means,  therefore,  that 
experience  adds  new(or  more  highly  specialized)  mooles  of  reaction 
to  me^  instjnc^ive  store  with  which  the  animal  begins. 

§  6.  Any  attempt  at  interpreting  the  process  of  learning  in  ani- 
mals must  involve  a  consideration  of  some  topics  which  are  more 
germane  to  the  next  chapter  than  to  the  present.  For,  as  a  matter 
of  fact,  the  formation  o£  associations  cannot  be  understood  without 
reference  to  processes  of  discrimination  and  atterTtTotf.  With  the 
quesHon  of  the  limits  of  animal^TCa*son  we  are  noFhere  specially 
concerned,  our  object  being  rather  that  of  understanding  the  sim- 
pler mental  processes  in  general.  In  the  experiments  already  de- 
scribed, the  method  by  which  the  animal  learns  to  master  a  maze  or 
a  puzzle-box  has  been  called  learning  by  "  trial  and  error."  We  pre- 
fer to  call  it,  "learning  by  varied  reaction  through  selection  of  the 
successful  variants."  Without  variation  of  reaction,  the  cat  would 
continue  trying  to  squeeze  between  the  bars  toward  the  food,  jus,t 
as  iron  filings  tend  along  fixed  lines  of  force  toward  a  magnet  from 
which,  perhaps,  they  are  separated  by  a  sheet  of  paper.  On  the 
other  hand,  without  some  sort  of  selection  from  among  the  varied 
reactions,  no  progressive  shortening  of  the  whole  time  of  reaction 
would  occur. 

The  principal  problems  which  arise  in  the  interpretation  of  this 
sort  of  learning  concern  (1)  the  source  of  the  varied  reactions  and 
the  manner  in  which  they  are  called  into  activity;  and  (2)  the 
mechanism  of  selection.  In  regard  to  the  source  of  the  variations, 
some  are  instinctive  and  others  are  derived  from  previous  experi- 
ence. But  not  all  of  previous  experience  is  utilized;  and  sometimes 
previously  learned  reactions  which  would  fit  the  new  situation  ad- 
mirably are  not  called  into  play  at  all.  The  problem  here  is  one 
which  has  passed  under  the  name  of  the  "transference"  of  the  effects 
of  training  in  one  situation  to  another  situation.  This  problem 
again  fereaks  up  into  two : — namely,  (1)  as  to  how  far  reactions 
learned  in  one  situation  can  be  directly  applied  to  another;  and  (2) 
as  to  how  reactions  which  might  well  be  applied  are  actually  re- 
called by  the  new  situation.  In  regard  to  the  first  point,  we  ob- 
serve that  there  is  often  something  specific  about  reactions  learned 
in  a  given  situation  which  unfits  them  for  direct  application  in  an- 
other situation,  even  though  the  two  situations  may  seem,  externally 
regarded,  to  be  essentially  the  same.  The  particular  combination 
of  movements,  for  example,  which  has  become  stereotyped  in  oper- 
ating a  fastening  in  one  cage  may  need  some  modification  to  work 
well  in  another  cage;  and  therefore  the  latter  may  need  to  be  mas- 
tered afresh.  Granted,  even,  that  a  mode  of  reaction  has  been 


PROCESS  OF  SELECTION  IN  LEARNING  551 

learned  which  is  directly  applicable  to  the  new  situation,  it  by  no 
means  follows  that  it  will  be  called  into  play;  for,  since  the  new  situa- 
tion differs  in  some  respects  from  the  old,  the  proper  reaction  de- 
pends on  the  chance  that  the  similar  features  of  the  two  situations 
shall  control  the  process  of  its  recall.  /In  every  new  situation,  how- 
ever, there  are  many  features  competing  for  prominence;  so  that 
the  familiar  feature  has  only  a  limited  chance  to  become  dominant./ 

The  nature  of  the  process  of  selection,  by  which  the  successful 
variants  of  reaction  are  strengthened  from  trial  to  trial,  and  the  un- 
successful eliminated,  is  indicated  to  some  extent  by  the  curves  of 
learning  (see  pp.548  f.).  Where  the  descent  of  these  curves  is  gradual, 
the  process  of  selection  must  itself  be  gradual;  but  where  the  curves 
show  a  sudden  drop  from  a  high  to  a  low  level,  the  process  of  selec- 
tion must  be  correspondingly  abrupt.  These  sudden  drops,  when 
(as  in  human  learning)  introspection  comes  to  our  aid  in  explain- 
ing them,  are  found  to  be  due  to  a  perception  of  certain  facts  about 
the  situation,  such  as  the  uselessness  of  some  reactions  or  the  value 
of  others.  This  perception  is  often  a  clearly  defined  event  in  the 
individual's  experience,  and  its  results  are  "stamped  in"  once  for 
all.  The  absence  of  sudden  drops  in  many  curves  of  learning,  even 
of  those  which  describe  graphically  the  attainment  of  a  high  degree 
of  skill  in  manipulation,  in  man's  case,  is  an  evidence  of  the  lack 
of  moments  of  clear  perception.  In  such  cases,  selection  must  be 
possible  in  some  more  gradual  and  mechanical  way;  but  this  gradual 
selection  must  be  highly  important  and  perhaps  even  fundamental  ' 
in  all  learning.  The  following  attempt  at  an  interpretation  of  the 
process  of  selection  in  learning  by  trial  andTerror  should  be  regarded 
only  as  tentative,  though  combined  of  elements  which  are  fairly 
sure  to  be  genuine.  ^ 

-  §  7.  In  the  first  place,  we  must  assume  in  the  animal  an  adjustment 
or  determination  of  the  psycho-physical  mechanism  toward  a  cer- 
tain end.  The  animal  desires,  as  we  like  to  say,  to  get  out  and  to 
reach  the  food.  Whatever  be  his  consciousness,  his  behavior 
shows  that  he  is,  as  an  organism,  set  in  that  direction.  This  adjust- 
ment persists  till  the  motor  reaction  is  consummated;  it  is  the  driv- 
ing force  in  the  unremitting  efforts  of  the  animal  to  attain  the  desired 
end.  His  reactions  are,  therefore,  the  joint  result  of  the  adjustment 
and  of  stimuli  from  various  features  of  the  cage.  EacL^single  re- 
action tends  to  become  associated  with  the  adjustment.  But  the 
unsuccessful  reactions  are  less  strongly  associated  than  the  success- 
ful, because  each  one  of  the  former  is  at  some  moment  given  up 
or  inhibited;  and  this  inhibition,  too,  being  made  under  the  influence 
of  the  adjustment,  tends  to  become  associated  with  it,  and  so  to  in- 
terfere with  the  association  between  the  adjustment  and  the  per- 


552       MEMORY  AND  THE  PROCESS  OF  LEARNING 

formance  of  this  particular  reaction.  In  the  case  of  the  successful 
reaction,  however,  the  phase  of  inhibition  does  not  occur,  and  the 
only  association  with  the  adjustment  is  of  the  positive  sort.  /Thus 
the  successful  reactions  must,  in  the  long  run,  gain  an  advantage 

ver  the  unsuccessful. 

•  The  preceding  explanation,  although  satisfactory  as  far  as  it 
goes,  is  not  fully  adequate  to  account  for  all  the  facts.  In  particular, 
it  does  not  take  full  account  of  the  pleasure  accompanying  success, 
and  of  the  often  strong  displeasure  that  attends  baffled  effort.  Such 
pleasure  and  displeasure  certainly  occurjn  humanjgajrning,  and_seem 
present" in  thft  fofohpr  animals.  Exactly  how  these  emotions  act  to 
strengthen  one  association  and  to  weaken  or  counteract  another 
cannot  readily  be  seen ;  but  it  is  safe  to  assume  that  they  correspond 
to  some  genuine  dynamic  process  of  great  efficacy.  The  displeasure 
of  failure  and  baffled  effort,  in  particular,  must  be  the  indication 
of  a  stronger  inhibitory  process  than  is  provided  for  in  the  preceding 
explanation.  ^ 

§  8.  Where  the  curve  of  learning,  instead  of  descending  slowly, 
passes  after  the  first  few  trials  from  long  to  short  times — where,  that 
is  to  say,  the  elimination  of  unsuccessful  reactions  occurs  suddenly, 
and  the  right  response  is  quickly  associated  with  the  situation — 
there,  we  may  assume,  something  like  a  clear  perception  of  the 
right  reaction  has  intervened.  But,  in  the  case  of  the  lower  ani- 
mals and  even  in  man's  case,  too  much  meaning  must  not  be 
read  into  the  word  "perception."  The  required  mental  act  need 
not  involve  any  insight  into  the  reason  why  one  reaction  is  successful 
and  the  other  unsuccessful;  there  need  not  be  any  moment  of  reflect- 
ive judgment,  such  as  might  be  expressed  in  the  words,  "  This  is  the 
way,"  or  "  That  is  not  the  way."  All  that  isjificessary  is_that  some 
feature  of  the  situation  should  jDrgxail  in  consciousness,  ancj  thaTthe 
reaction  to  it  should  have  aj^ertain  separatenesTrrom_the  total  series 
of  changing  reactions.  This  distinctness  of  a  certain  feature  of  the 
situation,  and  of  the  reaction  to  it,  probably  indicates  a  high  in- 
tensity of  the  correlated  neural  process,  and  so  a  favorable  condition 
for  a  strong  association. 

To  illustrate  this  quick  learning,  and  the  lack  of  real  insight  that 
may  accompany  it,  we  will  make  brief  mention  of  a  hitherto  unpub- 
lished experiment1  on  a  chimpanzee — a  species  which,  to  judge  by 
cerebral  development,  stands^  considerably  higher  than  the  smaller 
monkeys  (compare  p.  34).  The  specimen  tested  was  a  young 
female,  about  half  grown,  and  corresponding  in  relative  maturity, 
perhaps,  to  a  child  of  the  human  species  of  ten  or  twelve  years. 

1  The  experiment  was  performed  by  R.  S.  Woodworth  in  1902-03,  in  the 
laboratory  of  Professor  Sherrington  at  Liverpool. 


LEARNING  BY  TRIAL  AND  ERROR  553 

A  box  was  prepared,  having  a  slatted  front  with  a  door  closed  by  a 
button,  a  turn  of  which  through  90°  released  the  door.  The  chim- 
panzee, on  being  placed  in  front  of  this  box,  in  which  a  piece  of 
banana  had  been  placed  before  her  eyes,  quickly  came  to  devote 
most  of  her  efforts  to  the  door  (which  allowed  of  some  slight  mo- 
tion even  with  the  button  closed) — pulling  it  outward,  pushing  it 
inward,  and  shaking  it.  She  soon,  also,  attacked  the  button,  and 
alternated,  for  the  most  part,  between  this  and  the  door.  In  this 
way,  it  was  not  long  before  she  turned  the  button  through  90°, 
then  tried  the  door,  and  got  in,  thus  securing  the  food.  On  a  second 
trial,  the  chimpanzee  worked  almost  entirely  at  the  door  and  the 
button;  and  from  the  third  trial  on,  her  reason  was  uniformly 
prompt  and  correct.  After  several  more  trials,  a  second  button  was 
added  a  few  inches  from  the  first,  but  much  like  the  first  in  appear- 
ance, and  operated  in  the  same  manner.  The  chimpanzee  at- 
tacked the  box  as  before,  neglecting  the  second  button.  After 
once  turning  the  first  button,  and  pulling  the  door,  which,  of  course, 
did  not  yield,  she  turned  the  first  button  again,  so  locking  the  door; 
then  again  tried  the  door,  and  continued  in  this  way  for  a  long  time, 
before  passing  to  the  second  button  and  dealing  similarly  with  it. 
Entrance  was  finally  secured  by  a  chance  placing  of  both  buttons 
at  once  in  the  right  position.  In  the  course  of  several  trials,  no 
further  progress  was  made.  It  seemed  to  be  wholly  a  matter  of 
chance  whether  both  buttons  should  be  put  right  at  once  or  not. 
The  experiment  showed  then  a  prompt  narrowing  down  of  the  field 
of  effort  to  the  right  feature  of  the  situation;  but  this  important 
factor  in  the  process  of  learning  seemed  to  be  accompanied  by  a 
complete  absence  of  insight  into  the  mechanical  principle  involved. 

In  such  experiments,  one  of  the  features  of  a  situation  which  most 
readily  becomes  prominent  is  the  place  where  the  successful  issue 
occurs.  Often,  after  a  few  trials,  effort  will  be  concentrated  at 
the  right  place,  though  many  useless  movements  are  made  there. 

§  9.  A  contrast  is  usually  drawn  between  learning  by  trial  and 
error  and  learning  by  ideas, — the  latter  being  clearly  present  in 
much  human  learning,  but  only  doubtfully  present  in  the  learning 
of  animals.  "Ideas,"  if  available,  might  be  of  service  in  several 
ways.  First,  an  idea  called  up  by  some  feature  of  a  present  situa- 
tion might  suggest  some  reaction  which  would  not  be  directly  sug- 
gested by  the  situation  itself,  and  so  might  enlarge  the  range  of  the 
varied  reactions,  and  afford  greater  opportunity  for  success.  There 
is  some  evidence,1  not  wholly,  conclusive,  that  ideas  may  function 
in  this  way  in  some  animals.  Second,  where  a  reaction  has  been  pre- 
viously tried  and  found  unsuccessful,  it  might  be  mentally  rehearsed 
1  Thorndike,  Cole,  op.  cit. 


554       MEMORY  AND  THE  PROCESS  OF  LEARNING 

without  the  actual  motor  performance.  Such  mental  rehearsal  of  a 
reaction  certainly  occurs  in  human  behavior,  and  possibly  in  the 
monkeys,  which  seem  at  times  to  inhibit  movement  as  if  in  thought; 
but  there  is  little  evidence  of  it  in  dogs  and  cats,  whose  motor 
activity  is  perhaps  too  prompt  and  direct  to  permit  of  the  neces- 
sary inhibition.1  A  third  service  of  ideas  might  be  the  following: 
the  successful  reaction  and  its  result  might  similarly  be  rehearsed, 
and  thus  practice  in  dealing  with  a  situation  might  be  obtained  in 
the  absence  of  the  actual  situation.  This  use  of  ideas  occurs  fre- 
quently in  man,  but  in  animals  there  is  no  clear  evidence  of  it. 
Fourth,  by  a  combination  of  the  first  with  the  second  or  third  of 
the  above  uses  of  ideas,  some  feature  of  the  present  situation  might 
suggest  a  reaction  learned  in  previous  experience;  the  consequences 
of  this  reaction  might  be  mentally  rehearsed,  and  its  probable  suc- 
cess or  failure  in  the  present  situation  judged  without  actual  trial. 
This  would  be  equivalent  to  "thinking  out"  the  solution  of  a  pres- 
ent difficulty  without,  or  before,  actually  trying  it.  So  complex  a 
use  of  ideas,  while  perfectly  within  human  capacity,  occurs  seldom 
even  in  man,  in  cases  similar  to  the  maze  or  puzzle-box  experiments; 
for  the  prolonged  suspension  of  motor  reactions  which  it  requires 
is  disagreeable  to  most  men — especially  if  the  situation  permits  of 
immediate  reaction.2  Fifth,  the  ideas  employed  might  have  the 
character  of  general  principles,  from  which  the  necessities  of  the 
present  case  could  be  deduced,  and  so  the  situation  be  thoroughly 
and  surely  mastered  in  advance  of  motor  reaction.  Such  a  use  of 
ideas  occurs  in  specially  trained  human  individuals  within  the  range 
of  their  specialty;  but  otherwise  is  probably  rare,  at  least  in  any 
complete  form. 

§  10.  The  differences  between  different  animals,  and  between  man 
and  the  animals,  in  the  power  of  learning,  are  not  fully  accounted 
for  by  different  degrees  of  the  use  of  ideas.  Greater  fertility  in 
association  must  also  be  allowed  for  in  the  human  being.  That  dis- 
crimination favors  association  has  been  noted  above,  in  saying  that 
any  feature  of  a  situation  or  of  the  reaction  to  a  situation,  which 
stands  out  with  a  degree  of  separateness,  has  an  especially  good 
chance  of  becoming  associated  strongly  with  the  adjustment  to 
deal  with  the  situation.  That  association  favors  discrimination  has 
also  been  suggested  by  the  fact  that  a  previously  familiar  feature 
is  likely  to  be  singled  out  for  present  reaction.  Now  one  of  the 
differences  between  different  species,  and  also  between  different 

1  But  see  Hamilton,  "An  Experimental  Study  of  an  Unusual  Type  of  Reaction 
in  a  Dog,"  Journ,  of  Comp.  N enrol,  and  Psychol,  1907,  XVII,  329. 

2  See  Ruger,  "The  Psychology  of  Efficiency,"  Archives  of  Psychology,  No. 
XV,  1910,  p.  9. 


LEARNING  BY  IDEALS  555 

human  individuals,  lies  in  their  different  powers  of  discrimination. 
Greater  power  of  analysis  or  discrimination  belongs  to  man.  The 
terms,  as  heftrTTsect,""  Imply  reacting  to^slomenFeature  of  a  situation 
isolated  from  the  total.  Features  which  an  animal  cannot  isolate 
are  easily  isolated  by  man.  For  example,  in  dealing  with  Kinna- 
man's  combination  lock  (p.  549),  which  requires  several  fastenings 
to  be  undone  in  a  fixed  order,  the  cat  seems  wholly  lost,  and  the 
monkey  succeeds  only  with  difficulty,  not  because  they  cannot 
master  the  separate  fastenings,  but  because  they  do  not  take  these 
in  proper  sequence;  but  man  quickly  observes  that  a  certain  order 
is  necessary  in  dealing  with  the  parts  of  a  similar  mechanism. 
Whether  he  thinks  of  an  interlocking  of  hidden  parts,  or  does  not 
clearly  formulate  his  conception  in  any  way,  that  relation  of  parts 
or  of  movements,  which  we  have  designated  by  the  word  "  order," 
is  a  feature  which  his  powers,  dependent  largely  on  his  past  ex- 
perience, enable  him  to  single  out  and  make  the  basis  of  his  reac- 
tions. Man,  therefore,  reacts  to  relations  which  the  animal  passes 
by;  and  much  of  his  superiority  in  learning  to  deal  with  complicated 
situations  is  due  to  this  power. 

{/§  11.  Human  behavior  in  situations  resembling  those  employed 
ifi  experimenting  on  animals  has  been  studied  by  several  investi- 
gators. The  solution  of  mechanical  puzzles1  is  a  task  similar  in 
principle  to  that  presented  to  the  animal  in  a  puzzle  box,  but  of  a 
more  difficult  order.  The  behavior  of  a  man  on  his  first  attempt 
to  solve  such  a  puzzle  often  resembles  closely  that  of  the  animals. 
Under  the  influence  of  an  adjustment  to  solve  the  puzzle,  the  man 
usually  begins  promptly  to  manipulate,  according  as  parts  of  the 
device  catch  his  eye  and  lead  to  instinctive  or  previously  learned 
reactions.  This  random  procedure  may  continue  till  success  is 
accidentally  reached;  or,  continued  failure  may  lead  to  a  period 
of  inhibition  of  movement,  of  closer  examination  of  the  puzzle  and 
a  deliberate  attempt  to  "  think  it  out."  This  attempt  is  not  always 
successful,  and  random  manipulation  is  again  resorted  to.  In 
the  human  being,  however,  the  randomness  of  reaction  is  likely  to 
be  limited  by  certain  conceptions  as  to  the  probable  line  of  success- 
ful effort;  and  though  these  conceptions  are  sometimes  wrong  and 
a  hindrance  to  prompt  success,  at  other  times  they  are  of  great  value. 
The  first  successful  issue  comes  about,  accordingly,  in  various  ways, 
sometimes  by  a  pure  accident  which  remains  a  mystery  to  the  per- 
former; sometimes  by  an  accident  which  is  nevertheless  observed 
and  partially  understood;  sometimes  as  the  outcome  of  consciously 
trying  whether  a  certain  manipulation  will  succeed;  and  sometimes 
as  the  result  of  clear  insight  into  the  necessities  of  the  case.  The 

1  Ruger,  op.  cit. 


556      MEMORY  AND  THE  PROCESS  OF  LEARNING 

more  there  is  of  conscious  prevision  of  the  successful  movement, 
the  more  certainly  will  the  first  success  lead  to  prompt  and  correct 
reaction  in  all  subsequent  trials. 

Suppose,  now,  that  the  solution  of  the  same  puzzle  is  undertaken 
time  after  time,  with  a  constant  effort  to  increase  the  speed.  A  curve 
of  learning  then  appears,  which  may  vary  in  form  between  extremes 
such  as  are  represented  in  the  two  curves  for  animals  shown  above. 
In  general,  gradual  improvement  is  visible  in  the  midst  of  many 
oscillations.  As  soon  as  the  main  path  to  success  is  well  known, 
new  difficulties  of  manipulation  emerge,  and  each  of  these  becomes 
a  problem,  which  is  likely  to  be  solved  in  any  one  of  the  variety 
of  ways  which  appear  in  the  main  solution.  Decrease  in  the  total 
time  of  manipulation  occurs  sometimes  gradually,  from  the  increas- 
ing firmness  of  association  between  the  parts  of  the  performance, 
and  sometimes  suddenly,  from  the  adoption  of  a  "short-cut"  or 
better  form  of  manipulation.  The  steps  in  the  manipulation, 
which  in  the  early  stages  of  practice  are  separate  acts  and  require 
separate  attention,  come  later  to  be  combined  into  larger  units. 
It  thus  becomes^Dgssible  to  keep  the  attention  ahead  of  the  hands, 
and  to  prepare  ;HeBtally  for  each  step  before  the  time  for  it  arrives. 
In  this  way  tlmPIs  saved  in  passing  from  one  step  to  another,  and 
the  whole  process  gains  in  speed  and  smoothness. 

§  12.  Much  more  complex  than  the  solution  of  a  mechanical 
puzzle  are  some  of  the  performances  which  have  been  made  the 
material  for  experiments  on  practice.  Bryan  and  Harter1  have 
studied  the  process  of  mastering  telegraphy,  and  Book2  the  acqui- 
sition of  skill  in  typewriting.  In  contrast  with  the  maze  or  puzzle, 
which  can  sometimes  be  mastered  in  a  hundred  trials — though  it 
offers  a  surprising  amount  of  opportunity  for  continued  improve- 
ment in  matters  of  detail — telegraphy  and  typewriting  require 
years  of  practice  before  the  highest  expert  stage  of  skill  is  reached. 
Progress  consists  in  the  formation  of  numerous  specific  but  inter- 
related habits,  some  of  which  are  formed  early  in  the  process  of 
training,  and  may  be  called  habits  of  the  lower  order,  whereas 
others  are  superimposed  on  these  first,  and  may  be  called  habits  of 
the  higher  order.  All  in  all,  as  Bryan  and  Harter  state  the  case, 
mastery  of  such  an  art  consists  in  the  formation  of  a  hierarchy  of 
habits.  In  learning  to  telegraph,  the  first  step  is  to  acquire  some 
familiarity  with  the  alphabet  of  dots  and  dashes.  The  beginner, 
in  sending  a  message,  laboriously  spells  out  each  word  letter  by 
letter.  He  learns  to  make  a  particular  combination  of  dots  and 

1  Psycfwl.  Rev.,  1897,  IV,  27,  and  1899,  VI,  345. 

2  "The  Psychology  of  Skill,"  University  of  Montana  Publications  in  Psychol- 
ogy, 1908. 


ACQUIREMENT  OF  SKILL 


557 


dashes  in  response  to  each  letter  of  his  "copy";  and  the  associa- 
tion so  formed  between  the  several  letters  and  the  appropriate  finger 
movements  constitutes  his  habits  of  the  lowest  order.  Soon,  how- 
ever, he  comes  to  run  together  the  letters  of  familiar  words,  thinking 
rather  of  the  rhythm  of  the  whole  word  than  of  the  patterns  of  the 
constituent  letters.  As  his  skill  increases,  he  adds  more  and  more 
word-units  to  his  repertoire;  and  these  word-units  constitute  his 
habits  of  the  second  order.  He  does  not  stop  here,  however,  but 


Fia.  149.— Curves  Showing  the  Improvement  in  Sending  and  Receiving  Telegraphic 
Messages  (Bryan  and  Harter). 

begins  to  train  his  fingers  to  the  rhythm  of  phrases  and  other  fre- 
quently recurring  combinations  of  words.  The  progress  of  improve- 
ment in  sending  telegraphic  messages  is  shown  in  the  upper  curve 
of  Fig.  149,  in  which  a  rise  indicates  improvement.  The  main 
characteristic  of  the  practice  curve,  as  here  illustrated,  is  the  rapid 
rate  of  improvement  at  the  beginning,  and  the  gradual  slackening 
of  improvement  as  the  practice  continues,  till  the  "physiological 
limit"  for  this  particular  performance  is  reached,  and  no  further 
improvement  is  possible. 

The  curve  of  improvement  in  receiving  messages  over  the  wire 
is  more  complex,  and  the  whole  process  of  receiving  is  more  inter- 
esting to  study.  At  first,  greater  speed  is  possible  in  sending  than 
in  receiving,  and  this  difference  continues  for  a  long  period.  In 
sending,  the  operator  has  matters  in  his  own  hands;  he  can  adjust 
his  rate  to  the  difficulties  of  the  copy,  and  he  can  see  ahead.  In  re- 


558       MEMORY  AND  THE  PROCESS  OF  LEARNING 

ceiving,  he  has  none  of  these  advantages.  In  spite  of  this  early  ad- 
vantage of  sending  over  receiving,  in  the  long  run  the  speed  of  re- 
ceiving surpasses  that  of  sending,  so  that  an  expert  can  receive 
faster  than  either  he  or  any  other  man  can  move  his  fingers.  Habits, 
or  units  of  perception,  of  different  orders  are  formed  in  learning 
to  receive  much  as  in  learning  to  send.  The  beginner  must  atten- 
tively isolate  the  letters  from  the  series  of  clicks  which  he  hears; 
later  he  becomes  able  to  catch  the  pattern  of  frequent  combinations 
of  letters  and  of  common  words;  still  later,  he  apprehends  phrases 
and  even  short  sentences  as  wholes;  while  "the  real  expert  has  all 
the  details  of  the  language  with  such  automatic  perfection  that  he 
gives  them  practically  no  attention  at  all.  He  can,  therefore,  give 
his  attention  freely  to  the  sense  of  the  passage;  or,  if  the  message  is 
sent  accurately  and  distinctly,  he  can  transcribe  it  upon  the  type- 
writer while  his  mind  is  running  on  things  wholly  apart.  .  .  . 
The  feat  of  the  expert  receiver — for  example,  of  the  receiver  of 
press  despatches — is  more  remarkable  than  is  generally  supposed. 
.  .  .  To  bring  the  sender's  rate  up  to  that  of  the  receiver  abbrevi- 
ated codes  have  been  prepared.  The  receiver  must  translate  the 
code  into  English  words,  and  transcribe  these,  correctly  capital- 
ized and  punctuated,  upon  the  typewriter.  He  takes,  in  this  way, 
eighty  or  eighty-five  words  a  minute.  If  mistakes  are  made  by  the 
sender,  the  receiver  is  expected  to  correct  them  as  they  come,  and 
send  a  clean  copy  to  press.  The  work  continues  for  hours  without 
leisure  for  rereading,  the  pages  being  taken  away  to  press  as  fast 
as  they  are  finished." 1 

§  13.  For  an  understanding  of  the  psycho-physical  mechanism 
of  learning,  and  so  of  the  processes  of  association  and  discrimina- 
tion involved  in  learning,  it  is  important  to  attempt  an  analysis  of 
such  feats  of  skill  as  those  described  above.  An  essential  feature  of 
the  performance  of  transcribing  a  telegraphic  message,  is  that  the 
skilled  receiver  keeps  several  words  behind  the  message  as  he  hears 
it.  The  expert  learns  not  to  guess  ahead,  or  anticipate  what  is 
coming,  for  in  this  way  false  readings  are  sure  to  occur.  Yet  he  does 
not  by  any  means  spell  out  the  letters  as  they  come.  He  seems 
rather  to  let  the  pattern  of  the  clicks  "  soak  in  "  for  a  time,  gradually 
interpreting  those  that  come  earlier  in  the  light  of  those  which  fol- 
low. For  this  purpose,  the  expert,  in  receiving  a  connected  de- 
spatch, "prefers  to  keep  from  six  to  ten  or  twelve  words  behind  the 
instrument."  This  gives  an  index  of  the  size  of  the  units  in  which 
he  thinks. 

The  process  of  acquiring  skill  in  typewriting  has  been  even  more 
minutely  examined  by  Book,  who  employed  in  his  study  a  combina- 
1  Bryan  and  Barter,  Psychol.  Rev.,  1899,  VI,  352. 


ANALYSIS  OF  FEATS  OF  SKILL  559 

tion  of  objective  records  and  introspective  observations.  The  gen- 
eral course  of  practice  is  the  same  as  in  telegraphy,  and  the  same 
sort  of  hierarchy  of  habits  appears.  Many  habits  are  only  tempora- 
rily formed  and  used,  but  are  later  supplanted  by  the  development 
of  other  habits  better  adapted  to  a  higher  degree  of  skill  in  writing. 
Thus,  when  the  beginner  is  learning  by  the  "touch  method,"  and 
the  keyboard  is  concealed  by  a  screen,  the  first  associations  formed 
are  between  the  letters  and  their  place  on  a  plan  of  the  keyboard, 
which  may  be  held  in  mind  as  a  visual  image.  Later,  this  image 
or  plan  disappears  so  completely  that  a  skilful  performer  may  be 
unable  to  tell  the  location  of  the  letters  on  the  keyboard  without 
actually  writing  and  finding  out  where  his  fingers  automatically 
place  themselves.  In  such  cases  the  plan  has  already  become  un- 
necessary, because  of  the  direct  associations  which  have  become 
established  between  the  letters  and  the  appropriate  movements. 
When  such  associations  are  well  learned,  and  even  before  they  are 
perfected,  frequently  occurring  sequences  of  letters  begin  to  be 
associated  with  sequences  of  finger  movements,  so  as  to  constitute 
those  units  of  motor  reaction  to  which  reference  has  already  been 
made.  Prefixes  and  suffixes,  short  words,  longer  words,  and  even 
phrases  of  some  length,  become  in  this  way  reduced  to  specific^ 
motor  habits,  which  stand  ready  to  command. 

Since,  however,  the  possible  sequences  of  words  are  much  too 
numerous  ever  to  be  all  reduced  to  automatic  habits,  the  procedure 
of  the  finished  expert  does  not  consist  simply  in  writing  in  large 
units,  but  rather  in  utilizing  letter  associations,  word  associations, 
and  phrase  associations,  according  to  the  exigencies  of  the  copy. 
This  the  expert  accomplishes  while  still  maintaining  a  wholly  un- 
broken series  of  finger  movements  at  the  maximum  rate;  no  "units," 
higher  or  lower,  can  be  detected  in  the  rhythm  of  his  motor  reactions. 
The  means  by  which  this  result  is  accomplished  consists  in  keeping 
the  eyes,  on  the  copy,  several  words  ahead  of  the  fingers.  What  are 
the  marvellous  internal,  or  psycho-physical  processes  (mind  and 
brain)  which  occur  during  the  interval  between  the  reading  of  the 
copy  and  the  writing  of  it?  An  indication  of  the  answer  to  this 
question  is  afforded  by  the  fact1  that  when  some  special  difficulty 
appears  in  the  copy,  the  eyes  are  slackened  in  their  advance,  and  the 
hands  partially  overtake  them.  One  might  have  supposed  that 
the  hands  would  slacken,  and  the  eyes  get  still  farther  ahead,  but 
the  contrary  is  the  case;  and,  indeed,  the  hands  do  not  need  to 
slacken,  for  by  the  time  they  have  reached  the  difficult  spot,  the 
way  has  been  made  ready  for  them  by  special  attention  devoted 
to  the  details  of  the  coming  movements.  There  occurs,  then,  in 

1  Book,  op.  cit.,  p.  44. 


560      MEMORY  AND  THE  PROCESS  OF  LEARNING 

this  important  interval  between  the  reading  and  the  writing  of  a 
word,  an  organization  of  the  coming  reaction,  in  which  such  of  the 
various  ready-formed  phrase  associations,  word  associations,  and, 
if  need  be,  letter  associations,  are  utilized  as  fit  the  requirements 
of  the  case.  The  entire  procedure  is  an  elaborate  process  of  re- 
call of  many  associations,  and  of  appropriate  combination  of  these 
associations. 

§  14.  The  foregoing  studies  of  typewriting,  telegraphy,  and  the 
manipulation  of  mechanical  puzzles  serve  admirably  to  throw  into 
relief  the  importance,  for  skilled  and  efficient  action,  of  two  princi- 
ples which  were  found  necessary  to  a  satisfactory  theory  of  percep- 
tion, and  which  were  mentioned  in  a  previous  chapter  (p.  498): 
these  are  (1)  the  principle  of  higher  units,1  and  (2)  the  principle  of 
overlapping.  Overlapping  of  different  reactions  is  clearly  in  evi- 
dence when  the  reading  of  the  copy  precedes  by  several  words  the 
writing,  or  when  the  hearing  of  the  telegraphic  clicks  precedes  by 
several  words  the  full  interpretation  and  transcription  of  the  message. 
Such  overlapping  is  a  result  of  practice;  the  "span  of  attention," 
it  may  be  said,  is  lengthened  by  practice.  But  the  increase  in  the 
attention  span  does  not  mean,  mainly,  that  a  large  number  of  dis- 
tinct items  are  held  at  once  in  consciousness;  the  meaning  is,  the 
rather,  that  these  items  are  now  grasped  in  larger  units,  such  as 
words  and  phrases.  In  a  sense  it  is  true  that  the  expert  at  the  type- 
writer carries  in  mind  twenty  or  thirty  letters  at  once,  and  that  the 
expert  telegrapher,  in  receiving,  carries  in  mind  over  two  hundred 
clicks;  for  the  total  reaction  in  progress  at  any  moment  covers  this 
large  number  of  elements.  But  the  letters  or  clicks  do  not  remain 
separate  in  the  performer's  mind;  they  take  the  form  of  "units," 
consisting  of  words,  phrases,  etc.  The  lengthened  span  of  atten- 
tion,  and  the  extensive  overlapping  of  processes,  are  the  result  of 
the  higher  units  in  which  the  reaction  is  carried  on. 

§  15.  The  rate  at  which  improvement  progresses  with  practice  is 
seldom  steady.  Usually  the  curve  of  learning  shows  temporary 
descents  in  the  midst  of  the  general  rise.  Many  brief  relapses  can 
be  explained  by  poor  bodily  condition,  or  by  distractions  and  fail- 
ures of  interest  and  effort.  But  in  some  of  these  curves  there  ap- 
pear longer  periods  of  no  appreciable  gain,  which  can  scarcely  be 
explained  in  this  way.  For  weeks,  it  may  be,  the  learner  remains  at 

1  A  similar  demonstration  of  the  change  from  lower  to  higher  units  of  reaction 
has  been  obtained  by  Leuba  (Psychol.  Rev.,  1905,  VII,  351)  in  a  much  simpler 
performance,  which  consisted  in  transliterating  German  script  into  English 
script,  and  vice  versa — a  key  of  equivalents  being  at  hand  for  consulta- 
tion. At  first  the  German  letters  were  read  and  transliterated  one  by  one,  but 
later  words  were  reacted  to  as  units. 


THE  RATE  OF  IMPROVEMENT  561 

a  standstill,  and  seems  to  have  reached  the  limit  of  his  capacity. 
And  then,  if  he  perseveres,  a  sudden  advance  may  carry  him  to 
quite  a  new  level  of  efficiency.  He  apparently  begins  a  fresh  ascend- 
ing curve  on  the  top  of  his  old  one.  Such  a  compound  curve  is  seen 
in  Fig.  149  (the  curve  for  receiving  by  telegraph).  A  period  of  no 
gain,  followed  by  rapid  improvement,  has  been  named  the  plateau, 
and  the  study  of  the  process  of  learning  raises  the  question,  whether 
this  "plateau"  is  a  universal  and  necessary  feature  of  the  practice 
curve.  It  can  hardly  be  called  universal,  for  it  does  not  clearly 
appear  in  many  curves;  as,  for  example,  in  the  sending  curve  in 
telegraphy,  and  in  most  of  the  animal  curves.  In  practice  with 
puzzles,  however,  the  plateau  is  a  frequent  occurrence,  and  its  ex- 
ternal explanation  is  usually  to  be  found  in  the  adoption  of  some  new 
and  improved  method,  after  a  prolonged  period  of  practice  with  in- 
ferior methods.  In  the  case  of  the  long-protracted  plateaus  which 
appear  in  typewriting  and  telegraphy,  change  of  method  is  again  the 
explanation  of  the  rise  from  the  plateau.  The  plateau  occurs  where 

)  further  improvement  is  impossible  without  some  change  of  method. 
In  typewriting  and  telegraphy,  the  required  change  consists  in  the 
substitution  of  words  and  phrases  for  letters  as  the  units  of  perform- 
ance. In  typewriting  Book  found  two  plateaus,  one  at  the  transi- 
tion from  letter-reactions  to  word-reactions,  and  another  at  the  tran- 
sition from  word-reactions  to  phrase-reactions.  But  why  should 
not  these  transitions  occur  promptly,  without  the  long  period  of 
no  apparent  gain?  Bryan  and  Harter  believe  that  real  gain  is 
being  made,  below  the  surface,  in  the  further  perfection  of  the  more 

Elementary  reactions;  and,  in  support  of  this  view,  they  are  able  to 
show,  in  the  case  of  receiving  telegraphic  messages,  that  ability  to 
receive  isolated  letters  and  detached  words  does  slowly  improve 
during  the  plateau.  Until,  therefore,  these  simpler  forms  of  reac- 
tion  have  reached  a  high  degree  of  facility  and  promptness,  the  higher 
reactions  are  impracticable.  Swift,1  who  has  investigated  the  proc- 
ess of  learning  in  a  variety  of  activities,  including  the  learning  of  a 
language,  subscribes  in  the  main  to  this  view  of  Bryan  and  Harter, 
but  shows  that  the  higher  forms  of  reaction  do  not  all  wait  till  the 
end  of  the  plateau,  for  some  of  them  are  present  from  an  early  stage. 

i  Book's  observations  lead  him  to  a  somewhat  different  conclusion. 
He  found  that  the  beginning  of  a  plateau  was  marked,  introspec- 
tively,  by  some  loss  of  interest  in  the  work,  and  by  a  tendency  of 
the  mind  to  wander.  A  little  later,  the  learner  becomes  aware  of 
his  lack  of  progress,  and  may  increase  his  efforts;  but  at  this  par- 
ticular time  his  efforts  are  likely  to  be  ill  directed  and  result  in  no 
improvement.  What  he  needs  is  attention  to  the  details  of  work- 
1  Mind  in  the  Making,  p.  208  (New  York,  1908). 


562       MEMORY  AND  THE  PROCESS  OF  LEARNING 

manship;  what  he  is  apt  to  strive  for  is  speed  or  higher  methods 
which  are  at  that  time  not  wholly  within  his  grasp  and  only  dimly 
understood.  The  loss  of  interest  which  initiates  the  plateau  is 
easily  explained  by  the  comparative  facility  which  has  been  achieved 
in  a  certain  lower  form  of  reaction.  Attention  is  set  free  from  many 
of  the  details  which  have  hitherto  held  it.  What  is  needed  then, 
from  the  introspective  point  of  view,  is  to  find  new  objects  of  at- 
tention, and  new  modes  of  reaction  which  shall  nevertheless  lie 
writhin  the  learner's  powers  at  the  stage  which  he  has  reached. 
Specially  ingenious  individuals  may,  as  Book  found,1  come  quickly 
upon  such  modes  of  reaction,  and  avoid  the  plateau  altogether. 
Book  concludes  that  the  plateau  is  actually,  as  well  as  apparently, 
a  period  of  no  progress;  but  this  conclusion  does  not  take  account 
of  Bryan  and  Barter's  demonstration  of  progress  in  the  lower  habits 
during  this  period,  nor  does  it  explain  how  the  plateau  is  ever  left 
behind.  The  probability  is  that  continued  practice  does  perfect 
elementary  reactions  to  some  extent,  and  that  it  thus  becomes  easier 
to  enter  upon  the  more  inclusive  reactions. 

It  seems  to  us,  however,  that  all  explanations  of  the  process  of 
learning,  and,  especially  in  such  cases  as  we  are  now  considering, 
of  the  occurrence  of  the  so-called  "plateau,"  which  do  not  concen- 
trate attention  upon  what  is  probably  going  on  in  the  mechanism 
of  the  central  nervous  system,  are  surely  destined  to  prove  unsatis- 
factory. Perhaps  it  would  express  the  truth  better  to  say,  that  all 
explanations  are  satisfactory  only  in  so  far  as  they  involve  certain 
facts  and  laws  of  the  central  nervous  mechanism.  It  is  plain  that 
as  the  overlapping  of  the  psycho-physical  processes  proceeds,  and  the 
accompanying  higher  and  higher  psycho-physical  units  take  place, 
more  numerous  and  more  locally  distant  cerebral  elements  are  be- 
coming involved.  This  fact  implies,  of  necessity,  more  numerous 
chances  of  inhibitory  processes,  and  also  increased  difficulty  in 
effecting  the  required  co-ordination  of  these  elements;  or — to  speak 
figuratively — of  establishing  those  coherent  "dynamical  associa- 
tions" which  are  the  psycho-physical  basis  of  the  higher  unities. 
That  cerebral  associated  activities  of  this  complicated  and  locally 
diverse  character,  as  respects  the  nervous  elements  taking  part  in 
them,  should  advance  by  "fits  and  starts"  rather  than  always  at  a 
uniform  pace,  seems  to  us,  therefore,  to  accord  perfectly  with  the 
nature  of  the  central  nervous  mechanism  itself.  Indeed,  we  might 
well  venture  to  say,  that  such  a  way  of  development  accords  with 
the  nature  of  all  living  substance.  The  chemico-physical  changes 
in  which  the  life  of  this  substance  consists  seems  to  require  this 
irregular  manner  of  behavior. 

1  Op.  cit.,  p.  20. 


RELATION  OF  CONSCIOUSNESS  TO  LEARNING    563 

§  16.  The  relation  of  consciousness  to  the  process  of  learning  is 
an  important  topic  on  which  the  conclusions  of  different  investi- 
gators are  still  somewhat  at  variance.  Several  questions  can  be 
raised  under  this  general  head  as  to  the  participation  of  conscious- 
ness, first,  in  the  origination  of  new  methods;  second,  in  the  selec- 
tion of  one  method  and  the  rejection  of  others;  and  last,  in  the  later 
stages  of  practice,  when  automatism  has  become  the  condition  of 
the  acquired  skill. 

In  regard  to  the  first  question,  as  to  how  consciousness  is  con- 
cerned in  the  origination,  invention,  or  "  hitting-upon  "  of  new  meth- 
ods, there  is  little  doubt  that  this  process  is  essentially  the  same  as 
the  "varied  reaction"  observed  in  animals.  Each  new  method 
comes  first  as  a  spontaneous  variation.  But  in  human  learning  the 
variation  may  occur  in  the  form  of  a  recalled  "  idea,"  or  in  the  form 
of  a  clear  perception  of  some  hitherto  unnoticed  feature  of  the  situa- 
tion. In  other  cases,  however,  the  variation  occurs  first  as  a  motor 
reaction.  In  which  of  these  several  forms  it  shall  occur  depends  on 
the  nature  of  the  situation  in  relation  to  the  past  experience  of  the 
learner.  Where  the  difficulties  encountered  are  similar  to  those 
which  have  previously  been  met,  there  is  likelihood  that  past  ex- 
periences or  ideas  derived  therefrom  will  be  consciously  recalled. 
But  where  the  situation  is  very  unusual,  the  contributions  of  past 
experience  are  vague  and  not  suited  for  definite  ideational  recall. 
In  such  cases,  the  new  variation  has  to  be  achieved  in  the  thick  of 
the  fight,  so  to  say.  Then  the  individual  finds  himself  acting  in 
some  new  way,  which  he  did  not  plan  nor  foresee,  and  which  as  yet 
he  does  not  fully  understand. 

Unusual  situations  and  difficulties  are  often  created  by  the  process 
of  learning  itself;  for  when  once  the  learner  has  acquired  a  consid- 
erable degree  of  skill,  the  difficulties  which  next  confront  him  are 
such  as  do  not  occur  in  an  unskilled  performance.  Unusual  pre- 
cision of  manipulation  may  be  demanded,  or  overlapping  and  higher 
units  of  reaction  may  be  necessary  for  further  advance.  Past  ex- 
perience is  not  likely  to  provide  modes  of  reaction,  or  ideas,  which 
are  applicable  to  the  case  in  hand.  The  learner  may  indeed  have 
previously  acquired  high  skill  in  some  other  performance,  such  as 
the  use  of  language;  but  each  new  skilled  performance  must  be  pre- 
cisely adapted  to  the  particular  material  dealt  with,  and  previous 
experience  can  only  furnish  vague  hints  for  guidance.  This  vague- 
ness of  relation  of  the  present  to  the  past  situation  not  only  makes  it 
difficult  to  apply  past  acquisitions,  even  if  they  are  recalled,  but  it 
also  renders  the  recall  of  them  doubtful.  Consequently  the  new 
variations  which  lead  to  the  highest  skill  are  likely  to  make  their 
appearance  in  the  midst  of  actual  performance  and  without  the 


564      MEMORY  AND  THE  PROCESS  OF  LEARNING 

foreknowledge  of  the  learner.  This  fact  has  sometimes  been  rather 
ineptly  expressed  by  saying  that  the  new  variations  occur  "  uncon- 
sciously." By  this,  it  is  by  no  means  intended  that  they  occur  in  a 
condition  of  unconsciousness,  or  even  of  inattention;  on  the  con- 
trary, it  is  recognized  that  they  are  unlikely  to  occur  at  all  except 
in  a  condition  of  "  rapt  attention  "  to  the  work  in  hand.  Nor  are  they 
likely  to  occur  in  one  part  of  the  performance,  when  attention  is 
directed  to  another  part;  but  they  occur  where  attention  is  keenest, 
and  often  where  attention  is  striving  to  enlarge  its  span  or  to  per- 
fect its  grasp.  They  occur,  therefore,  in  the  very  focus  of  conscious- 
ness, but  are  "unconscious"  in  the  loose  sense  that  they  are  un- 
premeditated and  not  reflectively  observed.  They  are  not  thought 
about  at  the  moment  when  they  first  occur;  consciousness  is  too  much 
absorbed  in  doing  the  thing  to  classify  it  or  speculate  about  it. 

As  to  the  second  point — namely,  the  participation  of  consciousness 
in  the  selection  and  rejection  of  methods  of  reaction — here,  too,  no 
one  general  rule  is  applicable  to  all  cases.  Sometimes  new  varia* 
tions  of  method,  without  being  reflected  on  or  definitely  conceived, 
become  habitual  and  automatic.  In  such  cases,  the  learner  never 
really  knows  how  he  has  come  to  gain  the  skill.  At  other  times,  the 
variation  catches  the  eye  of  reflection,  is  definitely  made  note  of  and 
perhaps  formulated  in  words,  and  the  learner  can  later  tell  by  what 
means  he  has  improved.  At  times  this  reflective  attitude  proves  to 
be  of  value;  for  a  definitely  conceived  and  formulated  reaction  is 
more  readily  recalled  under  somewhat  changed  conditions. 

As  to  the  third  point,  there  is  no  doubt  that  attention  tends  to 
desert  a  reaction  which  has  become  well-drilled  and  facile,  and  to  \ 
pass  to  something  else;  in  illustration  of  this,  the  feats  of  skilled 
performers  on  any  instrument,  while  their  minds  are  occupied  with 
something  else,  are  well  known.  But  the  very  highest  skill  is  never  * 
manifested  except  when  the  performance  is  fully  occupying  the  at- 
tention. No  matter  how  far  the  elementary  reactions  have  become 
automatic,  there  are  always  higher  units  and  combinations  which 
demand  the  best  efforts  of  the  performer.  Given,  then,  a  constant 
degree  of  zeal  in  the  learner,  the  progress  of  practice  does  not  lead 
to  a  diminution  of  the  consciousness  attending  the  performance, 
but  to  a  change  in  the  distribution  of  consciousness.  The  expert 
finds  within  the  work  new  objects  of  attention  which  lie  entirely 
beyond  the  ken  of  the  beginner. 

The  changes  of  feeling  which  occur  in  the  course  of  practice  are 
also  noteworthy.1  A  new  occupation,  in  which  there  is  some  de- 
gree of  immediate  success,  is  interesting  and  pleasurable;  as  the 
near-lying  difficulties  are  overcome,  a  feeling  of  monotony  and  un- 

1  Book,  op.  cit.,  p.  71. 


TRANSFERENCE  OF  LEARNING  565 

pleasantness  ensues;  but  as  the  possibilities  of  higher  forms  of  re- 
action open  before  the  learner,  the  pleasure  in  the  work  returns,  and 
the  keenest  pleasure  of  all  may  be  felt  by  the  expert,  if  only  there  is 
sufficient  incentive  to  spur  him  to  his  best  efforts. 
-^flT?  How  far  skill  acquired  in  one  performance  can  be  "  trans- 
ferred" to  another  performance — how  far  success  in  dealing  with 
one  situation  is  an  equipment  for  dealing  with  other  situations — 
is  a  problem  of  evident  importance,  regarding  which,  however, 
great  differences  of  opinion  have  appeared.  Some  remarks  on 
this  matter  have  been  included  in  the  preceding  discussions  (p. 
550),  and  need  not  be  repeated  here.  The  history  of  the  question 
begins  with  Volkmann1  and  Fechner,2  who  reported,  in  1858,  ob- 
servations tending  to  show  that  training  one  function  might  result 
in  the  improvement  of  other  closely  related  functions.  In  Volk- 
mann's  case,  the  function  trained  was  the  discrimination  of  two 
points  applied  to  the  skin  (the  "  two-point  threshold,"  compare  p. 
402)  of  the  left  arm,  and  the  other  function  thus  improved  was  the 
same  discrimination  when  the  points  were  applied  to  the  right 
arm.  Also,  training  applied  to  one  finger  improved  the  power  of 
discrimination  in  the  other  fingers,  but  not  in  the  arm.  Fechner's 
observations  showed  that  teaching  a  boy  to  write  with  his  right  hand 
might  give  him  power  to  write  with  the  left  hand  also.  This  special 
form  of  the  transfer  of  acquired  skill,  between  symmetrical  parts  of 
the  body,  received  the  name  of  "cross-education"  from  Scripture,3 
who,  with  his  pupils,4  demonstrated  the  reality  of  it  in  a  variety  of 
cases,  such  as  speed,  force,  and  accuracy  of  movement. 

The  probable  explanation  of  cross-education  lies  in  the  fact 
(compare  p.  263)  that  the  same  portion  of  the  left  hemisphere 
(in  right-handed  persons)  is  concerned  in  skilled  movements  of 
either  hand,  or  in  the  skilled  use  of  sensory  data  from  either  side. 
If  an  essential  part  of  the  neural  mechanism  for  co-ordinating  a 
skilled  movement  is  the  same,  whether  that  movement  is  executed 
by  the  right  or  by  the  left  hand,  then,  naturally,  training  of  either 
hand  should  improve  the  other  hand  also.  But  since  some  of  the 
neural  connections  are  different  for  the  two  hands,  training  of  one 
hand  would  still  leave  something  to  be  accomplished  by  special 
training  of  the  other; — and  this  deduction  also  corresponds  to  the 
facts,  for  the  untrained  hand  does  not  at  once  show  all  the  skill  of 
the  trained  hand,  but  only  some  of  it. 

1  Berichte  d.k.-sachs.  Ges.  d.  Wissensch.,  math.-phys.  KL,  1858,  X,  38. 

2  Ibid.,  1858,  X,  70. 

3  Scripture,  Smith  and  Brown,  Studies  from  the   Yale  PsychoL  Lab.,   1894, 
II,  115. 

*  Davis,  ibid.,  1898,  VI,  6;   Scripture,  PsychoL  Rev.,  1899,  VI,  165. 


566      MEMORY  AND  THE  PROCESS  OF  LEARNING 

This  relatively  simple  case  may  contain  the  germ  of  an  explana- 
tion of  other  cases  of  transferred  training.  If  the  brain  functioned 
as  a  whole,  we  should  expect  that  training  in  any  special  perform- 
ance would  operate  equally  to  produce  the  improvement  of  all 
other  performances;  if  it  functioned  in  compartments,  correspond- 
ing to  "memory,"  "discrimination,"  or  to  other  so-called  "facul- 
ties" or  phrenological  organs,  then  we  should  expect  training  of 
any  special  performance  within  the  scope  of  a  faculty  to  benefit  alike 
all  other  performances  included  within  that  faculty.  But  since  the 
evidence  points  to  a  highly  detailed  localization  of  cerebral  func- 
tions, and  since  the  neural  mechanism  employed  in  any  performance 
cannot  be  wholly  identical  with  that  required  for  slightly  different 
performances,  though  it  may  be  partly  the  same,  training  in  one 
performance  would  not  be  expected  to  improve  another,  except 
in  so  far  as  the  neural  mechanisms  involved  were  in  part  identical 
— i.  e.,  employed  the  same  cells,  fibres,  and  synapses.  As  applied 
to  psychology,  this  would  mean  that,  in  order  for  a  transference  of 
skill  to  occur  from  one  performance  to  another,  there  should  be, 
between  the  two,  not  simply  likeness  in  the  abstract,  but  some  con- 
crete part-performance  in  common,  as  there  is  between  boxing  and 
fighting,  or  between  saying  "  boot"  and  saying  "  book."  In  general, 
since  the  neural  process  in  any  reaction  undoubtedly  has  more  de- 
tail than  appears  either  to  introspection  or  to  objective  observation, 
it  will  not  always  be  possible  to  point  out  the  common  features  of 
two  reactions;  but  that  there  should  be  features  in  common,  if  any 
transference  of  training  is  possible  from  one  to  the  other,  seems 
necessary  from  the  physiological  point  of  view. 

§  18.  But  leaving  these  speculations  for  the  present,  let  us  return 
to  the  investigations  of  fact.  James1  opened  a  new  line  of  inquiry 
by  seeking  to  determine  whether  training  the  memory  for  one  kind 
of  material  increased  the  power  of  remembering  other  kinds.  His 
method  was  to  practise  memorizing  the  poetry  of  one  author,  and 
to  test  beforehand  and  afterward  the  ability  to  retain  another  au- 
thor. His  results  showed,  on  the  whole,  a  lack  of  improvement  of 
the  memory,  except  for  the  material  on  which  it  was  trained.  The 
exceptions  he  believed  could  be  explained  by  improved  methods  of 
memorizing;  he,  therefore,  concluded  that  retentiveness,  as  such, 
was  not  susceptible  of  improvement  by  training. 

The  problem  of  memory  training  was  taken  up  again  with  the 
same  general  method,  by  Ebert  and  Meumann,2  who  made  more  ex- 
tensive experiments,  but  came  to  a  somewhat  different  conclusion. 
These  investigators  trained  their  subjects  in  memorizing  lists  of 

1  Principles  of  Psychology,  1890,  I,  666. 
8  Archiv  /.  d.  ges.  Psychol,  1905,  IV,  1. 


TRANSFERENCE  OF  LEARNING  567 

/ 

nonsense  syllables,  and,  before  and  after  this  course  of  training, 
took  a  "  cross  section  "  of  the  subjects'  powers  of  memory  by  testing 
them  with  lists  of  letters,  numbers,  disconnected  words,  vocabularies, 
passages  of  prose  and  of  poetry,  and  meaningless  visual  characters. 
In  all  these  tests  they  found  great  improvement,  but  much  more 
in  some  than  in  others.  It  seemed  to  them  that  the  improvement 
was  great  in  proportion  as  the  new  material  resembled  the  nonsense 
syllables  with  which  the  subjects  were  trained;  and  they  interpreted 
the  results  to  mean  that  memory,  trained  by  use  of  one  sort  of  mate- 
rial, is  thereby  trained  for  other  sorts;  not  however  equally  for  all 
sorts,  but  most  for  those  sorts  which  are  closely  related  to  the  ma- 
terial used  in  the  training.  They,  therefore,  conceive  of  several 
special  "memories"  in  place  of  the  old  faculty  of  memory,  and 
within  the  scope  of  each  of  these  sub-faculties  they  suppose  that  skill 
is  readily  transferred  from  one  performance  to  another.  This  is 
all  rather  mysterious,  and  it  is  more  to  the  point  to  note  that  many 
improved  methods  of  memorizing  were  invented  by  the  subjects 
during  their  training,  and  that  some  of  these  methods — such  as 
giving  a  rhythmic  form  to  a  list — were  readily  applied  to  different 
sorts  of  material.  Feelings  of  distaste,  strain,  and  doubt  gave  way, 
with  continued  experience  and  success,  to  pleasure  and  confidence 
in  the  work.  Distractions  and  useless  reactions  were  better  re- 
pressed; the  proper  direction  of  attention  was  better  understood; 
and  the  subject  came  to  know  his  own  powers  and  limitations. 

A  similar  line  of  experimentation,  with  similar  but  better-analyzed 
results,  was  undertaken  by  Fracker.1  He  trained  his  subjects  in 
an  unusual  feat  of  memory,  as  follows :  four  sounds,  differing  in  in- 
tensity, were  presented  in  varying  orders,  and  the  subject  had  to 
notice  the  order  of  each  presentation,  and  reproduce  it,  designating 
each  sound  by  a  number;  but  before  he  was  allowed  to  reproduce 
one  order,  he  was  required  to  observe  a  second;  and  the  second 
tended  to  interfere  seriously  with  the  first  and  cause  confusion. 
In  this  novel  situation,  the  subjects  tried  a  variety  of  means  to  es- 
cape from  the  difficulty,  and  most  of  them  hit  upon  some  successful 
device.  In  some  of  the  subsequent  tests  with  other  material,  the 
device  so  learned  was  readily  applicable  and  these  tests  profited 
by  the  transference  of  the  device.  But  to  other  material,  such  as 
poetry,  these  devices  were  not  applicable;  and  here  the  tests  showed 
no  improvement  as  the  result  of  the  special  training  in  memorizing 
the  order  of  sounds.  The  author's  main  conclusion  is  that  effective 
memory-training  consists  in  the  development  of  methods  of  memo- 
rizing. 

Most  of  the  studies  of  transference  of  acquired  skill  have  suffered 
1  Psychol.  Rev.,  Monogr.  Suppl.  No.  XXXVIII,  1908,  p.  56. 


568       MEMORY  AND  THE  PROCESS  OF  LEARNING 

from  an  obvious  defect  of  method,  which  lies  in  the  fact  that  the 
performances  which  are  supposed  to  receive  no  direct  training  must 
themselves  nevertheless  be  tested,  and  so  exercised.  But  even  a 
little  practice  in  a  novel  performance  may  cause  considerable  im- 
provement, and  therefore  the  amount  of  special  training  received 
by  the  "untrained"  performances  is  by  no  means  negligible.  In 
fact,  Dearborn,1  on  repeating  the  preliminary  and  final  tests  of 
Ebert  and  Meumann,  without  the  intervening  special  training, 
found  a  considerable  improvement,  and  thus  showed  that  the  results 
they  obtained  need  to  be  considerably  discounted. 

§  19.  It  was  a  much-needed  addition  to  the  methods  of  investiga- 
tion when  Bair2  tried  the  plan  of  following  up  the  training  of  one 
Eerformance  by  the  training  of  another,  to  see  whether  the  curve  of 
earning  of  the  later-trained  performance  would  show  any  effect  from 
the  previous  training.  In  his  experiments  he  made  use  of  a  type- 
writer, using  only  a  part  of  the  keys,  which  he  covered  with  different 
colors,  while  his  "copy"  consisted  of  series  of  color  stimuli,  to  which 
the  subject  reacted  by  striking  the  corresponding  keys.  After  one 
particular  series  of  colors  had  been  copied  time  after  time  till  great 
speed  and  accuracy  were  attained,  the  order  of  colors  was  changed 
and  a  new  learning  curve  obtained.  The  new  performance  showed 
from  the  start  the  influence  of  the  previous  training;  for  though  the 
new  series  was  not  at  first  written  as  rapidly  as  the  old  had  come  to 
be  written,  it  was  written  more  rapidly  than  the  other  had  been  at 
the  start;  moreover,  it  improved  rapidly,  soon  passing  the  limit  of 
efficiency  reached  in  the  first  series.  This  result  would  indeed  be 
expected,  for  the  single  reactions  remained  the  same  as  before;  the 
same  key  was  struck  in  response  to  each  color,  and  only  the  order 
of  the  colors  was  changed.  The  fact  that  there  was  some  loss  in  pass- 
ing from  one  order  to  another  proves  that  the  performance  first 
learned  consisted  partly  in  a  reaction  to  the  order  of  the  stimuli; 
and  this  was  an  untransferable  feature  of  the  performance.  In 
this  way,  then,  both  the  gain  in  general  facility  and  the  loss  in 
special  facility  are  accounted  for. 

Bair  now  interchanged  the  colors  on  the  keys,  so  that  a  new  re- 
action was  required  to  each  single  stimulus,  though  the  order  of 
stimuli  remained  as  it  had  been  just  previously.  This  change  in 
the  character  of  the  single  reactions  proved  to  be  more  of  a  dis- 
turbance than  the  previous  change  in  order;  yet  in  this  case,  too,  the 
new  performance  did  not  start  back  where  the  first  had  started,  but 
showed  from  the  beginning  some  of  the  skill  acquired  in  the  previous 
training.  From  the  point  at  which  it  started,  the  new  performance 

lPsychol.  Bulletin,  1909,  VI,  44. 

3  PsychoL  Rev.,  Monogr.  Suppl.  No.  XIX,  1902. 


TRANSFERENCE  OF  LEARNING  569 

also  made  more  rapid  progress  than  the  first  had  done — thus  show- 
ing, beyond  doubt,  a  transference  of  the  previously  acquired  skill. 
What  was  transferred  was  probably,  besides  acquaintance  with  the 
order  of  the  stimuli,  an  adjustment  to  the  apparatus  and  general 
conditions  of  the  experiment,  which  remained  the  same  throughout. 

The  method  of  Bair  has  also  been  employed  in  the  previously 
mentioned  researches  of  Leuba,  Book,  and  Ruger.  The  latter  noted 
several  kinds  of  transference:  transference  of  special  methods;  trans- 
ference of  more  "general"  methods,  such  as  transference  of  the  habit 
or  idea  of  analyzing  each  new  situation,  or  of  trying  to  induce  vari- 
ations instead  of  repeating  an  unsuccessful  reaction  time  after  time; 
transference  of  a  confident  and  self-reliant  attitude  toward  a  new 
situation;  transference  of  the  habit  or  idea  of  keeping  up  active 
attention  during  the  course  of  practice,  and  of  looking  for  improved 
methods  and  higher  units,  instead  of  settling  down  to  a  mediocre 
performance.  Transfer  is  readiest  in  the  realm  of  ideas;  and  the 
more  definitely  a  method  of  work,  either  special  or  general,  has 
been  conceived  and  formulated,  the  wider  is  the  field  of  its  prob- 
able usefulness.  It  may  be  remarked  in  passing  that  all  these  ad- 
mittedly possible  forms  of  so-called  "transference,"  when  taken  together, 
amount  to  a  tolerably  complete  summary  of  the  most  essential  factors  in 
what  is  popularly  included  under  the  training,  or  culture,  of  the  mind. 

The  conception  of  a  general  transference  or  spreading  of  the 
effects  of  training  was  called  in  question  by  Thorndike  and  Wood- 
worth,1  who  experimented,  by  a  method  similar  to  that  of  James, 
in  performances  involving  observation,  discrimination,  estimation 
of  magnitudes,  etc.,  and  found,  in  general,  rather  a  small  amount 
of  transference  from  any  specially  trained  performance  to  others 
which,  superficially,  appear  quite  similar  to  it.  This  led  them  to 
conclude  that  the  mind  works  in  great  detail,  adapting  itself,  of 
necessity,  to  the  particular  material  with  which  it  has  to  deal;  and, 
therefore,  that  training  in  one  performance  could  only  help  another 
when  the  two  had  elementary  factors  in  common.2  This  is  to  say 
that  what  has  been  found  to  be  transferred  has  always  been  some 
specific  habit,  or  reaction,  or  idea,  or  attitude;  although  it  need  not 
always  be  a  motor  reaction.3 

§  20.  Another  line  of  evidence  is  available  regarding  the  transfer 
of  training  in  memory.  It  is  not  necessary  that  the  kind  of  material 

1  Psychol.  Rev.,  1901,  VIII,  247,  384,  553. 

2  Compare  also  Coover  and  Angell,  Amer.  Journ.  of  Psychol,  1907,  XVIII,  328. 

3  For  a  discussion  of  the  present  state  of  the  problem,  and  of  its  practical  bear- 
ings, see  a  symposium  between  Professors  Angell,  Pillsbury,  and  Judd  in  the 
Educational  Review,  1908,  XXXVI,  1,  and  also  the  works  of  Meumann  referred 
to  on  p.  571. 


570      MEMORY  AND  THE  PROCESS  OF  LEARNING 

should  be  changed,  in  order  to  test  for  transference.  Does  learning 
one  list  of  twelve  nonsense  syllables  make  it  easier  to  learn  the  next 
list,  composed  of  other  nonsense  syllables?  Of  this  there  is  no 
doubt;  the  times  needed  for  learning  such  series  decrease  rapidly 
with  practice.  Now  the  particular  associations  formed  in  learning 
one  series  are  different  from  those  formed  in  learning  another;  and 
yet  the  learning  of  one  set  increases  the  power  to  learn  other  sets. 
This,  it  would  seem,  is  as  good  evidence  as  could  be  desired  of 
training  one  performance  by  practising  another.  But  there  is  an- 
other curious  fact  to  be  considered  in  connection  with  the  foregoing. 
The  learning  of  one  such  list  may  interfere  greatly  with  the  learn- 
ing of  another  similar  list,  especially  if  little  time  intervenes  be- 
tween the  two.  If,  immediately  after  one  list  has  been  learned, 
another  is  begun,  it  will  be  found,  later,  that  neither  list  is  as  well 
retained  as  if  either  one  had  been  attempted  alone.  Therefore  it 
is  not  the  particular  associations  of  the  one  list  which  favor  the  as- 
sociations of  the  other.  There  is  no  spread  of  associative  power 
from  one  association  to  another  unrelated  association;  but  there  may 
be  improvement,  through  practice,  in  the  process  of  memorizing.  - 

Let  us  consider  the  meaning  of  this  experience.  When  a  person 
first  takes  part  in  a  memory  test,  there  are  many  unaccustomed 
features  of  the  situation  to  which  he  must  react.  He  must  adjust 
himself  to  the  apparatus  and  procedure  and  to  the  peculiarities 
of  the  experimenter;  he  must  master  his  own  distaste  for  the  monot- 
onous work;  he  must  exclude  the  distractions  which  are  inherent 
in  the  circumstances  and  in  himself;  he  must  become  negatively 
adapted  to  these,  as  one  becomes  adapted  to  the  ticking  of  a  clock. 
The  material  to  be  learned  has  some  special  but  rather  uniform 
character,  and  he  readily  becomes  accustomed  to  that;  some  ways 
of  trying  to  memorize  it  are  good  and  others  bad,  and  he  has  a 
chance  to  learn  the  most  suitable  method.  In  a  word,  the  situation 
to  which  he  reacts  is  decidedly  complex,  and  his  total  reaction  is 
correspondingly  complex.  What  he  learns  is  not  merely  the  list  of 
syllables;  but  he  learns,  or  begins  to  learn,  how  to  react  to  that  par- 
ticular complex  situation;  and  when  a  similar  list  is  later  presented 
to  him,  a  large  share  of  the  situation  remains  the  same  and  can  be 
reacted  to  as  before.  Thus,  by  degrees,  he  comes  to  master  that  type 
of  situation;  and  even  if  the  situation  is  changed  somewhat  by  the 
introduction  of  a  new  sort  of  material  to  be  learned,  many  of  his  old 
partial  reactions  are  still  applicable  to  changes  in  the  material. 

It  would  seem,  then,  that  the  most  practical  sort  of  memory- 
training,  for  ordinary  purposes,  is  probably  to  be  obtained  by  con- 
necting together  things  that  belong  together  for  some  purpose  in 
hand,  and  so  building  up  a  system  of  valuable  associations.  The 


THE  PROCESSES  OF  MEMORIZING  571 

suggestion  of  Meumann1  to  the  effect  that  a  species  of  "  formal  dis- 
cipline" of  the  memory  might  lead  to  good  results,  is  not  without 
force;  since  what  he  means  by  formal  discipline  is  memory  work 
under  conditions  resembling  those  of  the  experiments  which  have 
proved  to  lead  to  greatly  improved  technique  in  the  memorizing 
of  certain  kinds  of  matter.  Experimental  conditions  are  stimulat- 
ing, largely  because  one  has  a  measure  of  one's  success  and  progress; 
and  the  habit  of  checking  up  one's  work  can  scarcely  fail  to  prove 
of  benefit  wherever  measures  of  success  and  failure  are  practicable. 
\/'  §  21.  One  other  most  important,  and  indeed  essential,  considera- 
tion must  certainly  be  borne  in  mind,  in  every  attempt  to  deal  with 
the  problems  of  memorizing  and  of  the  "transference"  of  acquired 
skill.  In  studying  the  development  of  sense-perception,  we  saw 
that,  from  the  introspective  point  of  view,  attention  and  discrimina- 
tion are  involved  in  all  human  knowledge  and  in  all  forms  of  human 
learning.  From  the  same  point  of  view,  these  both  appear  as  active 
forms  of  consciousness  in  all  memorizing.  But  it  is  "I"  that  at- 
tend, and  "I"  that  discriminate.  What  forms  of  cerebral  condi- 
tions, or  of  cerebral  changes,  correspond  to  these  conscious  activities, 
we  may  be  much  at  a  loss  to  point  out.  But  everything  which  we 
do  know  indicates  that  these  conditions  and  changes  must  be  of  a 
general  character  to  correspond  with  the  general  character  of  the 
mental  aptitudes  involved.  It  might  be,  then,  quite  appropriate 
to  speak  of  training  the  "faculty"  of  attention,  and  the  "faculty" 
of  discrimination,  so  as  to  pass  over  the  results  of  this  training  fr.om 
one  species  of  skilled  reactions  to  anothgr.  In  all  such  cases,  the 
well-known  distinction  between  the  speed  of  the  total  motor  reac- 
tion, or  of  the  total  act  of  committing  to  memory,  and  the  factor 
consumed  by  the  acts  of  attending  and  discriminating,  must  be  taken 
into  the  account.  For  example,  experiments  conducted  in  the  Yale 
Laboratory  showed  that  while  the  reactions,  under  customary  con- 
ditions, of  the  master  of  a  fencing  club  were  speedier  than  those  of 
any  of  the  members  of  the  club,  when  discrimination  was  required 
by  unusual  conditions,  his  discrimination-time  was  slowest  of  all. 
And  two  Yale  professors,  one  of  whom  had  never  used  foils,  while 
the  other  had  not  practised  fencing  for  many  years,  excelled  in  the 
speed  of  their  "discrimination-time"  every  member  of  the  club 
with  the  single  exception  of  a  gentleman  who  was  himself  both  a 
skilled  fencer  and  highly  educated.  Indeed,  to  claim  that  trained 

1  Vorlesungen  zur  Einfuhrung  in  die  experimentelle  Pddagogik,  I,  200  (Leipzig, 
1907) ;  Okonomie  und  Technik  des  Geddchtnisses,  p.  258  (Leipzig,  1908). 

Improvement  of  memory  with  practice  under  experimental  conditions,  has 
also  been  found  by  Winch  (Brit.  Journ.  of  Psychol,  1904,  I,  127;  1906,  II,  52; 
1908,  II,  284)  to  occur  in  school-children. 


572      MEMORY  AND  THE  PROCESS  OF  LEARNING 

powers  of  attention  and  discrimination  are  not  available  for  trans- 
ference to  unusual  situations  would  contradict  the  whole  round  of 
human  experience.  At  the  same  time,  it  must  be  admitted  that  a 
high  standard  of  specialized  skill  in  certain  lines  may  hinder,  rather 
than  help,  the  rapid  attainment  of  skill  in  other  non-cognate  lines, 
through  the  large  number  of  inhibitory  processes  which  it  may  in- 
troduce, if  in  no  other  way.  Concentration  of  attention  may  also 
be  opposed  to  nimbleness  of  attention.  In  a  word:  It  may  be  possi- 
ble, by  training,  to  increase  the  speed  and  improve  the  quality  of 
those  general  cerebral  conditions  and  forms  of  functioning,  to  which 
attention  and  discrimination  correspond  from  the  introspective  point 
of  view. 

§  22.  We  pass  now  to  a  study  of  that  complex  form  of  function- 
ing which  is  called  "memory,"  in  a  more  special  meaning  of  the 
word.  Investigations  of  this  subject  have  been  numerous  since 
Ebbinghaus  *  snowed  that  it  afforded  a  fruitful  field  for  experiment. 
Ebbinghaus  contributed,  first  of  all,  a  method  for  measuring  the 
degrees  of  memory  for  all  kinds  of  material;  he  further  introduced 
a  new  kind  of  material — namely,  so-called  "  nonsense  syllables,"  2 
which  possesses  the  advantage  of  being  comparatively  free  from 
ready-formed  associations;  and  he  applied  both  material  and  method 
to  the  study  of  some  fundamental  problems  of  the  formation  and 
retention  of  associations.  His  method,  which  passes  by  the  name 
of  the  "learning  method,"  consists  in  presenting  a  list  of  nonsense 
syllables  to  be  memorized,  and  in  determining  the  time,  or  the  num- 
ber of  readings,  necessary  before  the  list  can  be  recited  without  er- 
ror. Care  must  be  taken  that  the  list  is  not  "over-learned,"  i.e., 
that  more  study  is  not  given  to  it  than  the  bare  amount  necessary 
to  reach  the  standard  of  one  perfect  recitation  immediately  after  the 
study.3 

The  same  method  was  ingeniously  adapted  to  the  study  of  re- 
tention: after  a  given  list  of  nonsense  syllables  (or,  for  that  matter, 
any  other  material)  had  once  been  learned  up  to  the  above  stand- 
ard, further  work  on  it  was  suspended  for  a  certain  interval,  and  then 
it  was  relearned  to  the  same  standard,  the  time  or  number  of  readings 
needed  for  relearning  being  determined  as  before.  Two  important 

1  Uber  das  Geddchtnis  (Leipzig,  1885). 

2  A  nonsense  syllable,  as  used  by  Ebbinghaus  and  later  investigators,  con- 
sists of  a  vowel  or  diphthong  between  two  consonants. 

3  Some  investigators  have  indeed  chosen  a  higher  standard — namely,  twosuc- 
cessive  recitations  without  error;    and  have   found   that    considerable  further 
study  is  often  needed  to  reach  this  higher  standard.     See  Radossawljewitsch, 
Das  Behalten  und  Vergessen  bei  Kindern  und  Erwachsenen  nach  experimentellen 
Untersuchungen  (Leipzig,  1907). 


METHODS  OF  STUDYING  MEMORY  573 

facts  immediately  came  to  light  and  served  as  the  foundation  for 
further  use  of  the  method.  First,  when  a  list  was  learned  barely 
enough  to  permit  of  one  perfect  recitation  immediately  at  the  close 
of  the  learning,  an  interval  as  brief  as  twenty  minutes,  or  even  five 
minutes,  made  another  perfect  recitation  of  the  list  impossible;  and 
second,  after  a  much  longer  interval,  though  the  list  might  seem, 
introspectively,  to  be  altogether  forgotten,  the  time  necessary  to 
relearn  it  was  less  than  the  time  needed  to  learn  it  at  first.  This 
showed  that  the  associations  formed  in  the  first  learning  had  not  been 
entirely  obliterated;  there  was  a  partial  retention,  and  it  could  be 
measured  by  the  saving  (in  time  or  number  of  readings)  apparent 
in  the  process  of  relearning  as  compared  with  the  first  learning. 
If,  for  example,  the  first  learning  of  a  list  required  10  readings,  and 
relearning  after  a  week  required  only  8  readings,  the  saving  due 
to  partial  retention  was  2  readings,  or  20  per  cent,  of  the  original 
labor.  As  thus  applied  to  the  study  of  retention,  the  Ebbinghaus 
method  is  called  the  "saving  method." 

The  method  of  Ebbinghaus  was  improved  in  two  respects  by 
Miiller  and  Schumann.1  They  introduced  rules  for  the  prepara- 
tion of  lists  of  nonsense  syllables  which  should  be  as  nearly  as  pos- 
sible equal  in  difficulty;  and  they  provided  an  apparatus  for  expos- 
ing the  syllables  to  the  eye  at  a  fixed  speed,  so  making  the  entire 
procedure  more  uniform. 

A  second  method  of  experimentally  studying  memory  was  in- 
troduced by  Miss  Calkins,2  and  more  fully  formulated  by  Miiller 
and  Pilzecker,3  who  named  it  the  "Treffermethode."  This  name  has 
been  roughly  rendered  into  English  as  the  "method  of  hits  and 
misses,"  or  as  the  "scoring  method."  The  method  has  also  been 
named,4  and  perhaps  most  suitably,  the  "method  of  paired  associ- 
ates"; since  syllables,  words,  or  other  materials,  are  presented  in 
pairs,  the  effort  of  the  learner  being  to  associate  the  pairs.  Later, 
one  member  of  each  pair  is  presented  and  the  subject  responds,  if 
possible,  with  its  associate.  A  score  of  the  "  hits,"  or  right  responses, 
gives  the  measure  of  memory,  and  the  association  time  for  each  re- 
sponse can  also  be  determined.  This  method  has  certain  advan- 
tages over  the  other,  in  requiring  less  time,  and  in  permitting  a  more 
detailed  study  of  individual  associations.5 

1  Zeitschr.  f.  Psychol,  1894,  VI,  81,  257. 

2  "Association,"  Psychol.  Rev.,  Monogr.  Suppl.  No.  II,  1896. 

3  "  Experimentelle  Beitrage  zur  Lehre  vom  Gedachtniss,"  Zeitschr.  f.  Psychol., 
Ergdnzungsband  1, 1900;  also  Jost,  Zeitschr.  f.  Psychol,  1897,  XIV,  436. 

4  Thorndike,  Psychol.  Rev.,  1908,  XV,  122. 

5  For  the  latter  purpose,  Ebbinghaus  (Grundzuge  der  Psychologie,  1905,  I,  648) 
has  introduced  a  modification  of  the  learning  method,  which  may  be  called  the 
"prompting  method,"  and  according  to  which,  after  a  few  preliminary  readings, 
the  subject  attempts  to  reproduce  the  series  of  syllables,  and  is  immediately 


574      MEMORY  AND  THE  PROCESS  OF  LEARNING 

Two  other  simpler  methods1  are  of  use  for  some  purposes;  they 
may  be  called  the  "memory  span  method,"  and  the  "method  of 
retained  members."  The  latter,  which  might  also  be  designated 
as  a  method  of  measuring  the  accuracy  of  recall,  is  the  simplest 
in  principle  of  all,  and  consists  merely  in  measuring  how  much  of  a 
list  of  syllables,  or  of  any  other  material,  can  be  correctly  repro- 
duced. The  "memory  span"  is  the  largest  amount  of  any  given 
material  which  can  always  be  correctly  reproduced  immediately 
after  one  presentation.  For  example,  in  adults,  about  5-7  nonsense 
syllables,  8-11  one-place  numbers,  and  15-25  words  of  easy  con- 
nected prose,  can  be  so  reproduced.  The  span  is  determined  by 
starting  with  a  short  series  and  passing  to  longer  and  longer  series 
till  errors  begin  to  appear.  The  method  is  susceptible  of  various 
modifications.2 

§  23.  The  results  of  work  by  all  these  methods  are  well  worthy 
of  a  much  more  extended  analysis  than  space  will  here  permit.3 
We  may  consider  first  the  decline  of  retention  with  the  passage  of 
time.  Ebbinghaus  studied  this  matter  by  aid  of  his  saving  method, 
and,  as  the  result  of  many  experiments,  in  which,  however,  he  alone 
was  the  subject,  found  that  the  loss  of  retention  was  rapid  at  first, 
and  then  slower  and  slower.  If  retention  is  measured  by  the  per- 
centage of  the  original  time  which  is  saved  in  relearning,  the  follow- 
ing table4  shows  the  loss  of  retention  after  different  intervals: 

Interval  since  the  Per  cent,  of  saving, 

original  learning  or  of  retention 

20  min 58 

1  hour 44 

8.8  hours 36 

24  hours 34 

2  days '  ...     28 

6  days 25 

31  days 21 

prompted  or  corrected  when  he  halts  or  errs.  The  number  of  promptings  and 
corrections  gives  a  measure  of  the  degree  to  which  the  series  is  learned,  and  also 
shows  which  parts  of  it  are  learned.  The  process  can  be  repeated  till  no  more 
help  is  needed.  See  also  Ephrussi,  Zeitschr.  /.  Psychol,  1904,  XXXVII,  222. 

1  Jacobs,  Mind,  1887,  XIII,  75;   Pohlmann,  Experimentelle  Beitrage  zur  Lehre 
vom  Geddchtnis  (Berlin,  1906). 

2  Ebert  and  Meumann,  op.  cit.;  Kirkpatrick,  "Studies  in  Development  and 
Learning,"  Archives  of  Psychology,  No.  XII,  1909. 

3  Probably  the  best  general  treatment  of  the  subject  is  that  by  Ebbinghaus  in 
his  Grundzuge  der  Psychologic;  see  also  van  Biervliet,  La  memoire  (Paris,  1902). 
A  concise  account  of  experimental  results  is  given  by  Myers  in  his  Textbook  of 
Experimental  Psychology,  pp.  144-182  (London  and  New  York,  1909).     For  bib- 
liographies, see  Burnham,  Amer.  Journ.  of  Psychol.,  1888-89,  II,  39,  225,  431, 
568;  Kennedy,  Psychol.  Rev.,  1898,  V,  477;  Reuther,  Wundt's  Psychol  Studien, 
1905,  I,  93. 

4  Uber  das  Gedachtnis,  1885,  p.  103. 


THE  CURVE  OF  FORGETTING 


575 


These  results  may  be  plotted  into  a  "curve  of  forgetting "  (or  of 
retention),  by  making  the  distances  along  the  horizontal  axis  pro- 
portional to  the  time  elapsed,  and  the  vertical  distances  proportional 


144 

FIG.  150. — The  Curve  of  Forgetting  (Ebbinghaus).  The  numbers  along  the  horizontal  base 
line  give  the  hours  elapsed  since  the  time  of  learning;  the  numbers  along  the  vertical 
line  give  the  percentage  retained. 

to  the  percentage  of  material  retained.  For  comparison  with  this 
curve  of  forgetting,  a  typical  curve  of  learning  may  be  presented; 
in  which  case  a  certain  similarity  between  the  two  is  at  once  apparent, 
inasmuch  as  the  rate  of  change  in  each  is  rapid  at  first  and  then 


FIG.  151. — A  Typical  Curve  of  Learning.  The  curve  is  not  typical  in  one  respect,  since  it 
does  not  show  the  fluctuations  of  efficiency  which  always  appear;  but  it  is  intended  to 
show  the  general  increase  (rise)  of  efficiency  with  continued  repetition  of  the  performance. 
Horizontal  distances  denote  the  time  spent  in  learning,  and  vertical  distances  the  meas- 
ure of  efficiency. 


576    MEMORY  AND  THE  PROCESS  OF  LEARNING 

slower  and  slower.  On  the  other  hand,  if  the  two  curves  are  thought 
of  as  combined,  a  certain  discontinuity  between  the  fwo  is  evident, 
inasmuch  as  the  slow  increase  of  the  strength  of  association  toward 
the  end  of  the  process  of  learning  suddenly  gives  way  to  a  rapid 
loss  on  cessation  of  the  learning.  But  such  a  combination  of  the 
two  is  unjustified;  for  the  learning  wrhich  would  give  rise  to  such  a 
curve,  with  slow  gain  at  the  end,  is  carried  much  further  toward 
perfection  than  that  from  which  the  curve  of  forgetting  takes  its 
start.  Ebbinghaus  stopped  learning  as  soon  as  he  had  reached  such 
a  proficiency  that  he  could  barely  repeat  the  list  of  syllables  once 
correctly;  while  the  typical  learning  curve  supposes  the  learning 
to  be  continued  almost  to  the  point  of  automatism.  When  the  learn- 
ing is  thus  long  continued,  subsequent  forgetting  goes  on  at  a  much 
slower  rate.  Ebbinghaus  himself  showed  that  increasing  the  num- 
ber of  readings  of  his  nonsense  syllables  increased  markedly  the 
amount  retained  after  twenty-four  hours;  and  Radossawljewitsch,1 
who  required  the  lists  of  nonsense  syllables  to  be  studied  till  two  suc- 
cessive perfect  recitations  could  be  made  (instead  of  one,  as  in 
Ebbinghaus's  experiments),  found  the  loss  of  retention  to  proceed 
still  more  slowrly.  He  also  found  the  retention  of  poetry  to  decline 
more  slowly  than  that  of  nonsense  syllables,  as  is  shown  in  the  fol- 
lowing table: 

Interval  since  the  Per  cent,  of  retention  Per  cent,  of  retention 

original  learning  in  nonsense  syllables  in  poetry 

5  minutes 98 100 

20  minutes 89 96 

1  hour 71 78 

8  hours 47 58 

24  hours 68 .     .  79 

2  days 61  .........  67 

6  days 49 ;    ..     .  42 

14  days 41 30 

30  days 20  .........  24 

120  days 3 .     .     .         ? 

The  figures  given  above  are  the  average  result  from  several  indi- 
viduals. The  poor  retention  after  eight  hours  is  assigned  to  fatigue 
in  the  latter  part  of  the  day.  But  the  principal  thing  to  be  noted  in 
the  table  is  the  slight  loss  of  retention  after  five  and  after  twenty  min- 
utes. This  shows  that  after  some  "over-learning,"  the  onset  of 
forgetting  is  retarded,  so  that  the  transition  from  the  curve  of  learn- 
ing to  that  of  forgetting  would  not  be  wholly  abrupt.  When  the 
over-learning  has  been  carried  to  a  very  high  pitch,  as  in  the  experi- 
ments reported  above  with  the  typewriter — to  so  high  a  pitch  that 
a  very  large  share  of  the  associations  involved  have  been  reduced 

1  Op.  cit.,  p.  81. 


FORMING  OF  ASSOCIATIONS  577 

to  a  condition  of  automatic  efficiency — then  the  onset  of  forgetting 
is  extremely  slow.  Book  found,1  after  refraining  for  a  full  year 
from  all  use  of  the  typewriter,  that  his  speed  had  only  decreased 
by  8  per  cent.,  and  this  small  loss  was  fully  regained  in  forty  minutes 
of  fresh  practice. 

Material  having  "sense"  is,  as  the  last  preceding  table  shows, 
better  retained  than  nonsense  syllables.  Even  after  twenty-two  years, 
Ebbinghaus  found2  a  perceptible  retention  of  stanzas  of  poetry 
learned  only  once  to  the  point  of  one  perfect  recitation,  and  never 
since  seen.  Forgetting  can,  of  course,  be  stayed  by  learning  anew; 
and  after  each  new  learning  the  progress  of  forgetting  is  slower  and 
slower,  till  finally  recall  after  an  interval  is  possible  with  no  further 
study.3 

§  24.  Regarding  the  process  of  memorizing,  or  the  formation 
of  associations,  many  interesting  facts  have  been  established,  and 
some  advance  has  been  made  toward  their  explanation.  Some  of 
the  results  of  experiment  appear  rather  obvious,  from  the  fact  that 
they  are  met  with  in  ordinary  experience,  but  they  are  none  the  less 
curious  when  attentively  considered.  Among  such  may  be  men- 
tioned the  ease  with  which  a  short  list  of  words  or  syllables  is  memo- 
rized, and  the  rapid  increase  of  difficulty  which  appears  as  the  list 
is  lengthened.  A  certain  length  of  list,  corresponding  to  the  memory 
span,  can  be  immediately  reproduced  after  one  reading;  but  if  the 
list  is  lengthened  beyond  this  point,  several  readings  are  usually 
required.  Thus  Ebbinghaus4  could  recite  a  list  of  seven  nonsense 
syllables  after  one  reading;  but  it  took  him  13  readings  to  fix  a  list 
of  10  syllables,  17  readings  to  fix  a  list  of  12,  30  readings  to  fix  a  list 
of  16,  44  readings  to  fix  a  list  of  24,  and  55  readings  to  fix  a  list  of 
36.  Binet5  found  that  a  list  of  11  one-place  numbers  could  be  re- 
produced after  4  seconds  of  study,  whereas,  to  learn  a  list  of  13  num- 
bers, 38  seconds  were  needed;  and  75  seconds  for  a  list  of  14. 

When  a  list  is  too  long  to  be  learned  in  a  single  reading,  continued 
reading  does  not  develop  the  mastery  of  all  parts  of  it  with  equal 
speed ;  but  some  of  it  will  be  known  long  before  the  rest.  Generally 
the  first  and  last  of  a  list  are  known  long  before  the  middle,  which 
remains  like  a  shapeless  mass  after  the  ends  have  taken  shape  in 
the  mind.6  More  rapid  organization  of  the  middle  of  a  list — and 
therefore  more  rapid  learning  of  the  whole  list — is  accomplished 

1  Op.  cit.,  p.  75. 

3  Grundzuge  der  Psychologic,  1905,  I,  681. 

3  Ebbinghaus,  Uber  das  Gedachtnis,  p.  110. 

4  Op.  cit.,  pp.  64,  67. 

5  Psychologic  des  grands  calculateurs,  1894. 
6W.  G.  Smith,  Psychol.  Rev.,  1896,  III,  21. 


578      MEMORY  AND  THE  PROCESS  OF  LEARNING 

by  following  the  natural  tendency  to  accent  certain  members  of  the 
list  and  thus  impress  on  it  a  rhythmic  form.  It  is  not,  indeed,  an 
advantage  to  divide  the  list  into  parts  and  learn  these  separately, 
afterward  putting  them  together;  for  this  method  is  found  to  take  7 
more  time  than  learning  the  whole  list  together  by  reading  it  through  | 
and  through.1  But  dividing  it  into  measures  or  feet,  with  regular 
pauses  and  accents,  while  still  reading  the  list  straight  through,  is 
found  to  be  the  most  economical  device  for  memorizing  nonsense 
lists,2  and  still  more  for  memorizing  poetry.  If,  indeed,  the  material 
is  not  required  to  be  recited  as  a  whole,  but  in  parts,  as  in  the  case 
of  vocabularies,  or,  in  general,  in  experiments  by  the  method  of 
paired  associates,  then  division  into  parts,  and  prolonged  attention 
to  each  part  before  passing  to  the  next,  is  apt  to  give  the  best  results. 
Again,  where  the  members  of  a  list  are  individually  hard  to  grasp, 
it  may  be  economy  to  repeat  or  linger  on  small  parts  of  the  list.3 
Similar  considerations  apply  to  the  question  as  to  the  best  rate  of 
reading  a  list;  a  slow  rate  is  more  favorable  for  the  formation  of 
single  associations  between  the  pairs  of  which  the  list  is  composed, 
and  a  rapid  rate  of  reading  is  relatively  favorable  for  learning  the 
list  as  a  complete  list.4 

§  25.  A  partial  explanation  of  the  peculiarities  of  the  process  of 
forming  associations  is  afforded  by  the  three  following  considera- 
tions. 

(1)  Associations  are  certainly  formed,  not  only  between  the  suc- 
cessive members  of  a  list,  but  also  between  individual  members 
and   their  positions   in   the   series.     The   first   member   becomes 
strongly  associated  with  the  first  place,  and  the  last  member  with 
the  last  place;  and  when  the  list  is  rhythmically  organized,  the  ac- 
cented member  of  each  measure  is  associated  with  its  place  in  the 
rhythmic  pattern.     This  association  with  position  helps  to  explain 
not  only  the  advantage  of  the  rhythmic  form,  but  also  the  disad- 
vantage of  learning  a  list  in  disjointed  parts;  for  the  latter  method 
generates  false  associations  of  position,  since  the  last  term  of  each 
section  becomes  associated  with  a  final  position  and  a  "full  stop" 
which  do  not  belong  to  it  in  the  complete  list.     Any  such  false  asso- 
ciations must,  therefore,  be  broken  up  before  the  list  is  ready  to  be 
handled  as  a  whole. 

(2)  "Higher  units,"  similar  to  those  which  were  assigned  such 
importance  in  the  learning  of  an  act  of  skill,  are  present  also  in  the 

1  Lottie  Steffens,  Zeitschr.  f.  Psychol,  1900,  XXII,  321;    Pentschew,  Archiv 
/.  d.  ges.  Psychol,  1903,  I,  417. 

2  Miiller  and  Schumann,  Ebert  and  Meumann,  op.  cit. 

3Ephrussi,  Zeitschr.  f.  Psychol,  1904,  XXXVII,  161;    Pentschew,  op.  cit. 
*  Ogden,  Archiv  /.  d.  ges.  Psychol,  1904,  II,  93;   Ephrussi,  op.  cit 


FORMING  OF  ASSOCIATIONS  579 

process  of  memorizing,  both  when  the  material  to  be  learned  has 
meaning  and  when  it  is,  as  far  as  possible,  void  of  meaning.  Mean- 
ing organizes  the  words  of  connected  speech  into  phrases  and  still 
larger  groups,  which  are  grasped  as  units,  and  thus  it  is  that  prose 
or  verse  can  be  memorized  with  enormously  greater  ease  than  an 
equal  number  of  unconnected  words.  But  even  in  learning  a  list 
of  nonsense  syllables,  the  formation  of  higher  units  can  be  detected. 
The  syllable  is  itself  entitled  to  rank  as  a  higher  unit,  for  it  is  found 
that  many  more  letters  can  be  learned  in  a  given  time  when  they  are 
combined  into  nonsense  syllables  than  when  they  are  wholly  un- 
combined.  And  further,  the  measures  or  feet,  into  which  a  list 
of  nonsense  syllables  is  almost  inevitably  broken  up,  and  which 
greatly  facilitate  the  memorizing  of  the  list,  also  belong  in  the  class 
of  higher  units.  The  dynamic  unity,  or  "dynamical  association,"1 
of  the  measure  is  shown  by  the  facts2  that  relearning  of  a  list  is  made 
more  difficult  by  breaking  up  these  measures,  or  even  by  altering 
the  position  of  the  accent  within  each  measure;  while,  on  the  other 
hand,  a  list  built  up  out  of  complete  measures  taken  from  different, 
previously  learned  lists,  is  comparatively  easy  to  learn.  If  the  meas- 
ures are  short,  they  are  more  readily  joined  into  larger  groups  which 
have  a  certain  unity. 

The  further  fact  that  associations  are  formed  between  the  con- 
stituent syllables  and  their  positions  in  the  list  is  evidence  of  some 
degree  of  unity  in  the  grasp  of  the  entire  list.  Associations  are  also 
formed  between  the  single  measures  and  their  positions  in  the  list. 
Still  another  evidence  of  the  unitary  grasp  of  large  sections  of  the 
list,  or  even  of  the  whole  list,  is  afforded  by  the  fact  that  each  sylla- 
ble has  a  tendency  to  call  up,  not  only  the  immediately  succeeding 
syllable,  but  also  the  syllable  which  comes  next  but  one; — and,  in- 
deed, the  syllable  which  comes  next  but  two,  next  but  three,  etc. 
But  the  strength  of  these  "remote  associations "  decreases  with  the 
number  of  intervening  syllables.3  Associations  are  even  formed  in 
the  backward  direction,  so  that  the  later  members  have  some  ten- 
dency to  call  up  the  earlier,  at  least  within  the  same  measure. 
Enough  has  been  said  to  show  that  the  learning  of  a  list  of  nonsense 
syllables  is  very  far  from  being  simply  the  formation  of  a  chain  of 
serial  associations.  The  process  embodies  great  multiplicity,  and 
at  the  same  time  much  unity. 

(3)  Inhibition  or  interference  is  a  factor  to  be  reckoned  with, 
both  in  memorizing  and  in  recall.  A  distinction  may  be  drawn  be- 

1  For  an  explanation  of  this  phrase,  see  Ladd,  Psychology,  Descriptive  and  Ex- 
planatory, pp.  242  f.  and  384  f. 

2  Ebbinghaus,  Miiller  and  Schumann,  op.  cit. 

3  Ebbinghaus,  Miiller  and  Schumann,  op.  cit. 


580      MEMORY  AND  THE  PROCESS  OF  LEARNING 

tween  the  two  cases,  by  speaking  of  "associative  inhibition"  as  that 
which  occurs  in  the  process  of  learning,  and  impedes  the  formation 
of  associations;  and  "reproductive  inhibition,"  or  that  which 
hampers  the  process  of  recall  or  the  operation  of  associations.  A 
clear  instance  of  inhibition  appears  in  the  fact  that  lists  of  words 
or  syllables,  up  to  a  certain  length,  can  be  recited  after  one  reading, 
whereas  adding  one  or  two  more  members  to  the  list  makes  many 
readings  necessary.  Perhaps  an  individual  can  recite,  after  one 
reading,  a  list  of  six  syllables,  but,  on  attempting  a  list  of  seven, 
is  unable  to  give  more  than  one  or  two  of  them,  or  even  any  at  all. 
The  seventh  syllable  has  driven  away  the  others,  without  becoming 
fixed  in  their  place. 

§  26.  The  exact  nature  of  the  psycho-physical  mechanism  in- 
volved in  these  inhibitions  is  by  no  means  easy  to  "make  out.  The 
clearest  case  is,  perhaps,  that  in  which  a  given  stimulus  or  antecedent, 

A,  is  already  firmly  associated  with  a  certain  response  or  consequent, 

B,  and  now  the  attempt  is  made  to  associate  with  A  a  new  response, 

C,  A  leads  so  promptly  to  B  that  C  does  not  have  a  chance,  at  least 
till  after  B  has  had  its  turn;  and  thus  the  learning  of  the  sequence 
AC  is  impeded,  while  that  of  the  sequence  AB  may  be  further 
strengthened.     Even   if   the   overt  reaction   B,   or   the   conscious 
thought  of  B,  should  be  repressed,  A  would  still  exert  a  tendency 
toward  B,  and  might  sub-excite  it,  or  bring  it  into  "readiness," 
and  thus  hinder  the  formation  of  the  association  AC,  while  being 
itself  somewhat  strengthened.     If,  however,  AB  is  not  very  strong^ 
to  start  with,  it  is  likely  to  yield  to  the  pressure  toward  AC,  though 
interfering  somewhat  with  the  formation  of  the  latter  association. 
The  reality  of  such  interferences  has  been  shown  by  Miiller  and 
Pilzecker1  in-  the  case  of  learning  pairs  of  nonsense  syllables,  and 
by  Bergstrom2  in  the  case  of  associating  certain  movements  with 
certain  stimuli.     But  the  relations  of  interference  to  such  factors 
as  amount  and  recency  of  training  in  the  opposing  associations  are 
very  intricate  and  by  no  means  fully  worked  out.    It  was  shown  by 
Miinsterberg3  and  later  by  Bair,4  that  two  opposed  responses  to  the 
same  stimulus  might  both  be  learned  so  well  that  either  could  be  set 
into  operation  at  will.     They  had  ceased  to  interfere  with  each  other 
to  any  perceptible  degree. 

It  is  quite  possible  that  interferences  between  the  manifold  as- 
sociations formed  in  reading  over  a  list  of  nonsense  syllables  are  the 
main  cause  of  the  difficulty  of  learning  such  lists,  especially  when 
the  length  of  the  lists  is  increased  and  the  associative  tendencies 

1  Op.  cit.,  pp.  138  ff. 

2  Amer.  Journ.  of  Psychol,  1893,  V,  356;   1894,  VI,  433. 

3  Beitrdge  z.  exp.  Psychol.,  1892,  IV,  69.  *  Op.  cit. 


THE  CULTURE  OF  MEMORY  581 

are  thus  made  more  numerous.  Repeated  reading  strengthens  the 
principal  or  serial  associations  more  than  the  remote  associations, 
till  at  last  the  interferences  are  unable  to  prevent  the  correct  reci- 
tation of  the  list. 

§  27.  On  recurring  to  the  discussion  of  the  problem  whether  prac- 
tice in  learning,  in  one  rather  specialized  line  of  performance,  is 
available  for  the  culture  of  the  faculty  of  learning  in  general,  we  may 
gain  additional  light  on  the  reasons  for  the  negative  results,  from 
the  facts  just  disclosed  by  the  experiments  on  the  conditions  of 
successful  memorizing.  It  was  formerly  shown  that  great  skill  in 
making  certain  motor  reactions  can  be  attained  only  at  the  risk 
of  establishing  increased  chances  of  interferences  and  inhibitions, 
when  the  attempt  is  made  to  substitute  a  certain  changed  system  of 
such  reactions.  And  now  the  same  thing  appears  to  be  true  in 
the  cultivation  of  skill  in  memorizing.  It  is  a  well-known  fact, 
however,  that  there  are  certain  natural  or  acquired  aptitudes  in 
the  retentive  and  associative  factors  of  conscious  memory,  as  there 
are  aptitudes,  both  natural  and  acquired,  for  performing  highly 
specialized  feats  of  skill  in  motor  reactions.  Only  a  detailed  study 
of  each  particular  instance  of  failure  to  pass  from  one  kind  of  facility 
to  another  kind,  could  eliminate  the  influence  of  the  inhibitory  and 
interfering  processes,  which  belong  to  the  passage  itself.  An  over- 
cultivated,  and  so  implastic  mind,  or  brain,  in  some  special  kind  of 
learning — whether  it  be  in  the  form  of  nearly  automatic  motor  re- 
actions, or  of  conscious  attention,  discrimination,  and  deliberate 
memorizing — might  be  a  temporary  hindrance  to  "transference" 
of  the  facility  along  other  lines.  But  all  this  affords  no  satisfactory 
proof  against  the  almost  universal  assumption  that  the  power  of 
discriminating,  attending,  memorizing,  and  learning  in  general, 
can  be  cultivated. 

§  28.  Another  class  of  interesting  facts  regarding  the  process  of 
memorizing  may  now  receive  attention.  It  is  found1  that  a  list  can 
be  learned  in  fewer  readings  when  these  are  spread  over  several 
days  than  when  all  are  concentrated  into  a  single  learning  period. 
Since  a  similar  fact  appears  in  muscular  training,  the  explanation 
is  probably  to  be  sought  in  the  effects  of  activity  upon  the  nutrition 
of  the  organ  exercised.  Probably,  also,  the  lapse  of  time  allows  the 
minor  interfering  associations  to  die  out  more  than  the  principal 
associations,  and  thus  favors  the  latter  in  the  new  impression.  A 
similar  dropping  out  of  interferences,  and  consequent  rapid  im- 
provement after  an  interruption  of  practice,  has  been  noted  by  Book 
in  typewriting,  and  by  Ebert  and  Meumann  in  memorizing. 

1  Jost,  op.  cit.  A  similar  result  has  been  obtained  in  learning  other  perform- 
ances by  Leuba,  op.  cit.,  and  by  Kirkpatrick,  op.  cit. 


582       MEMORY  AND  THE  PROCESS  OF  LEARNING 

Another  fact,  which  has  been  brought  out  most  definitely  by 
Witasek,1  is  the  superior  value  of  active  recitations  of  the  matter  to 
be  memorized,  as  compared  with  the  more  passive  reading  of  the 
presented  lists.  Long  before  a  list  can  be  recited  entire,  it  is  possi- 
ble to  recite  parts  of  it,  and  if  the  learner  does  this,  relying  on  him- 
self as  far  as  possible,  but  being  prompted  when  he  hesitates,  the 
list  is  learned  in  fewer  repetitions  than  otherwise.  In  partial  ex- 
planation of  this  important  result,  it  is  a  plausible  conjecture  that 
the  passive  or  receptive  attitude  leaves  the  door  open  to  incidental 
and  interfering  associations;  while  an  active  reproduction,  so  far  as 
it  succeeds,  requires  the  suppression  of  interferences  and  the  selec- 
tion of  those  associations  which  contribute  to  success.  From  this 
point  of  view,  there  is  a  certain  analogy  between  learning  a  list  of 
nonsense  syllables,  and  learning  the  successful  response  to  a  situa- 
tion by  the  method  of  trial  and  error.  This  experience  also  illus- 
trates the  value  in  all  learning  of  a  cultivated  power  of  giving  at- 
tention, in  general. 

The  same  conclusions  are  further  enforced  by  the  fact  that  to 
achieve  quick  learning,  it  is  necessary  to  arouse  the  "will  to  learn."2 
If  a  subject  maintains  a  purely  passive  attitude  toward  the  lists 
which  are  shown  him,  his  learning  is  indeed  slow.  He  may  adopt 
a  decidedly  active  attitude,  but  if  this  is  rather  that  of  observation 
of  the  members  of  the  series  than  of  associating  them  so  as  to  be  able 
to  recite  the  list,  repeated  presentations  may  leave  him  without 
the  power  to  recite  it.3  It  is,  therefore,  clearly  possible  by  an  act  of 
will  to  set  up  an  adjustment  specifically  favorable  for  memorizing, 
though  it  is  by  no  means  clear,  introspectively,  in  what  this  adjust- 
ment consists.  There  is  even  some  evidence  that  the  adjustment 
may  be  different,  according  as  the  list  is  to  be  retained  for  a  long 
time,  or  only  for  a  few  moments.4 

§  29.  If  now  we  turn  from  the  "first  event,"  or  impression,  and 
pass  over  the  intervening  time,  during  which  the  disposition  left 
behind  by  the  impression  is  gradually  dying  out,  we  come  finally  to 
the  "second  event,"  the  so-called  "reproduction"  or  "recall" 
Neither  of  these  terms  is  perfectly  correct;  for  the  original  impression 
is  not  always,  and  perhaps  never,  fully  and  accurately  reproduced. 
Few  people  can  reinstate  an  impression  in  all  its  sensory  fulness 

1  Zeitschr.  f.  Psychol,  1907,  XLIV,  161,  246. 

*  Ebert  and  Meumann,  op.  cit.;  Meumann,  Okonomie  und  Technik  des  Ge- 
d&chtnisses,  1908,  pp.  24,  232. 

8  Miiller  and  Schumann,  op.  cit.,  p.  291. 

•Ohms,  Zeitschr.  f.  Psychol,  1910,  LVI,  73;  Henderson,  "A  Study  of  Mem- 
ory for  Connected  Trains  of  Thought,"  Psychol.  Rev.,  Monogr.  Suppl.  No.  XXIII, 
1903,  p.  53. 


PHENOMENA  OF  REPRODUCTION  583 

and  vividness;  many  can  accomplish  this  with  moderate  success; 
while  others  are  quite  incapable  of  seeing  their  breakfast  table  "in 
their  mind's  eye,"  as  if  it  were  actually  before  them,  though  they 
are  fully  capable  of  recollecting  aspects  of,  or  facts  about,  the  orig- 
inal experience.  This  difference  between  individuals  is  spoken  of 
as  a  difference  in  their  powers  of  imagery.1  Besides  this  deficiency 
in  fulness  and  vividness,  all  reproduction,  when  tested  carefully 
by  comparison  with  the  original  experience,  is  apt  to  be  found  in- 
fected with  certain  erroneous  factors. 

It  will  be  remembered  that  in  considering  the  topic  of  "associa- 
tion times"  (see  above,  p.  493)  we  found  the  stimulus  A  calling  up 
the  reaction  B,  although  both  stimulus  and  reaction  were  internal 
rather  than  sensory  and  motor.  This  was  explained  as  due  to  a 
"disposition"  left  behind  by  the  previous  experience.  Recall, 
then,  may — at  least  sometimes — be  considered  as  a  certain  type  of 
reaction.  And  concretely,  the  condition  would  seem  to  be  somewhat 
as  follows:  The  individual,  being  in  a  given  situation,  and  being 
adjusted  or  prepared,  voluntarily  or  involuntarily,  in  a  certain  di- 
rection, and  having  within  him  a  host  of  dispositions  or  reproductive 
tendencies  of  varying  strength  and  manifold  connections,  is  affected 
by  a  certain  stimulus,  and  reacts  to  it.  This  reaction  is  his  recall 
or  reproduction,  and  it  is  determined  by  the  stimulus,  by  the  indi- 
vidual's present  adjustment,  and  by  his  past  experience  as  retained  in 
reproductive  tendencies. 

§  30.  If  we  now  ask  what  determines  the  reaction  to  a  given 
stimulus — and  if,  for  the  present,  we  leave  out  of  account  the  im- 
portant factor  of  adjustment — our  previous  discussions  have  suffi- 
ciently shown  that  A  is  likely  to  recall  B  when  it  has  been  already 
associated  with  B;  and  that  the  likelihood  of  its  recalling  B  is  greater 
in  some  proportion  to  the  frequency  and  recency  of  this  association. 
If  A  has  been  previously  associated  with  both  B  and  C,  then  the 
likelihood  of  its  calling  up  one  rather  than  the  other  of  these  asso- 
ciates depends  on  the  relative  frequency  and  recency  of  the  two 
associations,  as  also  on  the  vividness  or  intensity  of  the  associative 
events.2 

Recall  also  depends  on  the  degree  of  success  or  pleasure  which 
has  attended  previous  reactions  to  A  by  B  or  C  (compare  pp.  547, 
552).  The  tendency  of  A  to  call  up  B  is  also  strengthened,  if  A  has 
been  immediately  followed  by  B,  and  if  attention  has  been  directed 
to  connecting  the  two;  but  in  a  less  degree,  if  A  was  preceded  in- 
stead of  followed  by  B,  or  if  it  was  only  indirectly  followed  by  B, 
or  if  both  A  and  B  formed  parts  of  a  single  group  or  "  higher  unit." 

1  Galton,  Inquiry  into  Human  Faculty,  p.  83  (London,  1883). 
8  Calkins,  op.  cit. 


584       MEMORY  AND  THE  PROCESS  OF  LEARNING 

In  all  these  cases,  we  may  speak  of  A  and  B  as  having  been  con- 
tiguous in  past  experience;  and  we  may  speak  of  the  present  recall 
as  resulting  from  "association  by  contiguity"  in  experience.1  Prob- 
ably it  is  not  so  much  a  mere  contiguity  in  "  experience"  that  counts, 
as  some  definite  contiguity  in  reaction.  Perhaps  we  may  express 
the  general  fact  best  by  saying  that  the  contiguity  must  effect  a 
"dynamic  association."  When  A  and  B  have  formed  parts  of  a 
single  unit  of  reaction — such  a  unit,  for  example,  as  one  of  the 
rhythmical  measures  in  which  a  list  of  syllables  is  learned,  or  such 
as  the  noting  of  a  relation  between  A  and  B — they  become  associated 
much  more  strongly  than  when  they  have  simply  been  present  to  con- 
sciousness at  the  same  time  or  in  immediate  succession.2 

§  31.  The  association  between  A  and  B  may  have,  at  a  given 
time,  any  strength  from  zero  to  a  maximum  such  that  A  will  recall 
B  with  certainty  and  promptness.  Below  a  certain  strength,  it 
cannot  be  utilized  for  purposes  of  recall.  In  other  words,  a  repro- 
ductive tendency  may  lie  below  the  "threshold  of  recall";3  or,  we 
may  speak  of  some  associations  as  being  "swfr-liminal."  The  ex- 
istence of  a  subliminal  association  may  be  shown  in  several  ways: 
sometimes  by  a  partially  correct  recall,  and  sometimes  by  the  feeling 
of  "being  near"  a  name  which  is  vainly  sought  for  in  memory. 
Its  activity  may  also  be  tested  by  the  following  experiment:  .Suppose 
the  association  AB  has  proved  to  be  subliminal  at  a  certain  time; 
that  is,  A,  being  presented,  has  not  been  able  to  recall  B.  Now  let 
B  itself  be  presented,  but  not  clearly;  let  it  be  shown  for  too  brief 
an  interval  to  allow  of  accurate  reading,  or  let  it  be  pronounced 
through  a  poor  telephone,  so  that  it  cannot  be  distinctly  heard; 
under  such  conditions,  it  has  been  found4  that  B  is  more  likely  to 
be  rightly  read  or  heard  if  it  has  just  been  put  into  a  condition  of 
"readiness"  by  the  use  of  the  subliminal  association  AB.  In  such 
a  case  the  subliminal  tendency  from  A  toward  B  facilitates  (com- 
pare p.  170)  the  arousal  of  B  by  another  stimulus.  It  may  also 
inhibit  the  tendency  to  some  other  response.  Two  associations, 

1  On  the  possibility  of  reducing  the  so-called  laws  of  association  to  the  princi- 
ple of  "contiguity  in  consciousness,"  see  Ladd,  Psychology,  Descriptive  and 
Explanatory,  pp.  263  ff. 

2  "  To  be  associated  it  is  not  enough  that  two  impressions  shall  occur  together 
or   in  immediate  succession.     B  may  follow  upon  A  as  a  physical  event  till 
doomsday,  but  it  is  only  as  A  and  B  are  apperceived  as  in  some  sort  one  and 
connected,"  that  A  will  afterward  give  rise  to  B.     J.  Ward,  Mind,  1894,  N.  S., 
Ill,  509.     This  statement  may  perhaps  need  some  qualification;  but  the  experi- 
mental work  on  memory  shows  abundantly  that  no  strong  associations  are 
formed  by  virtue  of  mere  temporal  contiguity  in  consciousness. 

3  Muller  and  Pilzecker,  op.  cit.,  p.  34. 

4  Ohms,  Zeitschr.  /.  Psychol,  1910,  LVI,  1. 


PHENOMENA  OF  REPRODUCTION  585 

AB  and  AC,  tending  in  different  directions  with  slight  and  about 
equal  strength,  may  inhibit  each  other  so  that  neither  B  nor  C  is 
recalled.1 

Reproductive  interference  between  different  associations  is 
probably  of  great  importance  in  explaining  the  difficulties  of  recall. 
Suppose,  for  example,  that  reproductive  tendencies  AB  and  AC  ex- 
ist in  an  individual  at  a  given  time,  but  that  AC  is  the  more  readily 
excited;  then  the  stimulus  A  may  lead  to  the  response  C,  though  B 
may  be  the  response  desired.  When  an  association  has  been  very 
recently  exercised,  it  is  left  in  an  excitable  condition  and  is  more 
quickly  re-excited  to  full  activity  than  an  old  and  perhaps  well- 
ingrained  association  which  has  not  been  recently  active.  In  other 
words,  the  association  time  for  recently  active  associations  is  es- 
pecially short.2  A  recently  active  association  is,  therefore,  likely 
to  "get  the  start  of"  an  older  and  perhaps  better-established  asso- 
ciation, and  thus  govern  the  first  response.  By  so  doing  it  acquires 
even  more  of  the  advantage  of  recency,  so  that  a  renewed  attempt 
to  arouse  the  old  association  may  simply  lead  to  the  recent  one  again. 
This  occurs  in  trying  to  recall  a  piece  of  music  which  resembles  a 
piece  just  heard;  it  is  hard  to  drive  one  air  out  of  the  mind  and  give 
the  other  a  chance.  The  same  difficulty  occurs  in  case  of  a  slip  in 
writing  or  spelling.  One  sometimes,  after  making  a  "slip  of  the 
pen,"  goes  back  to  write  the  sentence  correctly,  and  commits  the 
same  error  again.  In  playing  a  musical  instrument  or  in  a  highly 
skilled  athletic  performance,  an  error  tends  to  be  repeated  and  to 
put  the  performer  "out  of  form"  for  the  time  being,  or  until  the 
recent  bad  associative  tendencies  have  time  to  "cool  down."  The 
same  sort  of  trouble  occurs  at  times  in  trying  to  recall  a  familiar 
name;  unless  the  right  response  occurs  at  once,  it  appears  to  be  short- 
circuited  by  the  false  responses  which  occur,  and  the  best  plan  is 
often  to  abandon  the  search  for  a  time,  and  allow  the  interferences 
to  go  to  sleep.  It  would  seem  safe  to  say  that,  as  a  rule,  an  existing 
reproductive  tendency  AB  must  control  the  reaction  to  A  except 
for  the  interference  of  other  instinctive  or  associative  tendencies  set 
into  activity  by  A  or  by  some  other  stimulus  acting  at  the  same  time. 

§  32.  A  very  interesting  conception  of  the  interferences  operative 
in  preventing  the  recall  of  a  familiar  name,  and  in  causing  lapses 
and  slips  of  various  kinds,  has  been  put  forward  by  Freud.3  Without 
attempting  a  lengthy  exposition  of  this  theory,  we  may  get  a  glimpse 
at  its  character  in  the  following  way:  Let  us  suppose  that  a  name 
has  several  associates  which  tend  to  be  recalled  with  it.  It  may  be 
that  these  associated  ideas  are  unpleasant,  or  that  they  have  been  un- 

1  Miiller  and  Pilzecker,  op.  cit.  2  Miiller  and  Pilzecker,  op.  cit. 

3  Zur  Psychopathologie  des  Alltagslebens  (Berlin,  1904). 


586       MEMORY  AND  THE  PROCESS  OF  LEARNING 

pleasant  at  some  time  in  our  past  experience;  or  it  may  simply  be 
that  they  are  not  welcome  at  the  moment  because  they  would  dis- 
tract attention  from  the  present  object  of  interest.  In  any  of  these 
cases,  there  is  a  tendency  to  repress  or  inhibit  these  undesired  as- 
sociates, even  before  they  actually  break  into  consciousness.  But 
in  repressing  them,  we  may  repress  the  name  as  well;  or  we  may  only 
partially  suppress  it,  and  so  recall  a  name  similar  to  it.  Or,  again, 
some  of  the  associates  may  not  be  fully  suppressed,  but  may  be 
operative  in  recalling  a  substitute  name.  In  other  words,  a  for- 
gotten name  points  to  a  repressed  complex  of  ideas  and  emotions, 
and  what  is  thus  suppressed  may  be  discovered  by  removing  the 
suppression,  and  allowing  a  train  of  perfectly  free  association  to 
take  its  start  from  the  forgotten  name  when,  later,  it  has  been  found. 
By  this  method  of  "psycho-analysis,"  Freud,  and  others  following 
him,  have  been  able  to  bring  to  the  surface  submerged  "  complexes," 
which,  in  nervous  persons,  are  sometimes  the  source  of  much  mental 
inefficiency.  Freud  is  inclined  to  look  especially  for  old  emotional 
suppressions,  often  of  a  more  or  less  sexual  nature,  as  lying  at  the 
bottom  of  apparently  trivial  lapses  of  memory.  It  is  highly  im- 
probable that  all  interference  with  recall  possesses  this  highly 
elaborate  and  significant  character;  interference  could  hardly  act 
in  these  complicated  ways  did  it  not  also  act — as  has  been  abundantly 
shown  that  it  does — in  simple  ways;  but  it  may  very  well  be  that  such 
inhibitory  mechanisms  as  Freud  conceives  are  sometimes  operative.1 
§  33.  What  was  said  above  of  the  liability  of  a  recently  active 
association  to  re-excitation  leads  to  a  further  important  consideration. 
In  certain  abnormal  conditions  of  the  nervous  system,  a  tendency  is 
visible  to  repeat  an  act  time  after  time,  when  once  it  has  been  aroused 
by  some  appropriate  stimulus.  Or,  in  an  association  test,  which 
calls  for  responses  to  one  after  another  of  a  series  of  presented  words, 
there  may  appear  a  tendency  to  give  the  same  response  to  several 
words — a  response  which  was  evidently  called  up  by  association  in 
the  first  instance,  but  which  has  no  relevance  to  the  later  words. 
The  response,  once  excited,  persists  or  perseveres.  This  tendency 
to  "  perseveration "  is  not  confined  to  abnormal  conditions,  but  has 
been  demonstrated  by  Miiller  and  Pilzecker  in  normal  subjects, 

1  An  example  may  perhaps  make  this  clearer.  A  certain  author  has  frequently 
had  difficulty  in  recalling  the  name  "Wadelton."  When  he  applies  free  associa- 
tion to  this  name,  the  first  idea  that  occurs  to  him  is  that  of  waddling — a  notion 
which  does  not  in  the  least  comport  with  the  individual  so  named.  Moreover, 
this  cheap  punning  on  names  is  a  thing  to  be  suppressed,  and  the  author  has  been 
at  some  pains  to  suppress  it.  Now  it  may  be — to  speak  in  a  figurative  way — 
that  recall,  in  reaching  for  the  name,  got  hold  also  of  these  unsuitable  associates, 
and,  in  trying  to  drop  these,  dropped  the  name  with  them,  and  so  was  unable 
to  pull  it  up  into  consciousness. 


PHENOMENA  OF  PERSEVERATION  587 

when,  in  trying  to  recall  nonsense  syllables,  they  are  operating  with 
rather  weak  reproductive  tendencies.  Perseveration  may  then  ap- 
pear in  the  repeated  recall  of  the  same  syllable,  as  a  response  to 
syllables  with  which  it  has  not  previously  been  associated. 

Perseveration  is  most  apt  to  lead  to  the  recall  of  recent  experiences, 
and  is  most  apt  to  appear  when  attention  is  relaxed,  and  the  mind  al- 
lowed to  wander  freely.  Instances  of  perseveration  are  found  in 
the  running  of  a  tune  in  the  head,  soon  after  it  has  been  heard;  or 
in  the  flashing  of  a  scene  before  the  mind's  eye  soon  after  it  has  been 
actually  seen;  or  in  the  reminiscences  of  the  day  which  are  apt  to 
come  to  mind  as  one  is  dropping  off  to  sleep.  In  the  case  of  visual 
dreams,  these  may  be  caused  by  the  persistence  of  the  after-images 
in  the  "fundus"  of  the  eye. 

The  theoretical  importance  of  such  facts  lies  in  the  apparent  ab- 
sence of  a  stimulus  to  recall.  B  comes,  it  seems,  not  in  response  to 
any  A,  but  entirely  of  itself.  It  seems  to  spring  up  of  its  own  elas- 
ticity, when  the  repressive  force  of  other  interests  is  removed.  Per- 
severation seems  thus  not  to  belong  altogether  under  the  head  of 
association,  and  has  accordingly  been  assigned  an  independent 
standing  as  a  cause  of  recall.  We  may  conceive,  perhaps,  that  the 
cerebral  mechanism  which  takes  care  of  B,  after  being  strongly  ex- 
cited, does  not  at  once  lapse  into  quiet,  but  remains  subliminally 
active;  it  may  thus  become  the  most  active  part  of  the  brain  by  vir- 
tue of  a  decrease  in  the  activity  of  other  parts.  However,  it  is  not 
necessary  to  adopt  exactly  this  conception ;  for,  if  a  mechanism,  be- 
cause of  recent  activity,  simply  is  highly  excitable,  it  might  be  thrown 
into  activity  through  feeble  associative  links,  of  which,  as  the  pre- 
ceding pages  have  shown,  many  are  formed  between  things  which 
seem  only  remotely  connected  in  consciousness.  Such  a  condition 
is  especially  emphasized  when,  either  through  the  excitement  of 
fever,  or  the  relaxation  of  control  over  the  train  of  ideas  on  account 
of  exhaustion,  or  in  sleep,  a  perfect  hurly-burly  of  disconnected 
mental  images  takes  possession  of  the  conscious  mind.  Whatever 
be  the  explanation,  the  facts  indicated  by  the  name  perseveration 
are  important  in  any  sketch  of  the  succession  of  thoughts  which  pass 
through  the  mind. 

§  34.  Perseveration  is  not  the  only  apparent  exception  to  the  rule 
that  recall  proceeds  along  the  lines  of  previously  formed  associa- 
tions. The  law  of  association  requires  that  A,  in  order  to  recall  B', 
must  have  previously  been  associated  with  it.  But  often  A  is  new, 
and  so  never  previously  associated  with  B;  or,  though  A  and  B  may 
both  be  old,  they  may  never  have  been  present  together  in  experience. 
The  question  is,  then,  whether,  and  how,  A  can  recall  B  without 
having  been  previously  associated  with  it.  The  cases  in  which 


588       MEMORY  AND  THE  PROCESS  OF  LEARNING 

this  seems  to  occur  are  chiefly  of  two  kinds:1  first,  A  may  be  sim- 
ilar to  some  stimulus  which  was  previously  associated  with  B; 
and  second,  A  may  be  similar  to  B  itself.  The  first  case  is  ex- 
ceedingly common,  for  a  stimulus  seldom  recurs  exactly  as  it  was 
formerly  received;  its  quality,  intensity,  or  setting  may  vary,  and  yet 
the  same  reaction  be  evoked.  The  child,  having  learned  the  name 
"dog,"  for  example,  applies  it  readily  to  dogs  of  various  sizes  and 
colors,  seen  from  various  angles  and  in  various  attitudes;  and  even 
has  little  hesitation  in  applying  the  same  name  to  a  variety  of  other 
animals.  The  mastering  of  a  new  situation  by  aid  of  some  act 
learned  in  response  to  a  similar  situation  is  an  essential  factor  in 
learning.  This  form  of  recall  thus  takes  us  back  to  our  previous 
discussion  of  the  transference  of  a  learned  reaction  from  one  situa- 
tion to  another;  and  our  previous  analysis  of  transference  as  de- 
pendent on  the  existence  of  something  common  to  the  old  and  new 
situations  and  responses  is  still  applicable.  The  new  stimulus  need 
not  be  an  exact  copy  of  the  old;  neither  does  the  new  reaction  need 
to  be  an  exact  copy  of  the  old,  for  it  is  modified  by  the  exigencies  of 
the  present  situation;  but  both  stimulus  and  response  are  partly 
old,  and,  therefore,  connected  by  an  old  association. 

Less  common,  but  still  common  enough  to  be  of  constant  and 
great  importance  in  mental  life,  is  the  case  in  which  A  leads  to  a  B 
which  is  similar  to  A  itself.  This  is  the  famous  case  of  "  association 
by  similarity."  Although  it  differs,  descriptively,  from  the  pre- 
ceding case,  dynamically  it  is  much  the  same.  A  stranger,  for  ex- 
ample, reminds  us  of  an  acquaintance,  because  of  some  resemblance 
between  their  faces.  In  this  case,  A  is  the  sight  of  the  stranger,  and 
B  the  thought  of  the  acquaintance,  and  A  and  B  have  never  been 
associated  in  our  past  experience.  If,  now,  the  resemblance  had 
been  so  great  as  to  lead  us  to  call  the  stranger  by  the  name  of  the  ac- 
quaintance, the  response  would  clearly  belong  under  the  preceding 
case;  it  would  be  transference  of  a  reaction  rather  than  association 
by  similarity.  But  if  the  resemblance  is  not  strong  enough  to  lead 
to  a  transferred  motor  reaction,  it  still  exerts  a  tendency  in  that  di- 
rection, and  may  give  rise  to  an  "idea"  of  our  acquaintance.  The 
two  cases  differ  in  the  kind  of  reaction,  but  not  in  the  mechanism 
of  recall. 

Let  us  further  examine  the  question  in  an  instance  which  has  been 
experimentally  worked  out  with  some  detail.2  Suppose  a  collec- 
tion of  two-syllabled  "nonsense  words"  to  be  made  familiar  by  a 
moderate  amount  of  study,  and  a  few  minutes  later  let  similar  non- 

1  Compare  Ebbinghaus,  Grundzuge  der  Psychologic,  1905,  I,  pp.  636,  642. 

2  Peters,  "Uber  Ahnlichkeitsassociation,"  Zeitschr.  /.  PsychoL,  1910,  LVI,  161. 


VARIETIES  OF  ASSOCIATION  589 

sense  words  be  presented — the  similarity  consisting  in  the  general 
construction  and  in  the  possession  of  letters  in  common;  then  it  is 
found  that  a  word  has  considerable  power  to  recall  the  similar  word 
which  has  recently  been  made  familiar.  For  instance,  tolaf,  be- 
ing presented,  recalls  golap,  recently  studied.  Now  the  common 
group  of  letters,  ola,  was  an  essential  part  of  the  stimulus  which  led 
to  the  percept  golaf;  and  therefore  tends  to  recall  golaf,  even  when 
presented  as  part  of  another  complex.  If  the  association  formed 
between  A  and  B  were  exclusively  concerned  with  A  and  B  as  totals, 
it  would  be  difficult  to  conceive  how  anything  similar  to  A  could 
recall  B,  but  since  associations  are  formed  connecting  parts  of  A 
with  B,  the  recall  of  B  by  partial  recurrence  of  A  presents  no  real 
exception  to  the  law  of  association. 

§  35.  The  full  importance  in  intellectual  life  of  association  by 
similarity  does  not,  however,  become  evident  till  we  consider  that, 
though  similarity  may  be  analytically  regarded  as  due  to  the  pos- 
session of  features  in  common,  yet  the  detection  of  these  common 
features  is  not  always  easy  or  even  possible  in  concrete  cases.  It 
is  not  usually  easy,  for  instance,  to  analyze  the  resemblance  be- 
tween two  faces,  so  as  to  state  exactly  what  they  have  in  common; 
and  yet  one  face  may  remind  us  of  the  other.  Clearly,  therefore, 
association  by  similarity  does  not  depend  on  a  conscious  analysis 
of  the  stimulus,  and  a  conscious  discrimination  of  the  features  which 
have  power  to  lead  to  the  recall  of  some  other  complex.  The  recall 
of  similars  provides,  the  rather,  an  occasion  for  subsequent  analysis. 
A  recalls  B,  we  do  not  see  why,  though  we  are  vaguely  aware  of 
some  resemblance  between  them.  We  may,  next,  become  aware  of 
the  point  of  resemblance,  and  thus  be  led  to  an  analysis  of  what  would 
otherwise  have  remained  unanalyzed.  So,  when  a  new  situation 
arouses  either  a  thought  of  some  earlier  situation,  or  a  reaction 
which  was  made  to  it,  the  feature  common  to  the  two  situations 
may  become  isolated  in  thought,  and  thus  a  more  intelligent  grasp 
of  both  old  and  new  be  promoted.  Such  seems  to  be  the  mechanism 
— at  least  in  part — by  which  new  insights  and  discoveries  are  often 
achieved.  Thus  transferred  reactions  and  associations  by  similar- 
ity lead  over  from  memory  to  productive  imagination  and  thought. 

§  36.  Thinking  is,  in  very  large  measure,  a  process  of  recall, 
but  it  involves  one  factor  which  has  not  yet  been  fully  considered. 
It  has  been  shown  that  most  cases  of  recall  are  of  the  nature  of  a 
reaction ;  and  that,  besides  the  stimulus,  and  the  existing  stock  of 
reproductive  tendencies,  an  important  factor  in  the  result  is  the  men- 
tal adjustment  present  at  the  moment.  Recall  is  seldom  perfectly 
"free";  usually  it  is  governed  or  directed  by  some  present  inter- 
est or  state  of  mind.  Reproductive  tendencies,  formed  in  past 


590       MEMORY  AND  THE  PROCESS  OF  LEARNING 

experience,  are  numerous,  and  make  possible  many  reactions  to 
a  given  stimulus;  and  which,  in  particular,  of  these  reactions  shall 
occur  is  dependent  not  only  on  the  relative  strength  of  the  repro- 
ductive tendencies,  but  also  on  the  present  adjustment.  In  reading, 
for  instance,  the  context  so  "sets  the  mind"  that  the  appropriate 
*  meaning  of  each  word  is  at  once  recalled,  though  the  word  may  have 
many  meanings,  and  each  of  these  may  have  been  previously  as- 
sociated with  the  word.  In  arithmetical  work,  the  numbers  "  three 
and  four"  suggest  "seven,"  if  the  adjustment  is  to  add;  but "  twelve," 
if  the  adjustment  is  to  multiply.  Mental  work  of  any  kind  would  be 
a  hopeless  tangle  of  associations,  were  it  ncJTFor  the  directive  power 
of  preparatory  adjustments.  This  Factor  is  a  selective  one;  without 
increasing  the  stock  of  associations,  it  gives  the  preference  to  some 
and  suppresses  others. 

The  most  important  question  regarding  this  "selective  mechan- 
ism" is  whether  it  acts  before,  or  after,  the  reproductive  tendencies 
have  done  their  work.  Does  such  a  word  as  "fair,"  for  example, 
when  occurring  in  context,  first  call  up  several  of  its  familiar  mean- 
ings, from  among  which,  next,  that  which  suits  the  context  is  selected; 
or  does  the  selection  so  occur  that  only  the  appropriate  meaning 
comes  to  consciousness?  If,  in  multiplying,  we  meet  the  numbers 
7  and  9,  do  we  recall  both  their  sum  and  their  product,  and  after- 
ward select  the  product  ?  In  such  cases,  introspection  gives  a  clear 
answer:  usually  only  the  appropriate  response  appears;  and  in  fact 
the  reaction  is  objectively  too  prompt  to  allow  of  multiple  recall  and 
subsequent  selection.  The  preparatory  adjustment  here  facili- 
tates one  reproductive  tendency  and  Inhibits  the  others  from  the 
very  start  of  the  total  process.  Such,  indeed,  is  the  case  in  all 
smooth-running  mental  work.  On  the  other  hand,  there  are  not  a 
few  cases  where  the  appropriate  reproductive  tendency  is  weak, 
and  other  reproductive  tendencies  are  relatively  strong;  and  in  all 
cases  there  are  limits  to  the  power  of  the  adjustment.1 

§  37.  Recall  is  likely  to  be  attended  by  feelings  of  familiarity,  of 
recent  occurrence,  of  correctness  or  incorrectness,  of  certainty  or 
doubt;  and  these  feelings  are  often  the  chief  subjective  constituent 
of  what  is  termed  " recognition"  A  high  value  is  attached  to  them 
by  the  individual,  inasmuch  as  he  is  usually  willing  to  assert  that 
they  indicate  the  truth.  He  "feels  sure"  that  he  has  seen  this  face 
before,  or  that  he  has  recalled  this  name  correctly,  or  that  seven 
times  nine  is  sixty-three,  without  further  evidence  than  his  immediate 
impression.  Without  doubt  these  feelings  have  a  considerable  de- 
gree of  trustworthiness^_especially  when  they  are  strong  and  posi- 

1  Experimental  evidenpe  in  support  of  these  conclusions  has  been  presented 
in  a  previous  chapter,  pi 


THE  GUARANTEE  OF  MEMORY  591 

tive.  They  are  not,  however,  absolutely  trustworthy;  for  cases  are 
easily  found,  in  experiment,  in  which  a  false  recall  is  attended  with 
the  feeling  of  correctnes'"  and  confidence,  as  well  as  cases  in  which 
a  right  recall  is  attended  with  a  feeling  of  its  incorrectness.1  Testi- 
mony, whether  in  ordinary  intercourse  or  in  courts  of  justice,  re- 
garding events  experienced  shows  similar  cases;  and  in  fact,  where 
the  testimony  can  be  controlled  by  a  full  knowledge  of  the  facts, 
a  considerable  proportion  of  false  recalls,  attended  by  a  feeling  of 
correctness,  is  ordinarily  found.2  A  witness  in  court  may  even  be 
willing  to  testify  under  oath,  and  in  perfectly  good  faith,  to  the  oc- 
currence of  an  event  or  to  the  identity  of  a  person,  when  his  memory 
is  really  at  fault.3  Still,  the  feelings  of  recognition  are  on  the  whole 
worthy  of  confidence.  In  some  persons  they  are  highly  trustworthy; 
in  others  much  less  so,  according  to  the  temperament;  and,  again, 
they  are  much  more  reliable  as  to  certain  classes  of  facts  than  as  to 
other  classes. 

§  38.  No  subjective  guarantee  of  the  correctness  of  memory  is 
superior  to  these  feelings  of  recognition.  One  may  seek  outside  cor- 
roboration,  or  one  may  criticise  one's  recollections  on  the  basis  of 
probability;  but  in  so  far  as  one  must  rely  on  one's  own  memory,  the 
feelings  are  ultimate,  and  there  is  no  way  of  going  behind  them  to 
justify  or  to  discredit  them.  It  has  indeed  been  sometimes  sug- 
gested that  recognition  consists  in  more  recall,  i.  e.,  in  the  recall  of 
the  original  setting  of  the  particular  fact  recognized.  This  is  so 
far  true  that  recall  of  the  setting  of  a  fact  often  occurs,  and  permits 
of  a  more  exact  assignment  of  the  fact  to  its  place  in  past  experience; 
and,  besides,  recall  of  the  setting  reassures  us  as  to  the  correctness 
of  the  central  fact.  But  what  guarantees  the  correctness  of  the 
recalled  setting?  How  do  we  recognize  that?  If  a  setting  is  all 
that  is  required,  imagination  is  perfectly  capable  of  conjuring  one 
up  about  an  imagined  fact;  only,  such  a  setting  will  not  feel  genuine 
and  familiar.  The  recognitive  feeling  must  attach  to  the  setting 
to  give  it  subjective  validity.  Try  as  we  will  to  justify  memory  by 
indirect  evidence  from  within  ourselves,  we  have  always,  in  the  last 
analysis,  to  rely  upon  a  feeling  of  direct  certainty — though  the 
certainty  may  be  so  categorical  as  to  occasion  very  little  emotional 
tumult.  Such  rather  neutral  states  of  assurance  are  the  rule  where 
recognition  is  easy  and  unhesitating,  as  when  a  daily  companion 
is  seen,  or  a  well-known  date  recalled.  This  feeling  of  unquestion- 
ing assurance,  as  distinguished  from  feelings  of  uncertainty,  hesita- 

1  Miiller  and  Pilzecker,  op.  cit. 

2  Cattell,  Science,  1895,  N.S.  II,  760;  Stern,  Zur  Psychologic  der  Aussage  (Ber- 
lin, 1902). 

3  Urstein,  Zeitschr.  f.  PsychoL,  1906,  XLIII,  423. 


592      MEMORY  AND  THE  PROCESS  OF  LEARNING 

tion,  and  doubt,  is  the  correlate  in  consciousness  of  the  uninhibited 
and  smoothly  running  adjustment  and  unimpeded  dynamic  associa- 
tion of  the  cerebral  mechanism. 

That  recognition  does  not  consist  essentially  in  the  recall  of  a  set- 
ting was  asserted  by  Kiilpe,1  who  drew  a  distinction  between  the 
"mediate"  or  indirect  and  the  "immediate"  or  direct  types  of  rec- 
ognition. In  the  mediate  type,  the  setting  is  recalled  and  aids  in 
the  recognition;  but  in  the  immediate  type,  the  recognition  occurs 
before  the  setting  is  recalled,  if  indeed  it  is  recalled  at  all.  The  im- 
mediate type  occurs  when  recognition  is  prompt  and  sure.  Experi- 
ments on  the  recognition  of  odors,  undertaken  by  Gamble  and 
Calkins,2  showed  instances  in  which  an  odor  was  recognized  without 
recall  of  previous  experiences  of  it,  and  other  instances  in  which 
past  experiences  of  the  odor  were  recalled  only  after  the  odor  had 
been  recognized. 

1  Grundriss  der  Psychologic,  1893,  pp.  177. 

2  Zeitschr.  /.  PsychoL,  1903,  XXXII,  177,  and  XXXIII,  161. 


CHAPTER  IX 
THE  MECHANISM  OF  THOUGHT 

§  1.  Logic  treats  of  rational  discourse  as  made  up  of  syllogisms; 
of  these  in  turn  as  composed  of  propositions;  and  of  these,  finally, 
as  containing  terms  and  a  copula,  or  sign  that  the  terms  belong  to- 
gether. Or,  starting  from  the  terms,  logic  regards  the  proposition 
as  resulting  from  comparison  of  the  terms,  and  the  syllogism  as 
resulting  from  the  comparison  of  propositions.  Though  this  treat- 
ment is  based  on  the  analysis  of  the  linguistic  expression  of  thought 
rather  than  on  the  analysis  of  concrete  mental  processes,  it  may 
serve  to  indicate,  in  a  provisional  way,  the  topics  suggested  for  study 
in  the  field  of  thought. 

§  2.  The  so-called  "terms,"  or  "ideas,"  of  which' a  proposition 
or  judgment  is  supposed  to  consist,  are  supplied  either  by  perception 
or  by  recall.  This  fact  makes  desirable  some  further  consideration 
of  the  nature  of  perception,  although  the  topic  has  already  been 
discussed  (Chapters  V  and  VI  of  Part  II)  in  detail.  It  is  clear,  in 
the  first  place,  that  sense-perception  commonly  depends  on  past 
experience.  In  general,  each  thing  is  perceived  as  a  familiar  indi- 
vidual, or  as  belonging  to  a  familiar  class  of  things;  or  as  like 
something  already  known;  or  as  compounded  of  known  elements. 
Perception  thus  involves  recognition;  and  what  has  been  said  of  recog- 
nition applies  also  to  perception.  In  particular,  the  distinction 
between  mediate  and  immediate  recognition  is  applicable  in  the 
field  of  thought,  and  in  the  study  of  its  mechanism.  When  an  ob- 
ject appears  in  a  new  setting — and  every  setting  is  likely  to  differ 
somewhat  from  those  in  which  the  object  previously  occurred — the 
old  setting  may  be  recalled  and  apparently  aid  in  recognizing  the 
nature  of  the  object.  This  is  "mediate"  perception;  but  it  is  far 
from  being  the  common  type,  for  usually  an  object  of  a  familiar 
class  is  recognized  in  a  new  setting  promptly  and  directly;  and 
this  is  equally  true  if  the  present  object  differs  from  similar  ob- 
jects previously  experienced,  even  as  much  as  an  outline  drawing 
differs  from  an  actual  scene.  The  outline  drawing  is  readily 
"  understood,"  or  recognized  as  a  representation  of  a  certain  sort  of 
object,  without  the  recall  of  the  colors  or  background  which  an 
actual  object  must  have.  When  the  same  outline  drawing  can  repre- 
sent more  than  one  object — as  is  the  case  in  the  "staircase  figure" 

593 


594 


THE  MECHANISM  OF  THOUGHT 


and  other  ambiguous  figures — the  transition  from  one  way  of  seeing 
the  figure  to  another  is  not  attended  by  the  recall  of  a  new  setting.1 
The  observer  seems  to  see  only  the  figure  before  him,  but  to  see  it 
now  as  this  object  and  again  as  that.  In  perception  generally,  past 
experience  is  not  overtly  present  in  consciousness;  one  seems  to  live 

in  and  deal  with  the  pres- 
ent, while  all  the  time  one 
is  really  utilizing  previously 
learned  reactions. 

§  3.  A  percept  belongs 
without  doubt  to  the  class 
of  reactions  to  stimuli,  and 
usually  to  the  class  of  re- 
actions previously  learned.2 
Yet  introspection  can  sel- 


FIG.  152. — The  Staircase  Figure  (from  Wundt). 
a  can  be  made  to  appear  either  nearer  or  far- 
ther off  than  &. 


dom  distinguish  between  the 
stimulus  and  the  reaction  in 
a  perceptual  act.  It  is  im- 
possible to  lay  aside  habitual  modes  of  perception,  and  to  receive 
the  stimulus  as  a  "pure  sensation";  it  is  also  usually  impossible  to 
detect  by  introspection  an  interval  between  the  reception  of  the 
stimulus,  as  such,  and  the  recognition  of  it  as  some  definite  thing. 
Brain  pathology,  however  (compare  p.  252),  affords  reason  to  believe 
that  the  reception  of  the  stimulus  and  the  perception  of  it  as  some 
known  object  are  two  events;  and  even  that  an  interval  occurs 
between  the  beginning  of  the  first  event  and  the  beginning  of  the 
second;  but  this  interval  is  probably  a  small  fraction  of  a  second, 
and  is  not,  moreover,  an  empty  interval,  but  a  period  of  gradual 
change  in  consciousness.  Introspection,  therefore,  could  not  be  ex- 
pected to  be  aware  of  the  interval,  or  to  distinguish  the  earlier  from 
the  later  stages  of  the  total  process. 

In  cases  of  difficult  perception,  however,  it  is  often  possible  to 
observe  the  gradual  development  of  a  percept.  A  sudden  and  start- 
ling stimulus  may  call  out  an  immediate  motor  response,  attended 
by  mental  confusion,  which  then  gradually  gives  way  to  a  clear  per- 
ception of  the  nature  of  the  stimulus  or  of  the  object  indicated  by  it. 
At  other  times,  an  unfamiliar  stimulus,  especially  a  curious  sound, 
may  give  rise  to  a  rapid  succession  of  "trial  percepts"  which  are 
promptly  rejected,  each  for  the  next,  till  one  is  reached  which  sat- 
isfies the  mind.  A  few  seconds  may  see  the  whole  series  of  tenta- 
tive interpretations  gone  through  with,  and  a  satisfactory  percept 
reached.  In  such  cases,  perception  has  definitely  the  character 

1  Woodworth,  Journal  of  Philos.,  1907,  IV,  169. 

2  Ward,  Mind,  1893,  N.S.,  II,  355. 


PERCEPTION  AS  A  FORM  OF  REACTION  595 

of  response  by  "trial  and  error";  varied  reactions,  provided  by  in- 
stinct and  previous  training,  are  tried  in  succession,  till  one  is 
reached  which  gives  satisfaction.  On  the  other  hand,  the  percep- 
tion of  a  familiar  object  has  the  character  of  a  well-learned  reaction, 
which  is  recalled  swiftly  and  automatically  by  the  stimulus. 

More  definite  experimental  analysis  of  the  process  of  perception 
has  been  obtained  in  case  of  complex  visual  stimuli,  such  as  reading 
matter  or  "  nonsense  drawings."  The  method  used  in  these  studies 
is  that  of  exposing  the  object  for  too  brief  a  time  to  permit  of  a  com- 
plete and  accurate  perception,  and  then  repeating  the  exposure 
time  after  time,  noting  after  each  exposure  how  far  the  observer  is 
able  to  reproduce  the  object.  When  reading  matter  is  so  presented,1 
the  observer  is  able,  at  the  first  presentation,  to  read  a  certain  num- 
ber of  words  with  certainty;  to  distinguish  the  general  shape  of 
another  word  or  two;  and  to  hazard  a  guess  as  to  some  vague  char- 
acter of  the  next  word  or  two.  On  the  next  exposure,  his  eyes  be- 
ing directed  to  words  which  were  only  partially  grasped  the  first 
time,  he  sees  these  distinctly  and  his  area  of  indistinct  vision  is  ex- 
tended to  new  words  further  to  the  right.  It  moreover  appears  cer- 
tain that  the  indistinct  perception  of  words  is  of  value  in  the  subse- 
quent clear  perception  of  them.  The  indistinct  perception  renders 
the  clear  perception  more  prompt.  In  the  case  of  nonsense  figures, 
composed  of  seven  short  lines  and  curves,  Judd  and  Cowling2  found, 
with  repeated  exposures  of  ten  seconds  each,  a  gradual  development 
of  mastery  of  the  figure.  Some  subjects  got  at  first  the  general  out- 
line and  later  the  details;  while  others  neglected  the  general  out- 
line at  first  and  mastered  a  few  details,  adding  to  them  in  later  trials. 
In  all  cases,  there  was  a  shifting  of  attention  as  the  exposures  were 
repeated,  so  that  now  one  part,  and  now  another  came  into  promi- 
nence. Further,  in  this  more  deliberate  process,  as  distinguished 
from  the  rapid  succession  of  trial  percepts  spoken  of  above,  there 
was  not  a  strict  following  of  the  method  of  trial  and  error,  but  some 
individuals  adopted  a  definite  plan  of  operation  from  the  beginning; 
and  formulations  in  general  terms  of  the  way  the  figure  was  con- 
structed were  sometimes  in  evidence  before  the  details  were  mastered. 
On  the  whole,  then,  perception  presents  certain  features  essentially 
the  same  as  the  processes  involved  in  the  reaching  of  a  successful 
motor  reaction  to  a  situation,  as  the  latter  were  described  in  the  pre- 
ceding chapter. 

§  4.  Logic  shows  the  importance  of  general  and  abstract  terms, 
and  it  will  therefore  be  worth  while  to  consider  how  abstractions 

1  Hamilton,  "The  Perceptual  Factors  in  Reading,"  Archives  of  Psychol,  No. 
IX,  1907. 

2  Psychol.  Rev.,  Monogr.  Suppl.  XXXIV,  1907,  p.  349. 


596  THE  MECHANISM  OF  THOUGHT 

are  developed.  The  prominent  characteristic  of  an  abstract  con- 
ception is  that  it  neglects  many  of  the  features  of  a  concrete  situa- 
tion or  thing,  and  considers  only  certain  features.  A  thing  cannot 
exist  except  in  the  concrete,  and  in  a  concrete  setting;  but  the  con- 
crete setting  and  many  of  the  concrete  details  of  a  particular  thing 
can  be  neglected  for  the  purposes  of  thought,  and  so  the  thing  be 
treated  as  abstract.  Indeed,  if  the  mind  were  incapable  of  this  proc- 
ess of  so-called  abstraction,  all  the  generalizations  of  science  and 
of  practical  life  would  be  impossible. 

Logic  usually  speaks  of  ideas  or  concepts  only  as  being  abstract; 
but  psychologically  considered,  a  percept  may  be,  and  indeed  must 
be,  abstract;  since  every  percept  isolates  some  feature  of  a  thing  or 
situation,  and  for  the  moment  neglects  the  other  features.  A 
motor  reaction  does  the  same,  for  it  is  never  a  reaction  equally  and 
indiscriminately  to  the  whole  situation,  with  all  its  concrete  feat- 
ures, but  is  predominantly  a  reaction  to  some  particular  feature. 
Both  motor  reaction  and  perception  treat  a  situation,  therefore, 
as  the  bearer  of  the  feature  to  which  response  is  made;  and  thus  the 
situation  is  generalized,  while  the  feature  perceived,  or  reacted  to, 
is  abstracted,  though  it  actually  remains  concrete.  Every  percept 
is,  therefore,  a  partial  analysis  of  the  situation,  emphasizing  one 
feature  to  the  neglect  of  others. 

This  process  of  analysis  or  emphasis  is  often  considered  under 
the  name  of  attention',  and  some  of  the  results  of  the  study  of  atten- 
tion are  of  importance  in  our  present  inquiry.1  Attention  is  closely 
related  to  analysis  and  abstraction,  since  it  is  always  directed  to 
some  feature  of  a  situation,  others  being  neglected  for  the  moment; 
attention  is,  in  other  words,  selective.  Yet  attention  should  not  be 
identified  completely  with  analysis;  for  analysis  occurs  without  at- 
tention. A  motor  response  to  some  particular  feature  of  a  situation 
may  occur  automatically,  while  attention  is  elsewhere  directed ;  and 
perception  of  particular  features  may  also  occur  without  attention 
being  especially  directed  to  them.  While  attention  is  directed  upon 
some  visible  object,  sounds  which  occur  may  be  perceived  as  voices, 
footsteps,  and  the  like;  or,  while  attention  is  directed  to  some  "in- 
ternal" thought,  the  eyes,  if  open,  are  sure  to  be  accurately  di- 
rected upon  some  object,  and  this  object  is  apprehended  as  an  ob- 
ject (for  example,  as  a  word,  when  the  eyes  rest  inattentively  on  the 
page  and  the  thoughts  are  wandering).  What  is  not  especially  at- 
tended to,  therefore,  by  no  means  remains  "pure  sensation,"  or 
wholly  undifferentiated  from  the  total  situation. 

Moreover,  several  motor  or  perceptual  reactions  may  occur  at  the 

1  For  a  fuller  treatment,  see  Pillsbury,  Attention  (London,  1908),  and  Wirth, 
Die  experimentelle  Analyse  der  Bewusstseinsphdnomene  (Braunschweig,  1908). 


THE  SPAN  OF  ATTENTION  597 

same  time,  but  attention  is  usually  concerned,  in  any  special  way, 
with  only  one  of  these.  These  facts  make  it  difficult,  and  at  present 
impossible,  to  define  attention  as  a  special  function,  or  in  any  terms 
except  those  of  degree.  The  descriptive  definition  which  is  usually 
attempted  takes  a  figurative  form.  Of  all  the  contents  of  conscious- 
ness at  any  moment,  some  are  said  to  have  the  quality  of  "clear- 
ness," and  it  is  the  clear  contents  which  are  spoken  of,  in  other 
words,  as  the  objects  of  attention.  "Clearness"  has  here  a  special 
meaning,  which  can  only  be  defined  by  referring  back  to  attention. 
A  vague  or  obscure  impression  may  be  clear  in  this  sense,  since 
it  can  be  the  object  of  attention.  A  definition  in  terms  of  degrees 
of  consciousness  is  perhaps  admissible.  We  are,  at  any  moment, 
more  conscious  of  some  objects  than  of  others,  and  it  is  the  objects 
of  which  we  are  most  conscious  to  which  we  are  said  to  attend,  or 
which  are  said  to  be  "  clear."  Within  the  field  of  consciousness  there 
are  gradations,  and  the  most  conscious  part  of  this  broader  field 
is  the  narrower  "field  of  attention."1 

§  5.  The  attempt  to  fix  the  number  of  objects  which  can  be  at- 
tended to  at  once,  or  to  measure  the  so-called  "  span  of  attention," 
has  been  made  by  determining  how  many  small  and  discrete  units 
can  be  grasped  in  one  presentation.  Cattell 2  found  that  four  or  five 
short  lines  could  be  apprehended  correctly  in  a  brief  exposure;  be- 
yond this,  perception  was  not  clear  and  reliable,  though  approxi- 
mate judgments  were  possible  with  considerably  larger  numbers. 
The  span  was  about  the  same  for  disconnected  letters ;  but  when  the 
letters  were  combined  into  short  words,  about  four  words  could  be 
perceived.  Similar  results  have  been  obtained  in  other  senses.  It 
is  not  so  much  the  span  of  attention  as  the  span  of  apprehension  that 
is  measured  in  these  experiments ;  that  is,  it  is  not  so  much  the  extent 
of  the  field  of  clear  consciousness  as  the  extent  and  kind  of  object 
which  can  be  perceived  as  a  unit,  that  determines  the  result.  Train- 
ing increases  the  "  span,"  not,  probably,  by  increasing  the  amount 
of  clear  consciousness,  but  by  developing  "higher  units"  of  percep- 
tion. After  training  has  done  its  perfect  work,  apprehension  may 
proceed  by  large  spans,  while  attention  is  elsewhere  occupied.  The 
span  of  attention,  therefore,  measures,  not  so  much  the  area  of  the 
field  of  attention,  as  the  number  of  stimuli  which  can  be  taken  care 
of  in  a  brief  perceptual  reaction  (compare  p.  498), 

1  For  a  treatment  of  attention  as  one  of  the  "  most  general  forms  of  all  mental 
life,"  and  the  condition  and  accompaniment  of  all  consciousness,  see  Ladd, 
Psychology,  Descriptive  and  Explanatory,  chap.  III. 

2Wundt's  Philos.  Studien,  1886,  III,  94,  121.  This  is  one  of  the  oldest  ex- 
periments in  psychology,  having  been  performed,  in  a  relatively  crude  way,  by 
Bonnet  and  by  Sir  William  Hamilton. 


598 


THE  MECHANISM  OF  THOUGHT 


§  6.  The  shifting  of  attention  from  one  object  to  another  is  a  fact 
of  common  observation,  of  which  psychology  supplies  examples 
of  great  precision.  The  most  instructive  cases  are  those  in  which 
the  objective  situation  does  not  change,  and  yet,  from  internal 
causes,  now  this  feature  and  now  that  becomes  prominent.  A  good 
example  is  afforded  by  the  changes  in  appearance  of  the  staircase 
and  other  similar  figures,  which  have  several  times  been  mentioned. 
Perhaps  a  still  better  example  is  seen  in  McDougallV  "  dot  figure," 

the  particular  construc- 
tion of  which  can  be 
varied  ad  libitum.  If 
this  figure  is  steadily  ex- 
amined, ever  new  groups 
and  arrangements  ap- 
pear within  it;  but  it  is 
practically  impossible  to 
hold  any  one  for  a  long 
time.  "  Varied  reaction  " 
continually  occurs  in  a 
pronounced  degree. 
Changes  of  the  fixation 
point  of  the  eyes  are 
also  likely  to  favor 
change  in  the  group- 
ings; but  the  shifting  of 
appearance  is  by  no 
means  fully  explained  by  these  changes  of  fixation.  The  main  fact 
is  that  a  complex  stimulus,  which  can  be  apprehended  in  several 
ways,  or  out  of  which  several  features  can  be  isolated — all  equally 
easy  and  of  equal  significance — gives  rise  to  a  succession  of  differ- 
ent percepts. 

A  similar  instance  of  the  shifting  of  attention,  or  of  perception, 
is  found  in  "  binocular  rivalry." 2  Ordinarily,  both  eyes  receive 
only  slightly  differing  views  of  an  object,  and  these  views  are  com- 
bined or  blended  in  a  single  percept  (compare  p.  425).  But  if, 
artificially,  and  most  easily  by  aid  of  the  stereoscope,  very  different 
views  are  brought  before  the  two  eyes,  the  conditions  suitable  for 
single  vision  are  not  fulfilled,  and  there  is  likely  to  be  an  oscilla- 
tion, in  consciousness,  between  the  deliverance  of  one  eye  and  that 
of  the  other-.  If  a  plain  red  field,  for  example,  is  held  before  one 
eye,  and  a  plain  green  field  before  the  other,  we  see  red  and  green, 
not  usually  together,  but  alternately.  When  one  of  the  fields  is 

1  Mind,  1902,  N.  S.,  XI,  316. 

*  Compare  Helmholtz,  Physiologische  Optik,  p.  766. 


FIG.  153.— The  Dot  Figure  (McDougall). 


SHIFTING  OF  ATTENTION  599 

plain,  and  the  other  contains  a  figure,  the  figure  remains  seen  for 
most  of  the  time,  though  the  background  may  oscillate  as  in  the 
cases  of  the  other  figures.  Voluntary  attention  devoted  to  one  field 
gives  it  some  advantage  over  the  other,  by  bringing  it  back  more 
quickly  and  thus  abridging  each  appearance  of  the  other  field. 
This  is  true  to  some  extent  even  if  both  fields  are  plain;  but  volun- 
tary attention  has  much  more  effect  in  holding  a  field  which  pre- 
sents considerable  detail  and  so  allows  of  a  succession  of  percepts.1 

It  is  clear  that  binocular  rivalry  belongs  in  large  measure  to  the 
level  which  we  are  apt  to  distinguish  as  "  physiological,"  in  contrast 
with  the  mental;  but  it  is  equally  clear  that  the  value  of  each  of  the 
two  opposing  stimuli  is  important  in  giving  the  advantage  to  one  or 
the  other  of  them.  Binocular  rivalry  reminds  us,  on  the  one  hand, 
of  the  oscillation  of  attention  between  two  topics  of  interest;  and, 
on  the  other  hand,  of  the  alternation  of  two  reflexes  when  the  stimuli 
for  both  are  continuously  acting  (p.  173).  '  Apparently,  then,  we 
may  formulate  a  law  of  the  shiftings  of  attention  in  almost  the  same 
terms  as  those  of  the  law  for  the  alternation  of  reflexes.  When  two 
stimuli  act  simultaneously,  either  of  two  results  may  happen:  The 
two  may  facilitate  each  other's  action — that  is,  in  psychical  terms, 
they  may  give  rise  to  a  combined  or  blended  percept;  or  they  may 
be  incompatible  with  each  other,  in  which  case  they  will  not  give 
a  resultant  effect,  but  one  or  the  other  will  get  the  "right  of  way" 
and  inhibit  all  response  to  the  other  for  the  time  being.  This  in- 
hibition, however,  is  followed  by  a  "back  swing"  which  gives  the 
advantage  to  the  response  that  has  just  been  inhibited;  and  so  an 
alternation  of  two  responses  occurs.  Accordingly,  it  would  be  better 
to  substitute  for  "shifting  of  attention"  some  such  term  as  "shift- 
ing of  perceptual  reaction";  since  rivalry  between  the  eyes,  and  be- 
tween different  appearances  of  an  ambiguous  figure  as  well,  oc- 
curs even  if  attention  is  directed  to  something  entirely  apart. 

§  7.  What  have  been  called  "fluctuations  of  attention"  also  oc- 
cur, if  the  attempt  is  made  to  hold  continuously  under  observation 
something  which  is  just  above  the  threshold  of  perceptibility,  such 
as  the  ticking  of  a  watch  when  the  watch  is  held  at  a  distance  such 
as  to  be  barely  audible;  or  such  as  a  small  and  barely  perceptible 
speck  of  white,  or  a  patch  of  gray  just  distinguishable  in  brightness 
from  its  background.  In  these  cases,  the  watch  is  heard  for  a  time, 
then  becomes  inaudible,  and  then  is  heard  again;  and  similar  alter- 
nations occur  with  the  other  stimuli.  No  one  interpretation  of  these 
experiences  is  altogether  satisfactory;  for  oscillations  in  the  condition 

1  For  these  and  several  other  facts  regarding  this  important  phenomenon, 
see  Breese,  Psychol.  Rev.,  Monogr.  Suppl.  No.  XI,  1899;  also  Sherrington, 
The  Integrative  Action  of  the  Nervous  System  (New  York,  1906). 


600  THE  MECHANISM  OF  THOUGHT 

of  the  sense-organs  may  as  well  be  the  cause  as  oscillations  in  atten- 
tion, or  "perceptual  reaction";  and  the  fluctuations  in  perception 
do  correspond  to  some  extent  to  oscillations  in  the  blood  pressure 
and  in  respiration.1 

The  tendency  of  attention  to  shift  adds  another  factor  to  those 
spoken  of  in  the  preceding  chapter  as  determining  recall.  It  is  not 
simply  the  stimulus,  the  stock  of  reproductive  tendencies,  with  their 
varying  strength,  and  the  adjustment  of  the  moment,  which  are  ef- 
fective in  governing  recall;  but  the  tendency  to  change  operates 
counter  to  perseveration  and  to  the  factor  of  recency,  which,  with- 
out it,  would  probably  control  the  mental  train  so  as  to  make  mental 
development  difficult  or  impossible. 

§  8.  The  direction  of  attention,  or  of  analytic  perception,  is  de- 
termined in  part  by  dispositions  left  behind  through  past  experi- 
ence, as  was  said  a  few  paragraphs  back.  It  is  not,  however,  en- 
tirely so  determined;  for  certain  classes  of  stimuli  have  a  natural 
or  instinctive  hold  on  attention.  Very  intense  stimuli  compel  at- 
tention, as  do  sudden  stimuli,  sudden  changes  in  a  stimulus,  or 
moving  objects  in  the  field  of  view,  or  over  the  skin;  sharp  con- 
tours in  the  field  of  vision,  and  objects  contrasting  strongly 
with  their  background,  have  a  similar  attraction.  These  causes 
operate  even  in  young  children,  independently  of  previous  experi- 
ence, and  lead  to  some  degree  of  analysis  of  a  presented  complex 
of  stimuli.  Each  individual  also  possesses  native  interests,  which 
make  their  appearance  as  he  grows  older,  and  which  lead  him  to 
analyze  out  and  react  to  certain  features  of  situations  to  which  other 
individuals  may  remain  relatively  inattentive.  In  proportion  as 
these  instinctive  analyses  are  exercised  and  trained,  they  afford  a 
basis  for  further  analyses  of  a  more  varied  and  more  specialized 
character,  so  that  the  adult's  attention  becomes  directed  very  largely 
by  his  acquired  tendencies. 

§  9.  Psychology,  as  studied  from  the  physiological  and  experi- 
mental points  of  view,  can  make  at  present  only  meagre  contribu- 
tions to  a  theory  of  the  development  of  abstract  concepts.  But 
there  is  one  more  fact  which  should  be  mentioned  in  this  connection. 
Though  a  percept  involves  abstraction,  a  recalled  impression  is 
likely  to  surpass  its  percept  in  this  respect;  since  the  setting  is  more 
fully  neglected  in  recall  than  in  perception.  Recall  of  the  full  setting 
of  an  impression  is  seldom  possible  (compare  p.  582);  but  recall 
of  a  feature  which  has  previously  been  observed  often  occurs  without 
recall  of  the  setting.  Among  the  features  of  a  situation,  which  can 

1  Urbantschitsch,  Centralblatt  f.  d.  med.  Wissensch.,  1877,  p.  626;  Lange, 
Wundt's  Philos.  Stud.,  1887,  IV,  390;  Lehmann,  ibid.,  1894,  IX,  66;  Slaughter, 
Amer.  Journ.  of  Psychol,  1901,  XII,  313. 


THE  NATURE  OF  COMPARISON  601 

be  relatively  isolated  in  perception,  are  relations,  forms,  groupings, 
and  meanings  of  the  most  varied  character.  Each  of  these  factors  is 
actually  presented  in  some  concrete  setting;  but  it  is  partially  ana- 
lyzed out  of  the  situation  by  the  act  of  perception.1  Now  it  appears 
that  any  such  feature  which  has  been  observed  can  be  recalled  with- 
out its  setting;  or,  the  setting  may  be  only  vaguely  recalled  while  the 
feature  in  question  comes  to  clear  consciousness.2  Even  so  "ab- 
stract" a  feature  as  the  meaning  of  a  proverb  can  be  recalled  and 
identified  as  equivalent  to  that  of  another  proverb,  while  the  words 
in  which  it  first  occurred  are  altogether  forgotten.3  This  tendency 
is  of  importance  in  the  development  of  abstract  concepts. 

§  10.  Comparison,  or  the  judgment  of  likeness  or  difference,  is, 
like  simple  perception,  dependent  on  memory.  There  are  excep- 
tions to  this  rule,  especially  in  the  case  of  different  colors  or  degrees 
of  brightness  when  lying  immediately  side  by  side;  for  then  an  ele- 
mentary apprehension  of  difference  involves  little  more  than  the  per- 
ception of  the  line  separating  the  surfaces.  But  in  most  cases,  the 
objects  to  be  compared  must  be  examined  one  after  the  other,  so 
that  memory  of  the  first  one  examined  is  necessary.  Such  judgments 
have  been  frequently  subjected  to  psychological  experimentation, 
although  attention  has  usually  been  directed  to  the  accuracy  of  the 
comparison  rather  than  to  the  process  by  which  the  judgment  is 
reached. 

A  stock  experiment  in  judgments  of  this  sort  consists  in  the  com- 
parison of  two  weights,  first  one  and  then  the  other  being  lifted,  and 
the  judgment  being  announced  after  the  second  lift.  It  would  seem 
theoretically  necessary  that  the  experience  of  lifting  the  first  weight 
should  be  recalled  for  comparison  with  the  experience  of  lifting  the 
second;  but  Schumann,4  who  had  an  intimate  knowledge  of  this  ex- 
periment, asserted  that  this  is  not  the  case,  and  that  no  recollection 
of  the  first  weight  need  be  present  in  consciousness  when  the  sec- 
ond is  lifted.  Kulpe5  introduced  here,  as  in  the  case  of  recognition, 
the  distinction  between  mediate  and  immediate  judgments,  and  ex- 
periment6 has  abundantly  justified  this  distinction,  in  the  compari- 
son not  only  of  weights  but  also  of  other  successively  presented 

1  Compare  Ehrenfels,  "Uber  Gestaltqualitaten, "  Vierteljahrschr.  f.  wiss.  Philos., 
1890,  XIV,  249;  Griinbaum,  Arch.  /.  d.  ges.  PsychoL,  1908,  XII,  452. 

2  Compare  Wundt,  PsychoL  Studien,  1907,  III,  301;    Titchener,  The  Experi- 
mental Psychology  of  the  Thought-Processes  (New  York,  1909). 

3  Biihler,  Arch.  f.  d.  ges.  PsychoL,  1908,  XII,  24. 
'Zeitschr.  f.  PsychoL,  1898,  XVII,  119. 

5  Grundriss  der  Psychologic,  1893,  p.  212. 

6Bentley,  Amer.  Journ.  of  PsychoL,  1899,  XI,  1;  Whipple,  ibid.,  1901,  XII,  409, 
and  1902,  XIII,  219. 


602  THE  MECHANISM  OF  THOUGHT 

stimuli.  Martin  and  Miiller,1  who,  more  than  other  experimenters, 
have  sought  to  dissect  the  process  of  comparison  in  the  case  of 
weights,  also  found  that  no  clear  recollection  of  the  first  weight  need 
be  present  when  the  judgment  was  made.  Often  introspection 
revealed  nothing  like  a  comparison  of  the  two  weights — if  by  "  com- 
parison" be  meant  some  form  of  holding  them  together — but  the 
judgment  seemed  to  be  based  simply  on  the  impression  produced  by 
the  second  weight. 

If  now  we  attempt  an  interpretation  of  what  must  seem  at  first 
rather  a  mysterious  process,  we  can  make  use  of  conceptions  which 
have  become  familiar  in  preceding  discussions.  It  is  probable  that 
the  first  weight  arouses  an  adjustment  or  preparation  for  the  sec- 
ond, such  that  the  effect  of  the  second  will  be  different  in  dependence 
upon  its  being  either  heavier  or  lighter  than  the  first.  The  adjust- 
ment may  consist  in  regulating  the  muscular  force  with  which  the 
second  weight  is  lifted;  if,  for  instance,  the  force  is  so  gauged  that 
it  would  lift  the  first  weight  with  a  moderate  speed,  and  the  second 
weight  yields  tardily  to  this  force,  then  the  second  weight  appears 
heavier  than  the  first;  but  it  appears  lighter,  if  it  yields  with  great 
promptness.  Repeated  lifting  of  the  same  weight  perfects  the  ad- 
justment of  muscular  force  to  it;  so  that  when  this  weight  is  com- 
pared with  other  weights  a  little  heavier  or  lighter,  in  a  long  series 
of  trials,  the  delicacy  of  adjustment  to  the  central  weight  may  be  such 
that  lifting  one  of  the  lighter  weights  will  convey  an  "absolute  im- 
pression" of  lightness,  etc.,  and  upon  this  impression  the  judgment 
may  be  based.2  It  is  probable  that  this  conception  of  the  adjust- 
ment set  up  in  comparing  weights,  which  is  founded  on  the  theory 
of  Miiller  and  Schumann,3  is  not  fully  adequate;  and  certainly  an 
adjustment  of  this  motor  type  cannot  easily  be  conceived  as  available 
in  the  comparison  of  pitch,  brightness,  and  many  other  kinds  of 
stimuli.  Some  kind  of  cerebral  adjustment  is  likely,  however,  to 
be  set  up  in  anticipation  of  any  stimulus  the  character  of  which  is 
approximately  foreknown.  When  the  expectation  is  not  fulfilled, 
a  shock  of  surprise  is  experienced,  but  this  surprise  does  not  by  any 
means  imply  that  an  image  of  the  expected  event  is  present  in  mind 
alongside  of  the  actual  event,  but  only  that  something  happens 
for  which  we  are  not  ready,  i.  e.,  not  adjusted.  Innumerable  ex- 
amples of  this  truth  may  be  derived  from  daily  experiences,  which 
show  that  the  way  the  mind  reacts  to  any  new  event  requiring  judg- 

1  Zur  Analyse  der  Unterschiedsempfindlichkeit,  1899. 

2  Martin  and  Miiller,  op.  cit. 

3  Arch.  f.  d.  ges.  PhysioL,  1889,  XLV,  37.     Interesting  examples  of  the  reality 
of  motor  adjustment  or  "  Einstellung "  are  given  in  this  paper  and  in  a  supple- 
mentary study  by  Laura  Steffens,  Zeitschr.  /.  PsychoL,  1900,  XXIII,  241. 


THE  PSYCHOLOGY  OF  REASONING  603 

ment  depends  upon  a  multiplicity  of  pre-existing  conditions,  many 
of  which  are  barely  above  the  threshold  of  consciousness  or  even 
wholly  subliminal.  Such  experiences  vary  from  the  physical 
shock  which  we  encounter  on  miscalculating  the  width  or  the 
height  of  the  step  of  a  stair,  to  the  spiritual  shock  with  which  an 
insulting  look,  or  an  unpleasant  memory,  or  an  immoral  thought, 
is  greeted  in  consciousness. 

Fine  comparisons  require  a  much  more  delicate  adjustment 
than  is  present  in  many  cases  of  expectation,  but  the  inner  process  is 
probably  of  the  same  general  character.  Since  these  are  generally 
made  with  more  deliberation,  and  what  may  figuratively  be  called 
"weighing  of  evidence,"  we  often  find  ourselves  alternating  between 
mental  images  that  represent  the  more  recent,  and  the  more  remote, 
of  the  sensuous  impressions  that  are  to  be  compared.  But  if  the 
attempt  is  made  to  tell  "just  how  much"  the  more  recent  experi- 
ence differs  from  the  earlier,  a  voluntary  effort  to  recall  more  dis- 
tinctly the  memory  image  of  the  earlier  is  quite  sure  to  follow.  In 
all  these  cases  of  so-called  "comparison,"  when  considered  from 
the  introspective  point  of  view,  enough  of  recollection  is  involved  to 
account  for  that  recognition  of  the  character,  amount,  and  direction, 
of  the  change  in  the  sensation  elements  of  consciousness  on  which 
the  judgment  is  based. 

§  11.  The  psychology  of  reasoning  is  even  less  understood  than 
that  of  abstraction  and  comparison.1  Little  aid  can  be  derived 
from  logic  for  an  understanding  of  the  actual  processes  which  go 
on,  either  in  consciousness  or  below  the  threshold,  in  an  act  of 
reasoning.  The  formal  character  of  the  syllogism  is  undoubtedly 
a  travesty,  rather  than  a  description,  of  most  concrete  instances  of 
inference;  and,  in  particular,  the  major  premise,  or  general  principle 
involved,  is  almost  always  implied  rather  than  overtly  expressed 
in  the  thinking  process.  But  there  is  a  more  serious  exception  to 
be  made.  The  syllogism  treats  inference  as  though  it  were  a 
straight-ahead  process;  the  premises  are  supposed  to  be  given,  and 
the  conclusion  is  supposed  to  follow  from  them  by  "laws  of  thought" 
which  are  axiomatic  and  compelling.  If  this  were  a  true  account, 
rational  thinking  would  be  a  much  simpler  and  easier  matter  than 
it  is.  But  reasoning  is  not,  in  fact,  a  straight-forward  progress,  and 
does  not  proceed  by  rule;  neither  are  the  premises  customarily 
recognized  as  given.  This  is  true,  at  least,  in  a  large  proportion  of 
concrete  cases  of  the  discovery  of  causes  and  the  drawing  of  in- 
ferences. It  is  comparatively  unusual  to  ask:  "What  further  use 
can  be  made  of  the  knowledge  which  I  possess?"  The  usual 

1  See  Pillsbury,  The  Psychology  of  Reasoning  (New  York,  1910);  Dewey,  How 
We  Think  (Boston,  1910). 


604  THE  MECHANISM  OF  THOUGHT 

question  is,  "What  do  I  know  that  will  help  me  to  solve  this  par- 
ticular problem?"  What  is  given  is  a  problem.  The  solution  of 
the  problem  corresponds  to  the  conclusion  of  the  syllogism;  the 
minor  premise  corresponds  to  an  insight  into  the  problem,  i.  e., 
to  the  perception  of  some  feature  which  is  the  key  to  the  situation; 
the  major  premise  consists  of  previously  acquired  acquaintance 
with,  and  knowledge  about,  this  feature;  but  this  knowledge  may 
or  may  not  have  received  a  definite  formulation. 

As  a  hypothetical  example,  we  may  suppose  the  following  thoughts 
to  have  passed  through  the  mind  of  Ebbinghaus,  as  he  considered 
the  problem  whether  anything  once  learned  was  ever  forgotten; 
or  to  what  extent  and  at  what  rate  it  was  forgotten.  Clearly  what 
was  in  his  mind  to  start  with  was  something  more  like  the  conclusion 
than  like  the  premises  of  a  chain  of  reasoning;  he  could  formulate 
alternative  conclusions  from  the  beginning,  but  would  not  know 
which  was  true.  As  he  considered  the  problem,  it  may  have  first 
occurred  to  him  that  learning  produces  some  effect  on  the  mind, 
or  on  the  brain,  and  that  a  change  once  induced  ought  never  to  be 
entirely  lost;  and  therefore  what  has  been  learned  ought  never  to 
be  entirely  forgotten.  In  other  words,  he  has  isolated  from  the 
solution  the  feature,  "Change  produced  in  a  substance,"  and  has 
either  recalled,  or  newly  formulated,  a  general  principle  applica- 
ble to  this  feature.  But  he  may  also  have  had  his  attention  directed 
to  another  feature — namely,  that  he  has  now  to  deal  with  a  living 
creature,  and  that  atrophy  through  disuse  is  characteristic  of  such 
creatures;  and  thus  he  may  have  come  to  doubt  the  validity  of  his 
first  conclusion.  He  may  then  have  been  driven  to  abandon  the 
attempt  to  solve  the  problem  deductively,  and  have  sought  to  ap- 
proach it  inductively,  or  by  way  of  examining  concrete  instances. 
He  may  have  thought  of  the  first  lines  of  the  "^Eneid,"  which  he 
learned  fifteen  years  before — possibly  he  can  recall  some  of  them 
now.  This  failing,  he  was  not  satisfied  to  conclude  that  they  have 
been  all  forgotten,  for  it  might  be  that,  once  started,  he  could  con- 
tinue these  lines.  He  hunts  up  a  Vergil,  and  allows  himself  to 
refresh  his  memory  by  seeing  the  first  line;  but  on  turning  from  the 
book,  he  is  unable  to  continue.  Still  unsatisfied,  for  the  first  line 
had  a  tinge  of  familiarity  which  seemed  to  indicate  some  degree  of 
memory  of  it,  the  thought  may  have  occurred  to  him:  "It  might  be 
easier  to  relearn  these  lines  than  to  learn  an  equal  number  of  lines 
from  some  other  part  which  had  never  received  special  attention." 
He  then  tries  this  supposition,  and  finds  it  correct.  But  now  sub- 
ordinate problems  sprout  out  of  the  main  problem,  bearing  on  the 
best  methods  of  testing  this  matter  of  relearning;  till  finally  the  "sav- 
ing method"  and  the  use  of  nonsense  syllables  are  decided  on  as 


THE  SEARCH  FOR  PREMISES  605 

suitable  means  for  examining  the  degree  of  retention;  the  experi- 
ments are  tried  and  the  solution  reached.  Clearly  the  entire  process 
in  this  case  has  been  anything  but  straight-ahead;  it  has  depended  on 
"  turning  over  in  the  mind  "  the  situation  of  trying  to  recall  something 
long  forgotten,  and  being  "struck  by"  this  and  that  aspect  of  the 
situation  in  the  light  of  previous  experience.  Translated  into  psy- 
cho-physical terms,  it  has  been  distinctly  a  process  .of  "  varied  reac- 
tion"; and  both  the  inductive  and  the  deductive  parts  of  the  process 
are  alike  in  this  respect. 

§  12.  In  a  word,  reasoning  involves  hunting  for  premises,  or  for 
features  which  are  significant;  and  for  past  knowledge  of,  or  reac- 
tions to,  these  features.  As  in  other  processes  of  analysis  and  recall, 
the  course  of  thought  is  determined  in  part  by  the  present  adjust- 
ment, or  direction  toward  a  certain  problem;  partly  by  reproductive 
tendencies  of  varying  strength;  partly  by  the  concrete  situation  with 
its  many  incidental  and  often  confusing  features;  and  partly  by  the 
fundamental  tendency  to  vary  the  reaction. 

Though  the  above  example  is  hypothetical,  it  follows  along  the 
lines  of  actual  instances  of  rational  response  to  a  concrete  problem, 
especially  as  these  are  presented  in  such  trivial  but  easily  observed 
processes  as  the  solution  of  puzzles.1  In  many  such  cases,  it  is 
difficult  to  distinguish  reasoning  sharply  from  complex  cases  of 
"transferred  reaction ";[ the  reasoning  process  consists,  in  fact,  of 
the  transfer  to  a  new  situation  of  some  mode  of  response  previously 
acquired  -in  different  situations,  and  depends  on  some  degree  of 
isolatian-of  features  which  are  common  to  both  the  new  situation  and 
the  old^^he  transference  is  regarded  as  more  rational  in  propor- 
tion as  it  is  conscious  and  deliberate;  and  in  proportion,  also,  as 
the  varied  reactions  are  tried  out  mentally  instead  of  being  impul- 
sively put  to  an  immediate  motor  test.  Though  it  is  apparently 
impossible  to  lay  down  any  straightforward  procedure  for  approach- 
ing the  solution  of  all  problems,  a  few  general  maxims  emerge 
from  the  examination  of  concrete  processes  of  reasoning.  A  good 
supply  of  major  premises — in  the  form  of  reproductive  tendencies 
ready  to  be  brought  into  play — is  a  fundamental  desideratum. 
And  the  more  clearly  these  premises  have  been  thought  out  and  gen- 
eralized, the  more  likely  they  are  to  be  recalled  in  a  variety  of  situa- 
tions. Further,  the  importance  of  keeping  an  open  mind  for  feat- 
ures or  possibilities  which  do  not  at  first  appear — i.  e.,  the  necessity 
of  inventing  and  adopting  varied  reaction — is  so  great  that  it  may 
well  be  used  as  a  maxim  for  the  practical  guidance  of  reasoning. 
Finally,  as  an  offset  to  the  preceding  maxim,  another  may  be  based 
on  the  value  of  dwelling  on  each  feature  long  enough  to  afford  an 
1  Compare  the  previous  reference  to  the  work  of  Ruger,  pp.  555  f. 


606  THE  MECHANISM  OF  THOUGHT 

opportunity  for  any  reproductive  tendencies  connected  with  it  to 
be  brought  into  play. 

§  13.  Reasoning,  like  recognition,  perception,  and  comparison, 
may  be  "  immediate"  as  well  as  "  mediate";1  and  some  of  the  clever- 
est instances  of  reasoning  are  cases  in  which  a  new  situation  is  seen 
to  be  an  exemplification  of  some  general  principle,  without  conscious 
recall  of  the  former  situations  in  which  the  principle  was  learned, 
and  even  without  conscious  recall  of  the  principle  itself  as  inde- 
pendent of  the  case  in  hand.  An  abstract  general  principle,  or  a 
principle  as  exemplified  in  some  former  situation,  needs  always  to 
be  applied  or  adapted  to  the  novel  situation;  but  immediate  reason- 
ing short-circuits  this  process  by  seeing  the  principle,  not  as  "in 
itself,"  so  to  say,  but  as  adapted  or  exemplified.  If  we  possessed 
an  introspective  history  of  moments  of  new  insights  and  discoveries, 
we  should  probably  find  that  the  insight  usually  started  with  this 
immediate  or  implicit  reasoning,  which  was  followed  by  explicit 
statements  such  as  are  necessary  to  present  an  inference  in  lan- 
guage. 

On  glancing  back  over  the  preceding  sketch  of  the  psychology  of 
thinking,  meagre  and  disconnected  as  it  must  seem,  one  cannot  fail 
to  be  impressed  by  the  great  similarity  of  the  mental  process  as  it 
appeared  in  all  those  forms  of  perception,  analysis  and  abstraction, 
comparison  and  reasoning,  as  well  as  those  of  learning  and  recall, 
presented  in  the  preceding  chapter.  All  of  these  performances,  even 
the  most  intellectual,  though  they  differ  greatly  in  content,  preserve 
the  method  of  procedure  that  was  visible  in  learning  by  trial  and 
error.  In  all  cases  of  this  procedure  there  appears  the  controlling 
influence  of  some  problem  or  aim  which  so  sets  or  adjusts  the  psy- 
cho-physical mechanism  as  to  select  the  relevant  associative  tenden- 
cies and  make  them  prevail  above  others  which  are  irrelevant  to  the 
problem  in  hand.  In  all  cases,  too,  the  process  of  solution — with  the 
exception  of  persons  well  drilled  in  very  similar  problems — is  usu- 
ally far  from  straightforward;  it  has  rather  the  form  of  varied  and 
tentative  reaction;  and  in  all,  an  essential  fact  is  the  dealing  with 
parts  or  features  of  a  situation  by  emphasizing  each  in  turn  to  the 
temporary  neglect  of  other  features.  Such  thinking  appears,  in 
other  words,  as  a  species  of  observation  with  reaction;  and  the  de- 
grees of  its  intellectuality  and  practical  success  depend  largely  on 
what  is  observed,  i.  e.,  on  the  content  or  material  of  the  process  rather 
than  on  its  form.  It  is  a  question  of  what  features  of  a  situation  are 
isolated  or  analyzed  out,  and  made  the  starting-point  for  recall  and 
motor  reaction.  To  note  the  fall  of  an  apple  is  to  analyze  out  a  com- 

1  Storriug,  Arch.  f.  d.  ges.  Psychol,  1908,  XI,  1;  Pillsbury,  The  Psychology  of 
Reasoning,  pp.  119-120. 


IMMEDIATE  AND  MEDIATE  REASONING  607 

paratively  obvious  feature;  to  distinguish  falling  as  a  feature  of  the 
moon's  revolution  about  the  earth  is  still  analysis,  proceeding  in  much 
the  same  manner,  but  dealing  with  more  difficult  material  and  work- 
ing with  much  more  refined  and  elaborate  tools. 

§  14.  During  all  the  discussions  of  the  last  five  chapters,  and,  in- 
deed, throughout  the  entire  Part  II  of  the  book,  one  truth  of  su- 
preme importance  has  been  both  assumed  and  illustrated,  even 
where  it  has  not  been  made  perfectly  obvious.  This  truth  has  been 
embodied  in  the  language  which  we  have  found  ourselves  forced 
to  employ.  The  similarity  of  the  terms,  and  even  in  many  cases 
their  identity,  which  were  used  in  describing  the  histological  struct- 
ure and  physiological  functions  of  the  nervous  mechanism,  when 
studied  from  the  points  of  view  afforded  by  kindred  natural  and 
physical  sciences,  with  the  terms  used  in  describing  the  condi- 
tions and  activities  of  the  conscious  mental  life,  when  studied  from 
the  introspective  point  of  view,  can  scarcely  have  escaped  notice. 
This  "parallelism"  is  undoubtedly  of  the  highest  significance.  It 
is  true  that  some  of  these  terms  are  rather  highly  figurative,  and 
constructed  upon  a  basis  of  analogy  rather  than  of  observed  facts; 
it  is  also  true  that  many  of  them  only  imperfectly  suggest  what  we 
are  led  to  believe  are  the  actual  phenomena,  and  the  laws  of  their 
interdependence.  On  the  other  hand,  it  cannot  be  doubted  that  they 
describe,  on  the  whole  faithfully,  the  real  character  of  the  facts. 
But  this  is  to  say  that,  between  the  nervous  mechanism  and  the  phe- 
nomena of  conscious  mental  life,  as  respects  the  constitution  and  the 
behavior  of  both,  there  is,  in  man's  case  also,  an  extensive  and  in- 
tricate system  of  " correlations" 

Among  those  correspondences  of  language  which  are  significant 
of  correlations  in  fact,  the  following  are  some  of  the  more  important. 
In  describing  the  structure  and  functions  of  the  nervous  mechanism 
and,  as  well,  the  nature  and  development  of  mental  life,  it  was  found 
necessary  to  employ  terms  indicative  of  a  great  variety  of  highly 
differentiated  elements  combining  in  manifold  intricate  ways,  and  in 
different  degrees  of  intensity,  to  bring  about  more  and  more  com- 
plicated and  purposeful  results.  We  spoke  of  "stimuli"  and  "re- 
actions" to  stimuli,  in  the  case  both  of  mechanism  and  of  mind. 
But  the  one  is  a  physical,  the  other  a  psychical,  sequence.  We  took 
note  of  "transferences"  and  "consequences";  of  "facilitation" 
and  "inhibition,"  or  "interference";  of  the  acquirement  of  "habits 
of  reaction"  as  shown  in  the  recurrence  of  states  or  forms  of  action, 
essentially  similar  although  modified  in  various  details.  There 
were  tokens  of  the  laws  of  "  conservation "  and  of  "  perseveration " 
in  both  nervous  mechanism  and  mental  life.  But  especially  was  it 


608  THE  MECHANISM  OF  THOUGHT 

significant  to  see  how  the  development  of  both  mechanism  and  mind 
was  dependent  upon  the  speedy  and,  so  to  say,  accurate  formation 
of  "higher  units"  of  reaction  out  of  the  more  elementary  factors; 
and  how  in  the  formation  of  such  units  the  more  elementary  con- 
ditions and  reactions  were  absorbed  or  lost  out  of  sight.  Indeed,  if 
now  all  the  facts  and  laws  hitherto  treated  are  passed  in  review  as 
illumined  by  this  thought  of  the  actuality  of  the  relations  between 
mechanism  and  mind,  as  testified  to  in  the  very  language  employed 
to  describe  both — the  one  from  the  objective,  and  the  other  from  the 
introspective  point  of  view — the  significance  of  the  term  "physio- 
logical psychology"  will  be  enforced  and  illustrated  anew. 

§  15.  In  closing  this  Part  of  our  work,  therefore,  we  may  fitly 
attempt  to  summarize  some  of  the  results  in  the  form  of  established 
correlations  between  the  nervous  mechanism  and  mental  life,  while 
leaving  details  to  the  further  and  renewed  study  of  the  same  class  of 
facts  which  have  occupied  our  attention,  and  been  made  the  basis 
of  our  theories,  from  the  beginning  of  the  book.  The  general 
problem  with  which  we  have  been  concerned  may  now  be  formu- 
lated anew  in  somewhat  the  following  way:  What  conception 
can  be  formed  of  cerebral  action,  in  the  combined  light  of  the 
facts  of  anatomy,  physiology,  and  pathology,  as  these  were  set  forth 
in  the  first  Part  of  the  book,  and  of  the  facts  of  psychology  as  set 
forth  in  its  second  Part?  Without  necessarily  looking  to  cerebral 
action  as  the  explanation  of  mental  activity,  one  may  be  convinced 
that  the  brain  is  active  in  mental  operations,  and  may  seek  to  form 
scientific  conceptions  of  the  character  of  its  activity.  It  is  certainly 
to  psychology  that  the  neurologist  must  appeal,  in  large  measure,  for 
the  facts  of  behavior  on  which  his  conceptions  of  such  activity  must 
be  based.  It  will  be  admitted  at  once  by  the  psychologist  that  the 
data  supplied  by  him  are  deficient  not  only,  for  the  time  being,  in 
range  and  quantity,  but  also  in  minuteness  and  definiteness  of  de- 
tail. As  was  shown  in  considering  the  subject  of  localization  of 
cerebral  function,  the  brain  must  work  in  much  greater  detail  than 
is  visible  to  the  observation  of  the  psychologist,  whether  his  observa- 
tion be  of  the  introspective  or  of  the  objective  type.  From  the  latter 
point  of  view,  he  determines  the  reaction  of  the  subject  to  stimuli, 
and  obtains  a  rough  dynamics  of  behavior;  in  introspection,  he  seems 
to  have  some  insight  into  the  process  intervening  between  stimulus 
and  reaction,  and  so  to  penetrate  somewhat  to  the  inside  of  behavior. 
But  a  reaction,  as  judged  by  its  end-effect,  is  a  mass  action;  and 
introspection  shows  little  of  the  detail  which  must  be  present  in 
all  brain  action.  Hence  there  is  no  immediate  prospect  of  obtain- 
ing from  psychology  a  full  and  minute  account  of  any  single  men- 
tal performance,  or  of  reaching  by  this  means  anything  like  a  de- 


CEREBRAL  CONDITIONS  OF  CONSCIOUSNESS      609 

tailed  description  of  the  cerebral  process.  The  most  that  can  be 
hoped  is  to  gain  some  indications  of  the  general  character  of  cere- 
bral action. 

§  16.  Peculiar  difficulties  encompass  the  problem  of  determining 
what  cerebral  activities  are  the  indispensable  conditions  of  conscious- 
ness, in  general.  For,  not  only  do  we  find  what  must  appear  to  us 
as  abundant  signs  of  intelligent  and  purposeful  activity  in  animals 
of  a  simple  and  low  form  of  nervous  organization,  or  of  no  strictly 
nervous  mechanism  whatever,  but  also  in  plant  life,  and  even  in 
the  ova  and  protozoa,  and  in  the  individual  cells  which  are  aggre- 
gated to  form  both  vegetable  and  animal  tissues.  As  to  a  full  ex- 
planation of  consciousness  in  terms  of  brain  activity,  to  propose  the 
problem  is  to  expose  the  absurdity  of  an  attempt  at  its  answer.  But 
to  inquire  as  to  what  are  the  cerebral  conditions  of  consciousness  in 
man's  case,  is  a  legitimate  and  interesting  problem  for  physiological 
psychology.  Alas!  that  the  answer  to  the  problem  still  remains  so 
incomplete. 

Consciousness  may  reasonably  be  taken  as  indicating  brain  ac- 
tivity; in  other  words,  when  there  is  consciousness,  then  the  brain 
is  active.  And  degrees  of  consciousness  may  probably  be  taken  as 
indicative  of  degrees  of  brain  activity.  The  field  of  attention  may 
therefore  be  taken  as  an  index  of  the  field  of  greatest  brain  activity. 
Many  parts  of  the  cerebrum — many  systems  of  nervous  connec- 
tions within  it — are  likely  to  be  simultaneously  active;  it  is,  therefore, 
a  reasonable  supposition  that  the  most  conscious  part  of  conscious- 
ness is  an  index  of  the  most  active  part  of  the  brain;  so  that,  when 
sights  and  sounds  are  both  simultaneously  present  in  consciousness, 
but  sights  occupy  the  field  of  attention  and  sounds  are  relatively 
in  the  background,  at  such  a  time  the  visual  area  and  its  immediately 
connected  areas  are  more  active  than  the  auditory  area  and  its  con- 
nections. Even  this  supposition,  however,  is  not  altogether  free 
from  difficulties. 

§  17.  The  more  precise  question  now  arises  as  to  what  is  meant  by 
the  brain  activity  that  is  said  to  be  indicated  by  the  existence  of 
consciousness.  The  mere  transmission  of  nerve-currents  along  the 
numerous  "association  fibres"  of  the  brain  is  not,  probably,  what  is 
directly  correlated  with  consciousness.  The  activity  of  the  nerve- 
cells  in  the  cortex  would  be  a  better  guess.  But  on  account  of  the 
great  functional  importance  of  the  synapses,  a  still  more  likely  sup- 
position would  regard  the  brain  activity  indicated  by  consciousness  as 
occurring  at  these  junctions  between  neurones;  or,  perhaps,  both 
in  the  synapses  and  in  the  dendrites  and  nerve-cells.  The  activity 
of  the  brain  resolves  itself,  accordingly,  for  the  most  part  into  the 
passage  of  nervous  "currents"  or  "impulses"  across  the  junctions 


610  THE  MECHANISM  OF  THOUGHT 

between  the  terminations  of  axons  and  the  dendrites  with  which  they 
come  into  contact. 

According  to  this  view,  consciousness  serves  as  an  index  that  nerve- 
currents  are  traversing  synapses  in  the  brain;  and  incidentally  also, 
of  course,  passing  along  the  nerve-fibres  between  one  synapse  and 
another.  But  now  a  difficulty  arises  from  the  fact  that  well-trained 
or  habitual  reactions  occur  with  a  minimum  of  consciousness.  It 
even  happens  that  strong  sensory  stimuli,  if  commonplace  and  of 
no  momentary  significance,  are  neglected  by  attention  for  much 
weaker  sensory  stimuli;  and  that  fairly  energetic  motor  reactions 
may  be  carried  on  while  the  centre  of  consciousness  is  fixed  upon 
something  quite  different.  "  He  that  runneth  may  read,"  or  other- 
wise direct  his  attention;  though  no  doubt  motor  quiet  would  be  a 
more  favorable  condition  for  such  intellectual  activity.  On  the 
other  hand,  there  can  be  no  doubt  that  if  reading  were  originally 
learned  while  running,  the  act  of  running  would  facilitate  the  act 
of  reading.  The  difficulty  is,  then,  that  performances  which  in- 
volve the  cerebrum,  and  which  seem  to  require  a  considerable  ac- 
tivity in  the  parts  of  the  cerebrum  concerned  with  them,  may  yet  be 
attended  with  but  a  low  degree  of  consciousness.  We  may  note 
at  once,  however,  that,  though  intense  sensory  stimuli  and  energetic 
muscular  action  must  require  strong  activity  in  the  parts  of  the  cere- 
brum which  are  concerned  with  them,  this  is  no  proof  that  other  parts 
of  the  brain  may  not  be  simultaneously  in  even  greater  activity. 
To  compete  with  a  strong  sensory  stimulus,  another  object  of  atten- 
tion must  have  much  momentary  attraction  or  significance;  and  this 
probably  indicates  that  its  system  of  cerebral  pathways  is  in  a  con- 
dition of  very  high  activity.  The  focus  of  activity  need  not  always 
lie  in  the  sensory  or  motor  areas. 

§  18.  Important,  if  not  decisive,  objections  maintain  themselves  to 
each  of  two  opposed  theories :  one  of  which1  regards  clear  conscious- 
ness as  indicating  an  open  path  for  motor  discharge;  and  the  other 
of  which2  considers  that  the  difficulty  of  free  passage  stimulates 
attention  and  so  increases  the  degree  of  clear  consciousness,  or  that 
consciousness  itself  is  the  product  of  a  condition  of  tension  at  the 
synapses.3  The  former,  the  so-called  "Action  Theory,"  seems  dis- 
credited by  the  general  experience  that  well-trained  reactions,  which 
imply  open  motor-pathways,  are  almost  uniformly  only  dimly  repre- 

1  So  Miinsterberg,  Grundzuge  d.  Psychologic,  1900,  I,  525. 

3Dewey,  Psychol.  Rev.,  1894,  I,  553;  1895,  II,  13;  Philos.  Rev.,  1897,  VI,  43. 

3  Montague,  "Consciousness  a  Form  of  Energy,"  Essays  Philosophical  and 
Psychological  in  Honor  of  William  James,  1908,  p.  103.  Objections  to  Wundt'a 
theory  of  a  special  centre  for  attention  or  "apperception"  in  the  frontal  lobes 
have  already  been  brought  forward  (p.  274). 


THE  MECHANISM  OF  ATTENTION  611 

sented,  if  at  all,  in  consciousness.  The  supposition  that  such  path- 
ways do  not  lie  in  the  cerebral  hemispheres,  but  only  in  the  cord  and 
brain-stem,  is  opposed  to  the  phenomena  of  aphasia,  apraxia,  asym- 
bolia,  etc.,  which  follow  upon  injury  to  these  hemispheres. 

The  theory  which  emphasizes  the  difficulty  of  reaction  as  the 
cause  of  increased  clearness  of  consciousness  hardly  seems  to  com- 
port well  with  all  that  we  know  about  the  preference  of  objects,  both 
in  perception  and  recall — a  matter  which,  except  in  the  most  ex- 
treme cases  of  selective  attention,  and  largely  even  then — seems  to  be 
determined  by  influences  that  do  not  manifest  themselves  in  con- 
sciousness at  all.  Nor  can  it  be  claimed  that  the  "feeling,"  or 
other  evidence,  of  tension  is  anything  like  a  constant,  not  to  say 
indispensable,  precondition  of  clear  consciousness. 

§  19.  The  shifting  of  attention,  and  "varied  reaction"  in  general, 
next  claim  our  notice.  This  problem  has  already  met  us  in  the 
sphere  of  reflex  action  (pp.  165,  173),  in  the  form  of  that  alterna- 
tion of  responses  which  sometimes  results  from  a  single  stimulus. 
As  in  reflexes,  so  here  in  the  shifting  of  perceptual  reactions,  the  facts 
point  to  the  existence  of  branching  tracts  in  the  nerve-centres,  and 
so  of  alternative  pathways  open  to  a  nerve-current,  but  unequally 
open.  McDougall,  who  in  his  "Physiological  Psychology"1  has 
made  one  of  the  most  serious  of  recent  attempts  to  conceive  the 
neural  apparatus  involved  in  mental  action,  offers  a  theory  of  the 
mode  of  action  of  these  branching  pathways.  He  makes  use  of 
two  conceptions:  of  fatigue,  and  of  the  "drainage"  of  energy  from 
all  parts  of  a  neurone  into  any  synapse  across  which  the  current 
actually  breaks  its  way.  Suppose — to  illustrate— that  a  nerve-cur- 
rent reaches  a  point  where  the  paths  branch;  there  is  synaptic  re- 
sistance to  its  passage  into  either  of  these  paths;  the  nerve-current 
is,  for  an  instant,  dammed  up  against  the  entrance  to  each  of  them, 
and  gathers  tension  at  each.  At  whichever  entrance  the  resistance 
is  lower,  there  the  gathering  tension  will  soonest  be  sufficient  to 
force  a  passage,  and  when  once  an  outlet  is  thus  secured,  all  the 
energy  is  drained  off  into  it,  so  that  none  takes  the  alternative  path. 
But  now  the  synapse  across  which  the  current  is  moving  suffers  fa- 
tigue from  this  passage,  and  its  resistance  increases  till  further  pass- 
age is  blocked.  The  result  of  this  blocking  of  the  first  outlet  may 
be  that  the  alternative  outlet  is  forced,  and  so  the  current  takes  a 
new  path  and  gives  rise  to  a  different  reaction.  Such  a  shifting 
from  one  outlet  to  the  other  may  be  repeated  time  after  time.  This 
interpretation,  its  author  thinks,  is  readily  applied  to  the  alternation 
of  percepts  in  viewing  the  staircase  or  dot  figures,  and  probably  to 
all  cases  of  shifting  of  attention. 

1  London,  1905;  see  also  Mind,  1906,  N.  S.,  XV,  329. 


612  THE  MECHANISM  OF  THOUGHT 

There  are,  however,  what  appear  to  us  decisive  objections  to 
this  ingenious  hypothesis.  In  the  first  place,  it  does  not  explain 
the  high  excitability  of  a  pathway  which  has  just  been  inhibited.1 
Possibly  a  slight  modification  will  enable  the  theory  to  succeed  bet- 
ter at  this  point.  The  nerve-current  which  beats  at  the  door  of  a 
pathway  without  gaining  admission  is,  in  fact,  a  "subliminal  stim- 
ulus" at  that  point;  and  the  fact  of  "summation  of  subliminal 
stimuli"  shows  that  such  a  stimulus,  though  it  does  not  arouse  ac- 
tivity, induces  a  heightened  excitability.  Let  us  suppose,  then,  that 
a  nerve-current,  acting  on  a  branched  path,  not  only  breaks  through 
at  one  point  and  produces  activity  there,  but  raises  the  irritability 
of  all  synapses  against  which  it  impinges  without  being  able  to 
break  through.  It  would  therefore  leave  such  synapses  in  a  "  con- 
dition of  readiness"  for  any  soon-following  stimulus.  Stated  in 
this  modified  form,  the  conception  is  rather  attractive;  since  it 
would  apply  not  only  to  shifts  of  attention,  but  equally  well  to  the 
condition  of  readiness  observed  in  experiments  on  memory,  when  A, 
previously  associated  with  B,  does  not  now  recall  B,  and  yet  makes 
it  easier  to  reach  B  by  some  other  way.  In  such  a  case,  we  may 
imagine  that  the  nerve-current  started  by  the  presentation  of  A 
impinges,  at  some  point  in  its  passage  through  the  brain,  upon  a 
synapse  leading  to  the  response  B,  but  this  synapse  has  a  high  re- 
sistance, and  the  current  does  not  pass  that  way;  yet  it  raises  the 
excitability  (or  lowers  the  resistance)  of  this  synapse,  so  that  some 
other  nerve-current,  reaching  the  same  synapse  or  nerve-cell  by 
another  route,  enters  and  gives  rise  to  the  response  B. 

A  further  and  more  decisive  objection  to  the  conception  of  varied 
reaction  as  due  to  branching  paths  is  this :  unicellular  animals,  like 
Amoeba  and  Paramecium,  exhibit  varied  reactions,  but,  having  no 
nervous  systems,  can  hardly  be  credited  with  alternative  paths. 
Their  varied  reactions  are  not  executed  by  different  effectors;  but 
the  whole  cell  seems  to  participate  in  different  and  often  opposed 
movements.  Varied  reaction  is,  therefore,  much  more  primitive  and 
fundamental  than  branching  paths.  Indeed,  were  this  not  so,  the 
very  existence  and  development  of  animal  life  would  seem  impossi- 
ble. Besides,  fatigue  is  not  admissible  as  an  explanation  of  the 
shifting  of  response  in  these  unicellular  forms;  since  the  same 
structures  are  active  throughout  the  sequence  of  movements.  Such 
behavior  as  that  of  Stentor,  for  example  (p.  544),  in  response  to  a 
continued  stimulus  could  not  be  explained  in  terms  of  fatigue;  for 
if  a  cell  is  fatigued  by  its  first  gentle  response,  how  could  the  same 
cell  shift  at  once  to  a  more  vigorous  response  to  the  same  stimulus  ? 
Some  internal  change  certainly  occurs  in  the  cell,  in  consequence  of 
1  Compare  Sherrington,  op.  cit.,  p.  203. 


THE  POWER  OF  VARIED  REACTION  613 

the  first  stimulus  and  response,  and  by  virtue  of  this  change  repe- 
tition of  the  stimulus  gives  rise  to  another  response;  but  this  change 
cannot  be  of  the  nature  of  fatigue. 

§  20.  Although  we  have  no  certain  knowledge  as  to  its  nature, 
we  must  at  least  accept  the  gross  fact  that  some  change  occurs  in 
the  condition  of  the  cell,  such  that  the  same  stimulus,  on  being  re- 
peated, gives  rise  to  a  different  response.  It  is  not,  accordingly,  a 
wholly  unreasonable  attempt  to  view  the  nervous  system  of  man  in 
the  light  of  this  behavior  of  unicellular  animals.  If  we  suppose  that 
each  neurone  has  the  power  of  varied  reaction,  we  have  a  way  out 
of  some  of  our  difficulties.  The  neurone  has  hitherto  been  held  to 
have  only  one  form  of  reaction;  but  the  facts  of  inhibition,  refractory 
period,  etc.,  seem  to  indicate  that  it  has  at  least  two  opposed  modes 
of  reaction,  and  that  these  correspond,  in  a  manner,  to  the  two  op- 
posed reactions  of  the  amoeba.  Now  the  amoeba  sends  out  branches 
in  response  to  some  stimuli,  and  draws  them  back  in  response  to 
others.  Some  neurologists  have  believed  that  protraction  and  re- 
traction of  the  dendrites  occur,  and  have  based  explanations  of 
sleep,  etc.,  on  these  "amoeboid  movements"  of  the  neurone.  The 
balance  of  evidence  is  against  these  movements,  but  inner  changes 
of  a  chemical  or  electrical  nature  might  have  the  same  results. 
Protraction  of  the  dendrites  was  supposed  to  give  closer  contact 
at  the  synapses  and  so  to  lower  the  resistance  to  passage  of  a  nerve- 
current,  and  retraction  to  have  the  opposite  effect.  Similar  changes  in 
resistance  might,  however,  be  the  result  of  changes  of  surface  tension 
in  the  dendrites,  or  of  other  changes  not  involving  molar  motion. 
Suppose,  then,  that  each  neurone  retains  so  much  as  this  of  the  prime- 
val power  of  varied  reaction;  suppose  that  it  has  two  opposite  modes 
of  response,  positive  and  negative,  corresponding  to  excitation  and 
inhibition.  It  may  in  this  case  give  either  response  to  a  stimulus, 
according,  perhaps,  to  the  character  of  the  stimulus,  but  more  par- 
ticularly according  to  its  own  condition  as  determined  by  preceding 
stimuli  and  responses.  Each  positive  response  is  followed  by  a 
negative  phase,  and  each  negative  response  by  a  back  swing  toward 
a  positive  phase.  Aside  from  this  substitution  of  negative  response 
for  fatigue,  the  cerebral  action  in  varied  reactions  could  then  be 
conceived  as  before,  in  terms  of  branching  paths,  drainage,  and  sum- 
mation of  stimuli.  The  most  recent  researches  seem  to  favor  such 
a  view  of  the  "selective  affinities"  of  the  cells  composing  different 
structures  of  the  animal  body,  and  even  of  individual  cells. 

§  21.  On  recurring  to  what  was  said  concerning  the  known  facts 
and  laws  of  the  Presentations  of  Sense,  and  the  varying  qualities 
and  quantities  of  the  sensations  composing  them,  the  correspond- 
ences between  nervous  mechanism  and  mental  phenomena,  as 


614  THE  MECHANISM  OF  THOUGHT 

indicated  by  the  language  employed  to  describe  each,  will  be  found 
to  pervade  the  entire  treatment  of  the  subject.  It  is,  therefore, 
necessary  to  add  only  a  few  words  of  a  more  general  character  at 
this  point.  In  the  first  place,  the  distinctive  qualities  of  things  as 
apprehended  by  the  senses  are  undoubtedly  significant  of  specifically 
different  reactions  of  the  brain  to  the  various  kinds  of  stimuli  com- 
ing into  it  along  the  different  paths  from  the  different  end-organs 
of  sense.  Qualitative  distinctions,  as  observed  by  the  conscious 
mind,  imply  specific  differences  of  some  sort  in  the  correlated  brain 
activities.  Something  definite  as  to  special  localities  chiefly  con- 
cerned in  the  cerebral  activities  connected  with  the  different  senses, 
and  even  as  to  histological  differences  within  those  centres,  is  already 
scientifically  established.  And  we  seem  to  be  on  the  eve  of  knowing 
something,  or  at  least  of  having  some  ground  for  conjecturing  some- 
thing, about  the  chemico-physical  peculiarities  of  the  different 
states  and  forms  of  cerebral  functioning  concerned  with  the  different 
senses. 

Further,  the  whole  theory  of  "local  signs,"  as  justified  from  the 
introspective  point  of  view,  suggests  a  corresponding  theory  of  the 
highly  complex  character  of  the  brain  reactions  that  result  from  the 
simultaneous  discharge  into  the  cerebral  hemispheres  of  a  great  vari- 
ety of  weak  stimuli  from  a  correspondingly  great  variety  of  recep- 
tors. The  quantitative  relations  between  brain  activities  and  the 
conscious  estimate  of  the  intensity  of  our  sensations,  and  of  the  mag- 
nitude of  the  objects  of  sense,  are  too  obviously  in  place  here  for  more 
than  a  mere  reference  to  chapter  III  to  be  necessary.  The  same 
thing  is  equally  true  of  that  sequence  in  mental  events  of  which 
we  are  consciously  aware  and  the  objective  time  occupied  by  con- 
current or  sequent  events  in  the  areas  of  the  cerebral  hemispheres. 

In  all  these  cases,  the  correspondences  of  quality,  quantity,  and 
temporal  relations  are  no  simple  and  easily  analyzable  affair. 
Neither  do  they  lend  themselves  readily  to  the  discovery  and  con- 
fident announcement  of  so-called  "  laws,"  whether  it  is  proposed  to 
apply  the  term  to  the  activities  of  the  brain  or  to  our  conscious  ex- 
perience by  way  of  sense-perception.  Even  less  do  they,  up  to  the 
present  time,  enable  us  to  enunciate  a  system  of  fixed  and  inflexible 
terms  on  which  the  brain  and  the  mind  shall,  so  to  say,  pay  atten- 
tion to  each  other.  But  as  to  this  condition  of  general  correspond- 
ence, and  as  to  the  abstract  possibility  of  extending  our  knowledge 
of  it  more  and  more  into  details,  the  evidence  from  the  study  of  the 
development  of  sense-perception  in  man's  case  is,  of  all  the  available 
evidence,  quite  the  most  conclusive. 

§  22.  In  studying  the  presentations  of  sense  from  this  point  of 
view,  it  must  be  borne  in  mind,  that  they  can  never  be  regarded  as 


CEREBRAL  ACTION  IN  MEMORY  615 

merely  fortuitous,  or  most  strictly  necessitated,  aggregations  of 
sensations,  qualitatively  and  quantitatively  different.  They  all  im- 
ply such  activities  as  selective  attention,  more  or  less  conscious  dis- 
crimination, and  mediate  or  immediate  judgment  and  voluntary 
or  involuntary  recall.  They  are  also  creatures  of  habit,  so  to  say. 
But  attention,  discrimination,  judgment,  and  recall,  as  studied 
from  the  introspective  point  of  view,  of  themselves  invite  and  en- 
courage the  study  of  the  corresponding  brain  activities.  What  can 
be  made  a  matter  of  reasonable  conjecture  as  to  the  conditions  and 
functions  of  the  cerebral  hemispheres  in  connection  with  these  con- 
scious activities  belongs,  also,  to  our  legitimate  attempt  to  explain, 
or  illustrate,  the  nature  of  sense-perception  in  the  light  of  physiolog- 
ical psychology. 

§  23.  A  certain  antagonism  between  the  tendency  to  change  and 
the  tendency  to  persistence  or  "  perseveration  "  of  a  response,  has  al- 
ready been  remarked.  It  is  certainly  a  fact  that  a  response  just 
made  is  more  readily  evoked  a  second  time.  The  effects  of  great 
"recency"  in  favoring  recall  are  too  clear  to  be  doubted.  The  con- 
tradiction between  this  fact  and  the  fact  of  shifting  of  response  is 
only  apparent.  The  refractory  period,  or  negative  after-effect,  of 
response  is  of  variable,  but  usually  of  very  brief  duration  (compare 
p.  164),  and  gives  way,  in  turn,  to  a  condition  of  heightened  excita- 
bility. This  can  easily  be  demonstrated  in  case  of  the  heart  mus- 
cle, in  which  both  the  refractory  period  and  the  increased  excita- 
bility after  response  are  present  to  a  marked  degree.  Skeletal 
muscle  also  shows  both  effects,  as  do  unicellular  animals.  It  is  then 
a  legitimate  supposition  that  a  cerebral  neurone,  after  recent  ac- 
tivity, is  in  such  condition  that  it  is  readily  excited  again;  its  synapses 
have  a  low  resistance,  and  any  nerve-current  which  impinges  on  its 
dendrites  from  any  axon  is  likely  to  enter  and  give  rise  to  a  repeti- 
tion of  the  recent  reaction. 

§  24.  In  the  last  paragraph  we  have  entered  upon  the  attempt 
to  conceive  of  the  cerebral  action  in  Memory  and  Learning.  A  re- 
sponse once  made  leaves  behind  a  condition  favorable  to  the  repeti- 
tion of  the  same  response.  But  this  after-effect  is,  in  itself,  only 
temporary,  and  appears  in  low  organisms  which  show  no  sign  of  a 
permanent  modification  of  behavior.  Something  must  be  added  to 
our  conceptions  to  account  for  true  learning  and  for  the  formation 
of  habit.  The  "law  of  habit"  states  that  a  response,  once  made, 
is  more  readily  excited  again,  even  after  an  interval  of  complete 
quiescence.  Perseveration,  "warming  up,"  and  the  ready  recall 
of  very  recent  responses,  are  best  understood  as  due  to  a  persistence 
of  activity.  The  structures  concerned  have  not  entirely  cooled  off, 
or  gone  to  sleep.  The  recall  of  a  response  after  a  longer  interval 


616  THE  MECHANISM  OF  THOUGHT 

corresponds  to  the  awakening  of  organs  which  have  once  been  active 
but  have  since  been  asleep.  This  is  quite  another  thing,  and  de- 
mands a  quite  different  interpretation.  Some  comparatively  per- 
manent change  of  structure  has  been  induced.  Since  the  structures 
with  which  we  are  here  concerned  are  living,  the  change  can  best 
be  thought  of  as  having  a  metabolic  or  nutritive  character.  We 
know  that  activity  of  a  muscle  enables  it,  in  its  subsequent  rest, 
to  take  up  nutriment  from  >he  blood,  and  so  to  increase  in  size. 
There  is  evidence  (p.  58),^at  activity  of  the  brain  causes  a  growth 
in  the  fine  branphespf^the  axons  and  dendrites,  which  are  the  essen- 
tial structures  informing  connections.  Besides  this  growth  in  size, 
a  muscle  shows,  after  exercise,  an  improvement  in  its  inner  condi- 
tion; it  shows  this  in  the  fact  that  its  increased  strength  is  often  too 
great  to  be  explained  in  terms  of  increased  size.  The  exact  nature 
of  this  inner  change  is  unknown,  but  it  may  reasonably  be  supposed 
to  be  nutritive  or  metabolic.  It  is  not  unlikely  that  neurones  also 
improve  in  their  inner  condition  as  the  result  of  preceding  activity. 
Both  growth  of  the  fine  branches  and  improvement  in  internal  con- 
dition are,  probably,  factors  in  the  retention  of  a  response.  We  may 
regard  the  combination  of  neurones  which  are  concerned  in  carry- 
ing out  any  reaction  as  an  organ,  which  grows  and  improves  its 
condition  as  the  result  of  exercise,  and  which  is  thus  made  more 
ready  for  work  and  more  likely  to  be  called  into  activity.  That  an 
organ  improves  as  the  result  of  exercise  is  the  fundamental  physi- 
ological conception  in  the  doctrine  of  habit  and  learning.  In  connec- 
tion with  this  must  go  the  explanation  of  what  we  have  called  "  dy- 
namical associations." 

§  25.  Several  details  of  the  process  of  retention,  as  revealed  by 
psychological  experiment,  fit  easily  into  the  above  conception. 
That  retention  dies  out  with  the  lapse  of  time  seems  but  an  instance 
of  "  atrophy  through  disuse,"  or — to  speak  in  more  general  terms — 
of  the  tendency  to  recovery  and  "regulation."  That  repeated  ex- 
ercise gives  continued  improvement,  in  motor  skill  or  in  the  mastery 
of  a  series  of  nonsense  syllables  or  a  poem,  may  be  explained  as  due 
to  the  accumulation  of  the  nutritive  after-effects  of  a  single  period 
of  activity.  The  advantage  of  periods  of  rest  between  the  successive 
repetitions  of  an  act  to  be  learned,  is  what  would  be  expected  from 
the  behavior  of  other  organs,  such  as  the  muscles.  No  one  would 
hope  for  great  and  permanent  increase  in  the  strength  of  a  muscle 
from  prolonged  over-exercising  it  at  one  time;  common  experience 
teaches  the  advantage  of  spreading  out  the  exercise  over  many  days. 
A  physiological  explanation  of  this  fact  may  be  attempted  as  fol- 
lows :  the  nutritive  after-effect  of  exercise  occurs  largely  in  the  subse- 
quent period  of  rest.  After  a  little  exercise,  rest  improves  the  organ, 


7 


MECHANISM  OF  ASSOCIATIONS  617 

which  enters  on  the  next  period  of  activity  more  capable  of  deriving 
benefit  from  it.  It  is,  perhaps,  a  larger  organ,  and  so  able  to  absorb 
more  nutriment  in  its  next  rest.  Massing  all  the  exercise  in  one 
period  of  continued  activity  prevents,  in  large  measure,  the  summa- 
tion of  nutritive  after-effects.  But  since,  in  all  learning,  it  is  not  iso- 
lated but  co-ordinated,  and  often  widely  locally  separated  activities 
of  the  brain  which  are  involved,  something  of  a  more  dynamic  char- 
acter is  needed  to  account  for  the  phenomenon  of  "associated"  re- 
tention. 

§  26.  While,  then,  the  physiology  of  simple  retention,  as  in  the 
case  of  the  muscles,  falls  readily  into  accepted  biological  terms,  the 
physiology  of  the  formation  of  associations  meets  with  peculiar  diffi- 
culties, such  as  might  be  expected  from  the  fact  that  the  capacity 
for  initiating  connection  between  stimuli  and  responses  not  hitherto 
connected  with  them  is  peculiar  for  the  most  part  to  the  nervous 
system,  and  especially  to  the  brain.  The  problems  chiefly  suggested 
are  the  two  following:  (1)  What  is  the  physiology  of  learning  by 
"  trial  and  error,"  and  of  the  selection  of  the  successful  response  ? 
(2)  What  is  the  physiology  of  the  formation  of  an  association  be- 
tween two  presented  stimuli?  In  regard  to  the  mechanism  of  se- 
lection from  varied  reactions,  we  have  already  expressed  an  opinion 
(pp.  551  fL),  though  not  in  strictly  physiological  terms,  and  that 
matter  need  not  be  further  rehearsed.  The  second  problem  is  one 
of  so  great  importance,  as  to  lead  one  author  to  declare:  "  If  no  solu- 
tion can  be  found,  physiological  psychology  is  bankrupt." l 

The  problem  of  association  by  contiguity,  as  already  stated,  con- 
sists in  the  fact  that  A  and  B,  between  which  an  association  is 
formed,  do  not  stand  from  the  outset  in  the  relation  of  stimulus  and 
response.  A  does  not  originally  give  rise  to  B,  but  each  is  inde- 
pendently supplied.  A  difficulty  therefore  enters  here  which  is  not 
present  in  learning  by  trial  and  error.  The  animal,  striving  to  es- 
cape from  a  cage,  makes  a  series  of  different  responses  to  the  situa- 
tion ;  each  response  is  actually  called  out  by  the  situation  acting  as 
stimulus.  The  nervous  connections  between  the  stimulus  and  re- 
sponse are  traversed  by  nerve-currents,  and  therefore  strengthened 
in  accordance  with  the  law  of  exercise;  and  the  only  question  is  to 
explain  the  advantage  which  accrues  to  the  successful  reaction. 
But  in  the  case  now  before  us,  as  when  a  person  is  presented  and  his 
name  spoken,  neither  the  name  nor  the  face  of  the  person  comes  as  a 
response  to  the  other.  The  first  sight  of  the  person  does  not  suggest 
his  name,  nor  the  first  hearing  of  a  name  suggest  the  appearance  of 
the  person.  The  two  associates  come  independently,  one  by  way 
of  the  eye,  the  other  by  way  of  the  ear;  and  there  seems  no  occasion 
1  McDougall,  op.  cit.,  p.  126. 


618  THE  MECHANISM  OF  THOUGHT 

for  the  activity  of  any  neural  connection  between  the  parts  of  the 
brain  thus  independently  aroused  to  action.  And  yet,  later,  the 
name  recalls  the  face  or  the  face  the  name — showing,  in  accordance 
with  our  general  conceptions  of  the  brain  mechanism,  that  connec- 
tions have  been  established  between  the  parts  concerned  with  face 
and  name  respectively.  When  and  how  have  these  connections 
been  formed? 

§  27.  Such  is  the  problem  of  the  physiology  of  association  by 
contiguity.  A  solution  has  been  attempted  by  James,1  and  elabo- 
rated by  McDougall.2  As  the  latter  states  the  case,  a  certain  part 
or  system  of  neurones  in  the  brain  is  thrown  into  activity  by  A,  the 
sight  of  the  person,  and  immediately  afterward,  anoth'er  part  or 
system  is  thrown  into  activity  by  the  sound  of  the  name,  B.  The 
centre  of  activity  shifts  from  one  part  to  another,  as  it  does  in  other 
shifts  of  attention,  and  the  second  centre  of  activity  attracts,  and 
drains  off,  the  energy  of  the  first  centre,  so  that  there  is  really  a  trans- 
mission of  nerve-currents  from  the  first  to  the  second.  The  path 
from  one  to  the  other  is  thrown  into  activity,  as  truly  as  if  the  sight 
of  the  person  had  actually  called  up  the  name,  and  thus  receives  the 
nutritive  after-effect  of  activity  and  becomes  a  favored  path,  or 
path  of  low  resistance. 

This  explanation  probably  proceeds  in  the  right  direction,  so  far, 
at  least,  as  this  :  Nerve-currents  do  pass  between  different  centres 
of  activity  in  the  brain;  but  the  theory  seems  rather  too  elaborate 
and  highly  specialized  to  explain  some  cases  of  association  by  con- 
tiguity, while  it  is  not  elaborate  enough  to  explain  all  cases.  We 
have  grave  doubts  about  the  concept  of  "drainage,"  which  enters 
here  in  even  a  vaguer  form  than  in  the  case  of  the  alternation  of  re- 
flexes or  of  percepts  of  an  ambiguous  stimulus.  To  test  the  matter, 
let  us  recall  that,  in  looking  at  the  staircase  or  the  dot  figure,  the  ob- 
server shifts  completely  from  one  appearance  to  the  other;  so  that 
the  drainage  of  the  first-acting  system  by  the  second  should  be  com- 
plete, as  is  required  by  the  conception  of  drainage  in  the  formula 
of  McDougall.  But  there  is,  in  fact,  no  such  complete  shifting  in 
the  instance  of  the  face  and  the  name;  for  the  face  does  not  disappear 
when  the  name  is  spoken.  The  drainage  is  not,  then,  complete  in 
this  case.  In  general,  the  fact  that  the  field  of  consciousness  is 
broader  than  the  field  of  attention  goes  to  show  how  more  than  one 
system  of  neurones  may  be  simultaneously  active  in  the  brain; 
and  how  no  one  system  exerts  complete  attraction  on  the  energy  of 
the  other  systems.  Partial  drainage,  however,  is  a  much  less  clean- 
cut  conception  than  complete  drainage,  and  since  it  has  no  special 

1  Principles  of  Psychology,  1890,  II,  584. 
8  Op.  cit.,  p.  126. 


MECHANISM  OF  ASSOCIATIONS  619 

claim  to  acceptance  in  regard  to  the  activity  of  the  brain  as  a  whole 
(though  it  may  be  accepted  in  regard  to  a  single  neurone),  we  may 
profitably  seek  some  more  satisfactory  explanation. 

In  our  search  let  us,  first  of  all,  assume  that  the  neural  connections 
which  are  formed  in  learning  are  not  wholly  formed  at  the  moment 
of  learning;  for  it  would  clearly  be  impossible  to  suppose  association 
fibres  to  grow  all  the  way  from  the  visual  to  the  auditory  centre  in  the 
few  moments  needed  to  connect  a  face  with  a  name.  These  fibres 
already  exist  in  great  numbers,  and  connections  are  already  formed 
in  the  rough,  and  need  but  a  few  "finishing  touches,"  so  to  say,  to 
make  them  good  conductors.  Probably  the  finishing  touches  are 
applied  to-  the  synapses.  Thus  much  any  theory  must  assume, 
and  it  is  entirely  in  accordance  with  neurological  probability.1  Now 
let  two  centres,  thus  loosely  connected,  be  thrown  into  simultaneous 
or  nearly  simultaneous  excitement.  Each  centre  discharges  mainly 
into  some  previously  trained  channel,  giving  rise  to  motor  reactions, 
percepts,  or  associated  ideas.  But  after  each  has  thus  discharged 
itself,  its  activity  does  not  come  to  an  abrupt  end.  Each  probably 
continues  active  to  a  slight  extent,  and  each  is  also  in  a  condition  of 
heightened  excitability;  therefore  the  conditions  are  favorable  for 
the  passage  of  currents  across  the  imperfectly  formed  synapses 
between  them.  Each  has  something  to  give,  and  each  is  ready  to 
take,  and  so  an  interchange  takes  place  between  them.  The  fore- 
going is  intended  only  as  a  formulation  of  the  minimum  conditions 
of  association.  Memory  experiments  have  shown  the  formation 
of  "remote"  associations,  i.  e.,  associations  between  non-contiguous 
members  of  a  series  of  syllables,  etc.;  in  such  cases  no  shifting  of 
attention  occurs  directly  between  the  remote  terms  which  become 
associated;  and  the  conception  of  drainage  does  not  appear  applica- 
ble. The  conception  just  offered  would  better  suit  the  multiplicity 
of  associations  which  experiment  has  shown  to  be  formed  between 
other  than  directly  adjacent  members  of  a  series;  it  would  allow,  in 
fact,  for  the  formation  of  associative  paths  between  any  two  cerebral 
organs  which  might  be  thrown  into  activity  at  nearly  the  same  time. 
Taken  by  itself,  therefore,  the  drainage  theory  suffers  from  defect 
of  explaining  too  much;  since  it  allows  for  associations  which  are 
not  demonstrably  formed.  It  is  by  no  means  everything  present 
in  consciousness  at  the  same  or  nearly  the  same  time  which  be- 
comes associated  with  everything  else  so  present.  But  it  fails  to 

1  McDougall  also  makes  this  assumption  (p.  127):  "We  may  legitimately 
assume  that  all  parts  of  the  cerebral  hemispheres  are  connected  together  in  such 
a  way  that,  under  favorable  conditions,  the  excitement  of  any  sensory  neurone 
may  spread  to  any  part,  just  as  in  the  strychnine-poisoned  animal  the  excite- 
ment of  a  sensory  neurone  may  spread  through  all  parts  of  the  spinal  cord." 


620  THE  MECHANISM  OF  THOUGHT 

explain  the  strong  associations  formed  between  successive  members 
of  a  series  which  is  memorized,  and  to  show  why  the  forward  asso- 
ciations in  such  a  series  are  so  much  stronger  than  those  in  the 
backward  direction. 

§  28.  It  is  to  these  strong  serial  associations  that  the  drainage 
theory  has  been  most  applied  and  appears  most  applicable.  But 
its  applicability  here  is  mostly  an  illusion.  More  careful  analysis 
shows  that  this  theory  rests  on  two  assumptions:  (1)  that,  psycho- 
logically, serial  associations  are  formed  when  attention  shifts  from 
each  member  of  a  series  to  the  next;  and  (2)  that,  physiologically, 
shif tings  of  attention  are  always  attended  by  a  drainage  from  the 
centre  first  active  into  the  centre  next  active,  with  consequent  in- 
hibition of  the  first  centre.  Neither  of  these  suppositions  is  prob- 
ably true  for  all  cases;  and  the  cases  where  one  is  true  are  the 
cases  where  the  other  is  not  true.  In  those  cases  where  psychologi- 
cal facts  point  to  something  like  drainage  in  the  cerebral  mechan- 
isms— such  as  the  often-mentioned  cases  of  the  staircase  and  dot 
figures,  and  numerous  others  like  them — one  percept  vanishes  as  the 
other  appears;  that  is,  the  activity  of  one  centre  is  accompanied  by 
inhibition  of  the  other.  But  there  is  no  evidence  that  such  shifts  of 
attention  give  rise  to  a  serial  association  between  the  two  percepts; 
the  evidence  is  rather  to  the  contrary.  After  experience  with  an 
ambiguous  figure,  one  of  its  appearances  does  not  seem  to  call  up 
the  other;  the  observer  has  still  to  wait  for  the  process  of  varied 
reaction  to  cause  the  changes.  He  may  obtain  a  certain  degree  of 
control  over  them,  by  learning  the  best  fixation  point  to  bring  out 
either  appearance;  but  this  control  does  not  establish  an  association 
between  the  two  percepts.  It  is  also  true  that  the  alternative  views 
are  apt  to  succeed  each  other  more  rapidly  after  experience  with 
the  figure;  but  this  is  sufficiently  accounted  for  by  the  greater  fa- 
miliarity of  each  appearance,  taken  by  itself  as  a  response  to  the 
stimulus.  The  relation  between  the  alternative  percepts,  however, 
remains  essentially  the  same  after  experience  of  the  shifting  as  be- 
fore; and  the  shifting  of  percepts  itself  corresponds  always  to  the 
type  of  varied  reaction,  and  never  to  the  type  of  serial  association. 
Essentially  the  same  things  can  be  said  of  other  cases  which  belong 
under  the  type  of  varied  reaction.  The  result  of  learning  by  trial 
and  error  is  not  the  formation  of  serial  associations  between  the  suc- 
cessive reactions;  but  the  successful  reactions  become  associated 
directly  with  the  situation  and  with  the  "  adjustment,"  while  the  other 
reactions  tend,  on  the  whole,  to  be  dissociated.  Once  more,  the 
shifting  of  attention  that  occurs  with  a  sudden  interruption  to  the 
course  of  thought  does  not  result  in  a  strong  association  between  the 
thought  interrupted  and  the  interrupting  stimulus.  But  this  ex- 


PHYSIOLOGY  OF  THE  HIGHER  UNITS  621 

perience  is  pretty  nearly  equivalent  to  proving  that  cases  which  do 
correspond  well  with  the  drainage  conception,  are  not  cases  of 
strong  serial  association. 

On  the  other  hand,  the  cases  in  which  strong  serial  association 
does  occur  do  not  conform  to  the  type  required  by  the  drainage 
conception.  A  case  in  point  here  is  that  of  the  two  syllables  which 
form  a  single  measure  in  a  memory  series.  The  movement  of  at- 
tention from  the  first  to  the  second  syllable  of  such  a  measure  differs 
in  two  important  respects  from  the  movement  of  attention  in  exam- 
ining an  ambiguous  figure:  (1)  the  first  syllable  does  not  pass  en- 
tirely out  of  consciousness  with  the  coming  of  the  second;  and  (2)  the 
second  is  anticipated  while  the  first  is  being  presented.  The  move- 
ment of  attention  is,  therefore,  continuous  in  such  cases;  whereas 
in  viewing  an  ambiguous  figure — or  in  other  cases  of  varied  reaction 
— it  is  discontinuous.  The  reactions  to  the  two  syllables  are  not 
opposed  and  mutually  exclusive,  but  overlap  and  enter  into  a  "  higher 
unit"  of  reaction — namely,  into  the  measure.  Even  in  memoriz- 
ing by  rote  we  make  use  of  such  higher  units;  and  the  strongest 
associations  are  those  between  the  parts  of  such  units.  But  the 
drainage  theory  has  no  place  for  such  higher  units  as  extend  over 
two  or  more  successive  acts;  for  it,  each  act  is  antagonistic  and  in- 
hibitory to  its  predecessor.  Any  attempt  to  apply  the  drainage 
theory  to  the  cases  of  learning  typewriting  and  telegraphy — in 
which  overlapping  and  higher  units  of  reaction  are  so  much  in  evi- 
dence— would  reveal  beyond  doubt  its  utter  inadequacy.  This 
theory  also  has  no  room  for  adjustments  which  last  for  a  while  and 
control  a  series  of  acts;  but  the  process  of  learning  can  hardly  be 
interpreted  without  allowing  for  such  adjustments. 

§  29.  The  foregoing  considerations  have  increased  enormously 
the  complexity  of  the  problem  of  explaining  all  kinds  of  mental 
association  in  terms  of  cerebral  activities.  Nothing  so  simple 
as  the  shifting  of  activity  from  one  centre  of  the  brain  to  another 
can  satisfy  the  conditions  set  by  experience  of  the  facts.  It  is  best, 
however,  to  pause  before  attacking  the  physiology  of  "  higher  units," 
in  order  to  see  if  any  case  can  be  made  out  for  purely  serial  associa- 
tions, uncomplicated  by  such  higher  inclusive  units.  For  it  will 
soon  appear  that  the  concept  of  serial  association  is  needed  in  ex- 
planation of  more  complex  associations.  Suppose,  for  instance, 
that  attention  moves  from  A  to  B,  yet  not  by  way  of  transition  be- 
tween mutually  exclusive  terms,  but  with  a  certain  anticipation  of 
what  is  to  come,  or  condition  of  expectancy,  and  with  a  lingering 
of  what  has  just  gone.  Now  expectancy  means  a  suspension  of 
reaction  till  the  expected  has  happened;  that  is,  a  reaction  is  par- 
tially prepared,  but  is  not  "discharged"  until  the  coming  stimulus 


622  THE  MECHANISM  OF  THOUGHT 

shall  have  arrived.  In  physiological  terms,  expectancy  probably 
indicates  a  damming-up  of  nervous  energy  till  the  new  stimulus 
comes;  then,  the  accumulated  energy  is  set  free,  and  very  likely 
it  is  discharged  into  the  centre  aroused  by  the  stimulus.  If  one 
says,  for  example,  "Let  me  present  Mr.  A,"  the  nervous  energy 
aroused  by  the  sight  of  the  man  is  perhaps  held  in  check  till  the 
name  is  spoken,  and  then  discharged  into  the  centre  aroused  by  the 
auditory  stimulus,  which,  on  account  of  its  being  aroused  by  this  stim- 
ulus, would  the  more  attract  other  nerve-currents  having  a  partially 
open  path  into  it.  Such  a  condition  would  account  also  for  the 
special  clearness  with  which  an  expected  stimulus  is  perceived. 
In  other  words:  It  is  the  movement  of  expectant  attention,  rather 
than  the  shifts  between  antagonistic  reactions,  that  gives  rise  to  strong 
serial  reactions.  This  movement  of  expectant  attention  is  perfectly 
consistent  with  the  persistence  of  adjustments,  and  with  the  simul- 
taneous operation  of  higher  units. 

§  30.  To  pass,  now,  to  an  attempt  at  conceiving  the  neural  mechan- 
ism of  "higher  units."  They  may  best  be  conceived  of  after  the 
analogy  of  those  co-ordinating  mechanisms  of  which  convincing  evi- 
dence was  found  in  studying  the  spinal  cord.  These  mechanisms 
showed  the  existence  of  both  collecting  and  distributing  groups  of 
neurones.  Nerve-currents  from  different  sources  converge  by  means 
of  collecting  mechanisms,  and  diverge  by  means  of  distributing 
mechanisms.  In  the  case  of  learned  and  mental  performances,  it 
is  easier  to  conceive  of  the  working  of  the  distributing  mechanisms, 
and  of  the  development  of  the  collecting  mechanisms.  Let  us  begin, 
therefore,  with  the  distributing  mechanisms,  and  take  for  illustra- 
tion those  which  may  be  supposed  to  care  for  learned  combinations 
of  movement,  or,  in  other  words,  for  skilled  performances.  A  suc- 
cession of  simple  instinctive  movements  is  performed  as  a  single 
act;  it  is  a  veritable  unit,  in  spite  of  the  fact  that  it  appears  as  a 
succession  of  movements  which  may  enter  into  other  combinations. 

A  distributing  mechanism,  in  connection  with  serial  associations 
between  the  component  movements,  thus  seems  adequate  to  explain 
the  general  fact  of  co-ordination.  The  distributing  group  of  neu- 
rones calls  into  play  the  groups  of  cells  in  the  motor  area  which  pre- 
side directly  over  the  spinal  centres  for  the  component  movements. 
Serial  association,  however,  seems  necessary  to  account  for  these 
movements  occurring  in  the  proper  order.  The  serial  connections 
alone  cannot,  indeed,  account  for  the  co-ordination,  since  the  same 
component  movements  enter,  in  different  orders,  into  several  skilled 
acts,  and  serial  association  would  be  liable  to  go  astray,  and  lead 
off  into  the  wrong  series;  but  the  distributing  mechanism,  by  holding 
certain  motor  groups  of  cells  in  readiness,  brings  about  a  proper  se- 


PHYSIOLOGY  OF  THE  HIGHER  UNITS          623 

lection  among  the  various  possible  serial  orders.  A  good  example 
of  this  is  afforded  by  any  word  which,  as  spoken,  is  a  series  of 
articulatory  movements.  The  word  is  a  unit  to  consciousness  and 
is  also  a  unit  in  reaction.  Yet  it  is  composed  of  elementary  move- 
ments which,  in  other  words,  enter  into  an  enormous  number  of 
other  combinations.  A  distributing  unit,  corresponding  to  the  word 
and  holding  in  readiness  the  proper  groups  of  motor-cells,  plus  the 
established  serial  associations  between  the  groups  of  motor-cells, 
seems  to  afford  a  partially  satisfactory  conception  of  the  possible 
neural  mechanism. 

The  explanation  just  given  is  not  indeed  wholly  satisfactory, 
for  it  does  not  explain  the  differentiation  between  two  words,  such 
as  "cat"  and  "tack/*  which  are  composed  of  the  same  movements 
in  different  orders.  Of  the  two  mechanisms  postulated  in  this  ex- 
planation, the  distributing  unit  is  supported  by  the  best  evidence, 
psychological  as  well  as  neurological;  and  if  we  could  attribute  to 
such  a  unit  the  power,  not  only  of  distributing  its  nerve-currents 
to  selected  groups  of  motor  cells,  but  also  of  exciting  them  in  a  cer- 
tain order,  our  explanation  would  be  made  more  complete.  This 
last  is  a  difficult  conception,  however,  and  serial  association  seems 
to  be  indicated  by  certain  forms  of  lapses,  in  which  a  word,  a  melody, 
or  a  series  of  manual  movements,  though  rightly  begun,  runs  off  the 
track  by  switching  to  some  other  familiar  series  of  movements. 

§  31.  A  conception  of  collecting  mechanisms  is  specially  needed  for 
explaining  the  physiology  of  percepts.  A  number  of  items,  once  at- 
tended to  singly  (as  the  clicks  in  receiving  a  telegraphic  message), 
come  to  be  apprehended  in  groups;  it  would  seem,  therefore,  that  the 
nerve-currents  set  up  by  the  individual  items  of  stimulus  must  con- 
verge upon  some  unitary  mechanism,  which  can  discharge  as  a  unit, 
and  so  give  rise  to  unitary  motor  reactions,  and  other  unitary  se- 
quels. This  is  not  at  all  a  difficult  nor  improbable  conception;  but 
some  difficulty  arises  from  the  fact  that  we  may  pass  easily  from  such 
wholes  of  apprehension  to  the  items  which  arouse  them;  for  such 
passage  implies  distribution,  in  addition  to  collection.  In  like  man- 
ner, it  is  difficult  to  imagine  how  a  distributing  unit  could  acquire 
its  special  form  of  distribution,  except  on  the  supposition  that  the 
subsidiary  units  react  upon  it — which  would  make  it  a  collector 
as  well  as  a  distributor.  We  conclude,  therefore,  that  collecting  and 
distributing  mechanisms  always  exist  in  close  relation  one  to  the 
other. 

§  32.  Can  any  notion  of  controlled  association  be  formed  in 
terms  of  physiological  psychology?  This  would  require  an  instru- 
ment of  selection  among  the  numerous  paths  leading  out  from  a 
given  starting-point.  A  word,  taken  in  isolation,  may  suggest  any 


624  THE  MECHANISM  OF  THOUGHT 

one  of  many  meanings;  but  in  context  it  suggests  only  one.  To  put 
the  fact  in  neural  terms :  many  paths  lead  from  the  auditory  centre  for 
that  word  to  many  other  groups  of  cells  or  systems  of  neurones;  and 
all  of  these  paths  have  become  developed  by  previous  exercise.  Any 
one  of  them  is  a  path  of  low  resistance;  but,  under  the  given  con- 
ditions, only  one  of  them  is  traversed  by  the  current  starting  from 
the  auditory  centre.  Our  question  is,  What  cerebral  mechanism 
corresponds  to  the  context,  and  selects  the  proper  path  ?  It  is  not  al- 
together impossible  to  reach  a  rough  notion  of  such  a  mechanism. 
It  would  have  the  general  character  of  a  distributing  mechanism, 
holding  certain  paths  in  readiness,  and  facilitating  their  action 
above  those  not  so  prepared.  Some  special  cerebral  activity  is  in- 
dicated by  our  awareness  of  the  trend  of  the  meaning  of  a  passage; 
the  parts  thus  active  exert  their  influence,  not  promiscuously  over 
all  the  neurones  of  the  cortex,  but  selectively',  that  is,  they  favor  cer- 
tain responses  to  each  new  word  as  it  comes.  If  one  says  "  Add ! "  for 
example,  an  adjustment  is  set  up  which  favors  the  paths  developed 
in  learning  the  addition  table;  and  the  nerve-currents  started  by 
hearing  two  numbers  in  this  context  take  this  favored  path. 

A  certain  amount  of  controlled  association  would  thus  reduce, 
physiologically,  to  compound  or  convergent  association.  It  is 
probable  that  simple  serial  association  is  decidedly  the  exception, 
but  compound  association  the  rule,  in  all  mental  processes;  and  that, 
therefore,  the  cerebral  activity  indicated  by  mental  processes  is 
by  no  means  a  mere  succession  of  currents  passing  from  one  system 
of  neurones  to  another.  All  our  attempts  to  picture  the  process 
in  detail  must  be  ludicrously  inadequate  before  its  real  complexity. 
But  from  both  the  physiological  and  the  psychological  points  of 
view,  the  mechanism  involved  in  the  formation  of  so-called  "  higher 
units"  is  the  most  important  of  all  psycho-physical  mechanisms  to 
conceive  of  in  a  clear  and  definite  manner. 

§  33.  In  considering  the  process  involved  in  the  isolation  of  a  simple 
or  complex  feature  from  a  still  more  complex  situation,  our  previous 
studies  have  shown  that  such  analysis  is  most  apt  to  occur  when  the 
given  feature  has  been  previously  experienced  in  other,  somewhat 
different  situations.  How  does  it  come  about  that  a  stimulus  which 
acts  now  in  one  combination,  and  again  in  another,  may  come  to 
arouse  a  reaction  on  its  own  account  ?  We  must  suppose,  here  as  al- 
ways, that  the  cerebral  process  is  much  more  detailed  than  con- 
sciousness would  indicate,  and  that  when  a  combination  of  stimuli 
acts  on  the  organism,  each  component  stimulus  has  some  separate 
cerebral  effect,  though  consciousness  reveals  but  a  blended  total, 
and  though  motor  reaction  is  also  but  a  single  joint  response  to  the 
whole  mass  of  stimuli.  Notwithstanding  this  gross  effect,  the  brain 


LIMITATIONS  OF  ALL  EXPLANATION  625 

paths  directly  reached  by  the  incoming  sensory  currents  have  the 
benefit  of  exercise,  while  other  paths  not  so  excited  do  not  receive 
the  same  benefit.  When,  therefore,  a  new  combination  of  stimuli 
occurs  that  contains  some  old  components,  the  brain  paths  which 
receive  these  old  components  have  an  advantage  over  those  not  pre- 
viously exercised,  and  a  corresponding  especial  influence  in  deter- 
mining the  following  reaction.  They  may  even  determine  a  new 
reaction,  such  as  would  not  result  from  the  equal  balance  of  all 
the  stimuli  entering  into  the  combination. 

§  34.  In  closing  this  part  of  our  work  it  is  scarcely  necessary  to 
confess  anew  the  limitations  within  which  the  scientific  studies  of 
the  order  required  by  physiological  psychology  find  themselves  con- 
fined. Everywhere,  the  complexity  and  subtlety  of  nature's  proc- 
esses far  surpass  all  the  attempts,  however  successful,  of  human 
science  to  unravel  and  depict  them.  But  such  are  the  limitations 
of  every  form  of  science.  That  they  are  uncommonly  restrictive  in 
this  field  is,  doubtless,  in  large  measure  due  to  the  very  character  of 
the  facts  and  of  the  relations  which  are  to  be  examined. 

Nor  can  it  escape  the  insight  of  those  accustomed  to  reflect  upon 
all  the  phenomena  displayed  by  the  human  mind  in  its  higher  stages 
of  development,  that  many  of  the  most  important  and  distinctive 
classes  of  these  phenomena  have  really  not  been  treated,  from  the 
physiological  point  of  view — have,  indeed,  scarcely  been  mentioned 
at  all.  Such  are  the  phenomena  of  the  higher  degrees  of  selective 
attention,  and  of  deliberative  choice,  of  the  more  logical  forms  of 
judgment  and  of  reasoning,  of  the  thinking  involved  in  the  con- 
struction of  scientific  systems  and  philosophical  conceptions,  of  the 
feelings  of  the  more  strictly  ethical  and  sesthetical  order,  and  of  the 
ideals  of  art,  duty,  and  religion.  But  all  these  so-called  higher  forms 
of  functioning  do  seem  to  be  involved,  as  it  were,  in  a  sensory- 
motor  basis,  to  which  they  are  responses  of  a  secondary  and  derived 
order,  and  so  require  the  assumption  of  the  activity  and  develop- 
ment of  that  conscious  subject  of  them  all  which  we  call  the  Soul, 
or  Mind. 


PART  THIRD 

THE  NATURE  OF  THE  MIND 


CHAPTER  I 
GENERAL  RELATIONS   OF  BODY  AND  MIND 

§  1.  Without  entering  in  a  definite  and  detailed  way  into  the  field 
of  metaphysics  proper,  there  are  certain  quasi-metaphysical  prob- 
lems to  which  psychology,  when  studied  from  the  physiological  and 
experimental  points  of  view,  can  scarcely  avoid  making  a  constant 
reference;  and,  indeed,  about  the  most  plausible  solution  of  which 
it  cannot,  in  fidelity  to  its  own  completeness,  fail  to  express  some 
opinion.  Such  of  these  problems  as  we  are  now  proposing  briefly 
to  discuss  may  be  conveniently  grouped  under  two  heads.  We  ask, 
in  the  first  place :  In  what  terms  shall  we  conceive  of  the  most  gen- 
eral relations  between  the  Nervous  Mechanism  and  the  Subject  of 
the  psychical  or  mental  phenomena  ?  And  in  the  second  place :  In 
what  terms  shall  we  conceive  of  the  nature,  or  most  essential  and 
permanent  characteristics,  of  this  Subject  itself?  Or  more  popu- 
larly stated:  How  are  Body  and  Mind  essentially  related,  as  re- 
spects their  development  and  the  forms  of  functioning  assigned  to 
each  ?  and  What  may  be  affirmed  as  to  the  reality  and  unity  of  the 
so-called  Soul  or  Mind?  Both  of  these  questions,  however,  are 
to  be  considered  only  so  far  as  some  indications  leading  to  a  cor- 
rect answer  seem  to  exist  on  scientific  grounds. 

At  this  point  it  is  necessary  to  recall  what  was  said  at  the  begin- 
ning, and  what  has  been  amply  illustrated  and  enforced  by  the  facts 
and  the  conclusions  of  the  entire  treatise.  Our  problem  concerns 
the  very  conception  of  physiological  and  experimental  psychology. 
The  express  object  of  this  science  is  to  investigate  the  relations  be- 
tween the  constitution  and  functions  of  a  certain  mechanism,  the 
human  Nervous  System,  at  the  various  stages  of  its  development  and 
under  the  excitement  of  various  forms  of  internal  and  external 
stimuli,  and  the  changes  effected  in  consciousness,  whether  of  a 
temporary  or  more  permanent  character — in  a  word,  the  behavior 
and  development  of  human  Mental  Life.  Since  these  relations  ap- 
pear, at  least,  to  maintain  themselves  in  both  directions,  as  it  were; 
and  since  sometimes  the  physical  phenomena  are  antecedent  to, 
and  sometimes  consequent  upon,  the  mental  phenomena,  they  may 
be  spoken  of  as  "co-relations."  By  speaking  of  co-relations,  how- 

629 


630       GENERAL  RELATIONS  OF  BODY  AND  MIND 

ever,  we  are  only  adopting  a  term  which  summarizes  in  a  convenient 
way  a  working  scientific  hypothesis. 

§  2.  The  nature  of  this  hypothesis  needs  further  explanation. 
For  this  purpose  a  brief  reference  to  two  particulars  is  sufficient  at 
present.  In  the  first  place,  any  piece  of  scientific  research,  no  matter 
how  much  it  may  abjure  all  definite  metaphysical  theories,  is  com- 
pelled to  make  certain  assumptions  and  to  indulge  certain  beliefs, 
which  are  really  of  a  metaphysical  character.  It  is  quite  right,  how- 
ever, that  these  assumptions  should  be  those  which  have  a  prima 
facie  evidence  for  all  human  thought;  and  that  these  beliefs  should 
be  such  as  accord  with  the  so-called  "common  sense"  of  mankind. 
Science,  as  long  as  it  remains  science,  does  not  undertake  to  furnish 
a  system  of  critical  metaphysics;  its  metaphysics  is  naive  and  pop- 
ular, rather  than  the  metaphysics  of  any  of  the  schools  of  philoso- 
phy. It  is  this  kind  of  metaphysical  attitude  which  we  purpose  to 
adopt  at  the  beginning  of  the  discussion  of  both  classes  of  the  prob- 
lems just  proposed.  And,  for  the  most  part,  we  intend  to  main- 
tain this  attitude  throughout  the  entire  discussion. 

§  3.  Now  there  can  be  no  doubt  as  to  what  "  naive  metaphysics," 
which  is  the  commonly  and  properly  adopted  attitude  of  science,  has 
to  say  as  to  the  general  relations  of  body  and  mind.  Its  theory  is 
what  the  expert  in  philosophy  and  its  history  would  call  "an  un- 
critical dualism."  Of  course,  everybody,  from  the  most  untutored 
savage  to  the  most  accomplished  student  of  psycho-physics,  when- 
ever the  subject  is  not  taken  as  a  purely  scholastic  question,  believes, 
speaks,  and  acts  as  though  there  were  two  existences,  a  body  and  a 
mind;  and  as  though  each  one  of  these  two  had  much  to  do  with  the 
behavior,  the  welfare,  and  indeed,  the  very  life  of  the  other.  In 
fact,  the  body — as  respects  both  what  it  is  and  what  it  is  doing — 
is  always  thought  of  as  acting  upon  and  influencing  the  mind ;  and 
in  like  manner,  the  mind  is  always  yielding  to  or  resisting  the  bod- 
ily influences.  Moreover  it  habitually  uses  the  bodily  organs  as 
though  they  were  its  ministers  or  tools.  It  is  as  an  embodiment  of 
this  dualistic  theory,  which  is  commended  both  by  common  sense 
and  by  science  as  a  working  hypothesis,  that  we  have  adopted  the 
term  "correlations." 

It  must  be  said  in  the  second  place,  that  no  one  word  can  serve 
equally  well  to  define,  or  even  to  describe,  all  those  reciprocal  in- 
fluences which  furnish  conditions  to  man's  complex  life  and  devel- 
opment as  an  ensouled  body  or  an  embodied  soul.  It  is  not,  however, 
at  all  strange  that  such  should  be  the  fact.  For  in  their  very  nature, 
the  relations  between  neural  and  mental  phenomena  are  of  all  others 
about  the  most  abstruse  and  complex;  and  in  man's  case,  the  beings 
between  which  they  are  assumed  to  maintain  themselves  are  of  all 


NATURE  OF  UNCRITICAL  DUALISM  631 

physical  and  psychical  existences,  about  the  most  complex  and  diffi- 
cult of  access. 

Our  course  would,  therefore,  seem  to  be  plainly  marked  out  for 
us,  so  far  as  the  problems  are  concerned  which  fall  more  properly 
under  the  titles  of  these  chapters.  However,  at  once  we  are  met 
by  differences  of  method;  and  finally,  by  two  rival  and  contrary 
ways  of  replying  to  the  general  inquiry.  One  of  these  denies  that, 
in  order  to  account  for  mental  phenomena,  we  need  assume  the  ex- 
istence of  any  reality  other  than  the  material  substance  of  the  living 
and  active  nervous  system  (especially,  or  wholly,  of  the  brain).  The 
other,  on  the  contrary,  claims  that  no  explanation  of  mental  phe- 
nomena is  possible  without  referring  them  to  a  non-material  or  spir- 
itual entity  as  the  real  subject  of  them  all.  Both  of  these  ways  of 
explanation  admit  of  various  modifications.  A  third  view,  which 
regards  both  the  so-called  "brain"  and  the  so-called  "mind"  as 
merely  phenomenal  aspects  of  some  one  reality  that  is  like  neither, 
but  manifests  itself  in  both,  requires  for  its  discussion  so  much  of 
subtle  metaphysics,  and  is  so  foreign  to  all  the  scientific  material 
with  which  we  have  thus  far  been  dealing,  that  it  is  for  the  present 
passed  by  with  a  bare  reference. 

In  the  remaining  part  of  our  discussion  we  shall  be  chiefly  occu- 
pied with  considering  which  one  of  two  theories  best  accords  with 
all  the  facts.  These  facts,  which  are  to  test  the  theory,  are  facts  of 
the  nervous  mechanism,  and  of  the  correlations  between  this  mechan- 
ism and  the  phenomena  of  consciousness.  The  question  before  us 
may  then  be  stated  in  the  following  provisional  form:  Do  the  phe- 
nomena of  consciousness  require  for  their  explanation  nothing  more 
than  a  statement  of  those  changes  in  the  material  mechanism  with 
which  they  are  obviously  correlated;  or  do  they  also  require  the  as- 
sumption of  one  real  and  non-material  being  as  the  subject  and 
ground  of  them  all  ? 

§  4.  How,  then,  we  now  inquire,  are  body  and  mind  related,  as 
respects  their  development  and  the  forms  of  functioning  assigned 
to  each?  The  two  series  of  phenomena,  as  has  already  been 
said,  when  looked  at  superficially,  appear  correlated — reciprocally. 
What,  however,  is  signified  by  these  apparent  or  obvious  facts  as  to 
the  real  connection  of  the  two? 

Various  attempts  have  been  made,  from  different  points  of  view, 
to  sum  up  in  some  single  word  the  relations  that  maintain  themselves 
between  the  body  and  the  mind.  Thus,  the  body  has  frequently 
been  spoken  of  as  the  "seat"  or  "organ"  of  the  mind.  Looking  at 
these  relations  from  the  more  strictly  mechanical  point  of  view,  men- 
tal phenomena  have  been  regarded  as  the  "products"  of  the  func- 
tional activity  of  the  brain.  More  highly  figurative  terms  even  have 


632       GENERAL  RELATIONS  OF  BODY  AND  MIND 

often  enough  been  employed,  especially  in  the  supposed  interests 
of  morals  or  religion.  The  body  has  then  been  called  the  "prison" 
or  "tenement"  or  "tabernacle"  of  the  soul.  Not  seldom,  also,  has 
the  mind  been  represented  as  mastering  and  controlling,  and  even 
"moulding"  the  body — somewhat  as  the  rider  subdues  and  guides 
his  horse,  or  the  worker  in  clay  and  metal  shapes  the  product  of  his 
toil.  One  form  of  the  doctrine  of  "  animism"  has  held  that  the  mind 
is  identical  with  the  vital  principle,  which  is  busy  from  the  very  im- 
pregnation of  the  ovum  in  shaping  its  increasing  molecules  accord- 
ing to  an  unconscious  or  dimly  conscious  plan.  Much  debate  has 
also  been  held  as  to  whether  the  conception  of  "cause"  is  applica- 
ble to  any  of  the  relations  in  which  body  and  mind  stand  to  each 
other — whether,  indeed,  it  must  not  rather  be  held  that  what  hap- 
pens in  one  is  only  the  "occasion"  on  which  some  underlying  cause, 
common  to  both,  operates  to  produce  a  change  in  the  other. 

§  5.  The  inquiry  in  what  sense,  if  at  all,  the  brain  can  be  said  to 
be  the  "seat"  of  the  mind  is  more  easily  answered  in  a  negative 
than  a  positive  way.  Nothing  but  the  crudest  notions,  both  of  the 
nervous  mechanism  and  of  the  mind,  would  be  consistent  with  any 
of  the  more  literal  and  direct  interpretations  of  this  word.  No  one 
would  seriously  regard  the  mind  as  a  special  entity,  whether  con- 
structed of  ordinary  material  atoms  or  constituted  in  ethereal  form, 
that  maintains  a  sitting  or  other  posture  amidst  the  cerebral  masses. 
Nor  is  it  any  more  correctly  conceived  of  as  thinly  diffused  over  the 
entire  mechanism  of  nerve-cells  and  nerve-fibres,  or  as  wandering 
about  among  the  cerebral  elements  to  find  its  temporary  "seat" 
where  occasion  seems  to  require  its  presence.  And,  although  some 
of  the  two  classes  of  phenomena  perhaps  admit  very  well  of  being 
brought  under  the  conception  of  an  atom,  acting  and  acted  upon  in 
varying  relations  to  other  atoms  of  kinds  different  from  itself,  no 
essential  gain  is  made  by  the  attempt  to  regard  the  mind  as  in  real- 
ity anything  of  the  sort.  In  brief,  there  is  no  literal  meaning  of  the 
words  in  which  we  can  speak  of  the  mind  as  seated  in  the  brain. 

The  phrase,  the  brain  is  the  "seat"  of  the  mind,  is,  however,  very 
well  adapted  to  raise  the  whole  question  of  the  spatial  qualities  of 
the  mind,  and  of  its  alleged  spatial  relations  to  different  portions  of 
the  central  nervous  system.  We  shall,  then,  briefly  consider  the 
grounds  for  the  use  of  this  figurative  term.  There  can  be  no  doubt 
that  science  justifies  ordinary  language  in  speaking  of  the  soul  as 
in  the  body,  in  some  sense  in  which  this  term  does  not  apply  to  any 
other  collection  of  material  atoms.  The  human  soul  is  in  the  human 
body  as  it  is  not  in  the  bird,  the  tree,  the  house,  the  star.  Even  that 
way  of  regarding  the  mind's  nature  which  does  not  hesitate  to  speak 
as  though  it  were  a  thinly  diffused  and  half-spiritualized  form  of 


THE  BRAIN  AS  "SEAT"  OF  THE  MIND  633 

matter,  assents  to  the  necessity  of  asserting  a  special  relation  in  space 
between  it  and  the  body.  Hence  some  old-time  philosophies  rep- 
resented the  soul  in  perception  as  streaming  out  through  the  avenues 
of  sense  in  order  to  get  the  sensuous  object  into  its  embrace;  or  else 
pictured  some  etherealized  copy  of  this  object  as  streaming  into  the 
soul  by  the  same  avenues.  Modern  vagaries,  in  the  form  of  theories 
of  an  "astral  body,"  or  of  "spiritualistic  materializations,"  are  fa- 
miliarizing us  with  such  representations  anew.  But  even  such  a 
view  of  the  nature  and  activities  of  the  mind  is  based  upon  the  claim 
that  the  body  is,  in  some  sort,  the  peculiar  dwelling-place,  or  "  seat," 
of  the  mind. 

A  correct  account  of  the  process  by  which  the  world  of  things  be- 
comes known  also  shows  that  all  our  experience  is  connected  with 
the  establishing  and  justifying  of  a  similar  claim.  There  are  no 
"  things  "  known  to  experience  except  as  our  sensations,  or  modes  of 
being  affected,  are  both  localized  and  projected  extra-men  tally.  In- 
ducements and  considerations,  such  as  have  already  been  treated 
in  great  detail,  irresistibly  urge  on  science  to  arrange  all  phenomena 
under  two  classes — phenomena  which  are  qualities  of  outside  things, 
and  phenomena  which  are  mere  states  of  internal  experience.  But 
the  same  inducements  and  considerations  compel  it  to  look  upon 
certain  phenomena  of  the  first  class  as  related  to  those  of  the  second 
class  in  a  peculiar  way.  The  world  of  things  outside  always  (at 
least  in  ordinary  experience)  affects  us — is  perceived  by  us  or  modi- 
fies our  consciousness — through  the  body.  The  mind  is,  therefore, 
said  to  be  in  the  body. 

The  conclusion  from  the  foregoing  general  experience  is  confirmed 
by  certain  experiences  of  a  special  order.  The  feelings  of  pleasure 
and  pain,  which  have  so  immediate  and  incontestable  a  value  for 
the  life  of  consciousness,  are  all  connected  with  sensations  more  or 
less  definitely  localized  in  the  body.  So  close  is  the  connection  be- 
tween the  localized  sensations  and  the  painful  or  pleasurable  states 
of  the  mind,  that  the  mind  actually  seems  to  be  suffering  in  that  part 
of  the  body  where  the  sensations  are  localized.  When  the  localiz- 
ing of  sensations  connected  with  feelings  of  strong  "tone"  is  very 
indefinite,  as  it  is  in  cases  where  the  feelings  arise  from  the  condi- 
tion of  large  areas  of  the  internal  organs,  the  soul  seems  to  be  suffer- 
ing in,  and  throughout,  almost  the  entire  body. 

Furthermore,  both  ordinary  experience  and  scientific  observation 
require  us  to  regard  the  mind  as  standing  under  certain  special  re- 
lations to  certain  parts  of  the  body.  The  ancients  located  the  soul 
in  the  heart  or  lower  viscera,  because  of  marked  connections  be- 
tween conscious  states  and  the  condition  of  these  organs.  But  the 
obvious  connection  of  the  head  with  the  more  obtrusive  sensations 


634      GENERAL  RELATIONS  OF  BODY  AND  MIND 

of  the  perceptive  order  tends  to  confirm  the  belief  that  the  mind,  as 
perceptive,  has  its  "seat"  in  that  region  of  the  body.  For  reasons 
already  given  in  detail  modern  scientific  researches  justify  us  in  nar- 
rowing more  precisely  the  local  domain  within  which  we  can  affirm 
the  mind  to  have  its  seat.  The  mind  is  certainly  in  the  nervous 
system,  in  a  sense  in  which  it  is  not  in  any  other  of  the  systems  of 
the  animal  body.  More  precisely  yet,  it  is  pre-eminently  in  the 
brain;  and,  among  all  the  complex  groups  of  encephalic  organs,  the 
final  and  special  claim  of  the  cerebral  cortex  to  be  the  "seat"  of  the 
mind  is  most  easily  maintained.  Here,  in  this  convoluted  rind 
which  forms  the  interlaced  "projection-systems"  of  sensory  and 
voluntary  motor-impulses,  here — if  anywhere — must  it  be  held  that 
the  subject  of  the  states  of  consciousness  has  its  peculiar  dwelling- 
place  and  home.  And  yet,  recent  experiments  in  cerebral  surgery 
upon  human  subjects  have  demonstrated  the  fact  that  the  conscious 
mind,  when  the  sensory  areas  of  the  brain  are  stimulated,  itself 
localizes,  or  seate,  its  own  sensations  and  feelings,  not  in  the  brain 
at  all,  but  in  the  appropriate  areas  of  the  periphery. 

§  6.  On  the  contrary,  the  results  of  modern  scientific  inquiry 
become  unfavorable  to  the  effort  yet  more  precisely  to  designate  a 
material  "seat"  for  the  mind.  Is  there  any  one  mathematical 
point,  or  minute  area,  in  the  cerebral  cortex  that  is  most  especially 
of  all  the  dwelling-place  of  mind  ?  If  so,  might  it  not  be  properly 
conceived  of  as  ordinarily  remaining  at  this  point  to  receive  the 
messages  despatched  to  it  from  the  various  parts  of  the  periphery; 
and  as  executing  its  will  over  those  peripheral  parts  by  sending  back 
to  them  corresponding  messages  despatched  from  the  same  central 
point?  The  pineal  gland  has  undoubtedly  lost  the  significance 
which  Descartes  gave  to  it  as  the  special  seat  of  the  soul.  But  can 
no  substitute  be  found  to  take  and  hold  so  important  a  place  ?  The 
answer  of  cerebral  histology  and  physiology  to  the  foregoing  ques- 
tions is,  on  the  whole,  a  decided  negative. 

At  this  point  it  is  customary  to  greet  with  peculiar  satisfaction, 
as  though  we  had  found  the  solution  of  a  puzzling  metaphysical 
problem,  the  discoveries  of  cerebral  localization,  in  recent  times. 
Certain  areas  of  the  cerebral  cortex  do,  indeed,  appear  to  have  a 
particular  connection  with  the  execution  of  certain  functions  of  the 
mind.  But  the  very  phenomena  on  which  reliance  is  placed  for 
establishing  this  connection  forbid  us  to  regard  the  mind,  in  its 
special  relations  to  the  brain,  as  limited  to  any  point  or  small  area 
of  the  cerebral  cortex.  Both  gross  and  microscopic  anatomy  show  us 
that  the  cortical  part  of  the  brain,  like  all  its  other  parts,  is  not  con- 
structed on  the  plan  of  having  its  uses  for  the  mind  concentrated  in 
any  one  minute  circumscribed  spot.  In  any  sense  in  which  the 


THE  BRAIN  AS  "SEAT"  OF  THE  MIND  635 

mind  can  be  said  to  have  its  "seat"  in  the  brain  at  all,  in  that  same 
sense,  and  with  equal  propriety,  may  the  entire  cerebral  cortex, 
with  its  vast  complexity  of  nerve-fibres  and  nerve-cells,  be  said  to 
be  entitled  to  something  of  the  same  distinction.  Moreover,  the 
combined  testimony  of  physiology  and  surgery  establishes  the  truth 
that  the  brain  never  localizes  the  mental  products  of  perception 
within  its  own  areas.  The  simple  yet  essentially  mysterious  truth 
is — as  has  just  been  pointed  out — that  when  certain  cerebral  areas 
are  properly  stimulated,  the  mind  localizes  its  own  sensations,  either 
in  the  corresponding  peripheral  parts  of  the  body,  or  somewhere 
external  to  the  body. 

§  7.  And  now  the  puzzling  question  recurs:  What  that  is  intelli- 
gible can  be  meant  by  designating  the  supreme  central  organs  of 
man's  nervous  mechanism  as  the  "seat"  of  his  conscious  mind? 

The  only  solution  for  such  a  puzzle  as  the  foregoing — if  solution 
it  can  be  called; — must  always  consist  in  calling  attention  anew  to 
the  essential  facts  of  the  case.  Certain  particles  of  very  highly 
organized  chemical  constitution,  when  grouped  into  nerve-fibres 
and  nerve-cells,  and  when  further  associated  into  organs,  may  be 
acted  upon  by  appropriate  stimuli.  These  material  particles  are 
locally  in  the  cranial  cavity,  and,  more  precisely,  in  this  or  that  area 
or  organ  of  the  cranial  contents.  Moreover,  a  large  and  important 
part  of  the  phenomena  of  consciousness  consists  in  localized  bodily 
sensations  of  a  painful  or  pleasurable  character.  To  these  facts  in- 
vestigation adds  the  inference  as  based  upon  experiment  and  ob- 
servation in  the  case  of  others,  that  the  localized  sensations  are 
themselves  ultimately  dependent  upon  the  behavior  of  the  afore- 
said material  molecules  in  the  brain.  That  is  to  say,  we  directly 
localize  many  of  our  mental  affections  in  this  or  that  part  of  the 
body;  by  remote  processes  of  observation  and  argument  we  infer 
that  the  last  material  antecedent  of  them  all  is  the  behavior  of  cer- 
tain invisible  parts  of  the  body  within  the  brain.  Therefore  we  say: 
The  mind  is  in  the  brain;  or  the  seat  of  the  mind  is  in  the  brain.  By 
this,  nothing  further  can  be  meant  of  an  assured  or  intelligible 
character  than  the  emphatic  repetition  of  the  same  principal  facts: 
the  sensations  which  we  localize  at  the  periphery  of  the  body,  or  pro- 
ject from  the  body  in  space,  all  have  a  sui  generis  connection  with  the 
condition  and  action  of  that  portion  of  the  same  body  which  is  con- 
tained in  the  cranial  cavity. 

As  to  the  possibility  of  such  a  sui  generis  relation  between  mate- 
rial elements  which  exist  in  space,  and  the  localizing  and  other  ac- 
tivities of  a  being  not  to  be  conceived  of  as,  strictly  speaking,  in 
space,  only  experience  is  entitled  to  pronounce.  Such  a  relation  is 
an  accomplished  fact.  The  fact  is,  therefore,  not  to  be  disputed  on 


636       GENERAL  RELATIONS  OF  BODY  AND  MIND 

any  so-called  a  priori  grounds.  It  does  not  follow,  however,  that 
the  relation  of  the  mind  to  the  brain  is  any  more  essentially  myste- 
rious than  that  of  the  molecules  of  the  brain  to  one  another.  Nor 
does  it  form  an  insuperable  objection  to  the  former  relation  that 
it  is  not,  like  the  latter,  a  relation  of  changes  of  position  in  space. 
For  who  shall  undertake  to  affirm  that  beings  which  are  not  ex- 
tended and  movable  in  space,  because  their  very  nature  is  of  another 
order,  cannot  exist  in  relations  of  any  kind  to  beings  which  are  thus 
extended  and  movable  ?  It  is  precisely  in  this  way  that  the  mind  is 
actually  related  to  the  brain.  To  speak  of  the  mind  as  having  its 
"seat"  in  the  brain  is  only  a  figurative  way  of  affirming  the  reality 
of  such  relations. 

§  8.  The  term  "organ"  (or  instrument)  of  the  mind,  as  applied 
to  the  body,  is  particularly  calculated  to  emphasize  the  relation  of 
the  ideas  and  volitions  which  arise  in  consciousness  to  the  control 
of  the  muscular  apparatus.  But  the  same  term  may  also  be  used, 
though  with  less  propriety,  to  describe  the  relation  of  the  brain  to 
the  mind  in  sensation  and  thought.  Thus  we  may  be  said  to  feel 
or  think  with  the  brain,  in  some  manner  supposed  to  be  analogous 
to  that  in  which  the  workman  accomplishes  his  task  by  availing 
himself  of  a  particular  tool  or  instrument.  It  is  obvious,  however, 
that  the  figure  of  speech  suggested  by  these  terms  also  will  not  admit 
of  a  literal  interpretation.  We  cannot  conceive  of  the  mind  as  a 
peculiar  kind  of  material  entity  which,  when  it  desires  or  wills  to 
move  the  bodily  members  in  a  certain  way,  lays  a  clutch — as  it 
were — upon  the  nervous  substance  of  the  central  organs,  and  so 
makes  the  body  serve  as  an  "  organ"  of  the  desire  or  volition.  Even 
less  are  we  to  conceive  of  the  brain  as  a  complex  tool  or  mechanism 
which  the  mind  uses  in  thought  and  feeling,  somewhat  as  senses  and 
fingers  avail  themselves  of  a  calculating  machine  or  of  a  musical 
instrument. 

In  producing  changes  of  shape  and  position  in  masses  of  matter 
outside  of  our  own  bodies,  we  ordinarily  find  it  convenient  to  use 
some  material  medium  between  those  masses  and  the  various  mov- 
able parts  of  our  own  bodies.  We  can,  by  means  of  complicated 
mechanisms,  accomplish  a  great  variety  of  changes  which  it  would 
be  quite  impossible  to  accomplish  without  such  aid.  On  the  other 
hand,  we  sharpen,  define,  and  multiply  our  sensations  and  percepts 
of  things  in  similar  manner.  The  deaf  man  hears  with  a  trumpet 
or  other  acoustic  contrivance;  and  the  scientific  observer  contrives 
an  instrument  for  observing  the  absolutely  simple  tones  as  analyzed 
out  of  the  composite  clang;  and  with  a  prism  the  optician  beholds 
the  colors  of  the  spectrum. 

It  is  characteristic  of  all  the  most  skilful  use  of  tools  and  instru- 


THE  BRAIN  AS  ORGAN  OF  THE  MIND  637 

ments  that  they  come  to  seem  to  the  observer  like  a  part  of  his  own 
bodily  mechanism.  By  feelings  of  "double  contact"  the  workman 
comes  to  know,  with  the  chisel,  the  wood  or  metal  which  he  is  carv- 
ing— just  as  the  blind  man  seems  to  extend  his  conscious  life  to  the 
very  end  of  the  stick  he  is  accustomed  to  carry.  In  these  cases  the 
mental  picture  before  the  practised  mind  is  not  that  of  the  hand  and 
the  way  it  must  be  moved,  but  of  the  graving  tool  and  the  motion  to 
be  imparted  to  it — as  though  the  instrument  itself  were  immediately 
subject  to  volition. 

The  conception  of  an  "organ"  or  instrument  may,  with  a  cer- 
tain propriety,  be  extended  so  as  to  cover  the  relation  which  exists 
between  the  nervous  system  and  the  muscular,  and  between  the 
central  and  peripheral  parts  of  the  nervous  system.  Thus  it  may 
be  said  that  the  spinal  cord  and  brain  move  the  limbs  "with  the 
use"  of  the  afferent  nerves,  or  that  the  cerebral  hemispheres  employ 
the  lower  ganglia  of  the  brain  in  effecting  certain  co-ordinations  of 
sensation  and  motion;  it  may  even  be  said  that  the  end-organs  of 
sense  communicate  with  the  supreme  central  organs  "by  means  of" 
the  afferent  nerve-tracts  and  the  lower  ganglia.  All  such  language 
expresses,  correctly  enough  for  popular  usage,  the  undoubted  fact 
that,  in  the  complicated  relations  of  position  and  motion  which  are 
maintained  among  the  different  members  of  the  nervous  system,  a 
certain  order  of  action  is  constantly  preserved.  Changes  originate 
in  one  part,  and  are  propagated  to  other  contiguous  or  more  distant 
parts.  In  such  propagation  of  the  changes  a  regular  tract  of  the 
advancing  motions  is  assumed  always  to  exist;  and  thus  to  science 
the  parts  that  lie  between  the  extremes  may  be  looked  upon  as  means 
or  media — i.  e.,  as  instrumental  to  the  completion  of  the  process. 
To  the  uninformed  person,  however,  the  result  seems  to  be  an  "  im- 
mediate" effect  of  the  will. 

§  9.  It  is  obvious  from  the  foregoing  remarks  that  one  part  of 
the  nervous  mechanism  can  be  said  to  be  the  "organ"  or  instru- 
ment of  another  part,  in  the  meaning  of  the  word  which  cannot  prop- 
erly apply  to  the  relation  of  the  brain  and  the  mind.  In  a  certain 
justifiable  meaning  of  the  word,  all  the  rest  of  the  body  may  be 
said  to  be  the  organ  of  the  brain.  That  is  to  say,  those  changes  in 
the  molecules  of  the  brain's  substance  which  arise  there — whether 
because  of  certain  ideas  and  volitions  of  the  mind,  or  because  of 
changes  in  the  character  of  the  blood-supply,  or  of  sensory  impulses 
thrown  in  from  the  periphery  or  other  lower  nervous  centres — get 
themselves  expressed  through  the  other  members  of  the  body.  One 
part  serves  as  an  instrument  or  "organ"  for  another,  because  the 
changes  in  it  effect  changes  elsewhere,  not  directly,  but  through  con- 
tiguous and  connected  parts.  If  the  necessary  contiguous  parts  are 


638       GENERAL  RELATIONS  OF  BODY  AND  MIND 

wanting  or  their  relations  disarranged,  if.  the  connection  is  inter- 
rupted or  destroyed,  then  the  work  cannot  be  done;  the  "organ," 
"instrument,"  or  "means,"  is  lacking. 

But,  in  truth,  only  a  part  of  the  real  relations  existing  between 
mind  and  brain  can  properly  be  described  under  such  terms  as 
"organ,"  "instrument,"  etc.  The  brain,  with  its  appropriate  func- 
tions, is  an  indispensable  medium  between  certain  changes  in  the 
peripheral  parts  of  the  body  and  corresponding  changes  in.  the  states 
of  consciousness.  As  much  as  this  is  true  of  all  the  efferent  tracts 
which  lead  from  the  cerebral  cortex  through  the  lower  portions  of 
the  encephalon,  along  the  spinal  cord,  and  out  to  the  particular 
groups  of  muscles.  Something  more  and  special  is,  however,  true 
of  the  brain.  It  is  the  first  of  the  indispensable  physical  links  in 
the  whole  chain;  it  stands  nearest,  as  it  were,  to  the  mind.  All 
the  other  steps  in  the  execution  of  the  ideas  and  volitions  of  the 
mind  depend  upon  what  takes  place  in  the  brain.  In  this  sense, 
at  least,  the  brain  is  the  particular  organ  of  the  mind;  it  is  the  most 
intimate  and  indispensable  means  for  the  execution  of  all  its  ideas 
or  volitions  of  motion. 

It  does  not  appear  that  the  foregoing  statement  by  any  means 
exhausts  the  description  of  the  experience,  reflection  upon  which 
induces  us  to  regard  the  brain  as  the  "organ"  of  the  mind.  For 
the  brain  seems  to  serve  as  the  special  physical  basis  of  the  ideas 
and  volitions  of  motion  themselves.  Experiments  with  animals, 
by  extirpating  the  cortical  areas,  and  observation  of  human  patho- 
logical cases — especially,  perhaps,  in  certain  forms  of  aphasia — 
seem  clearly  to  show  that  a  much  more  intimate  "organic"  relation 
exists  between  the  brain  and  the  mind.  With  the  destruction  or 
derangement  of  certain  cerebral  areas,  the  power  even  to  form  cer- 
tain ideas  and  volitions,  or  to  have  certain  feelings,  seems  to  be  im- 
paired or  lost.  We  cannot  say,  to  be  sure,  that  the  mind  has  lost 
a  part  of  its  general  faculty  to  conceive,  to  feel,  and  to  will.  It  has, 
however,  suffered  in  respect  of  its  power  to  frame  a  certain  set  of 
definite  ideas  and  volitions  for  the  purpose  of  controlling  the  mo- 
tion of  the  peripheral  members.  This  class  of  facts  is  certainly 
calculated  to  emphasize  strongly  our  conception  of  the  brain  as 
being,  in  a  special  sense,  the  indispensable  means  through  which  the 
states  of  consciousness  are  related  to  changes  in  the  position  of 
molecules  and  masses  of  matter. 

§  10.  There  is  another  most  important  class  of  facts  which  may 
be  partially  described  under  the  same  terms  as  the  foregoing.  The 
brain  is  the  indispensable  means  for  furnishing  the  mind  with  its 
sensations,  and  so  with  its  presentations  of  sense  or  perceptions  of 
things.  This  statement  is  not  to  be  understood  as  though  the  brain 


A  BOND  BETWEEN  BRAIN  AND  MIND  639 

could,  of  itself,  construct  the  sensations  and  perceptions  and  hand 
them  over  ready-made,  as  it  were,  to  the  mind.  Sensations  are 
states  of  consciousness,  not  modes  of  the  brain ;  and  even  when  they 
are  synthetically  united,  localized,  and  projected  to  the  periphery 
of  the  body,  or  into  surrounding  space,  they  are  brought  under  no 
essentially  new  relations  to  the  nervous  mechanism.  Sensations 
are  not  nerve-commotions,  "etherealized"  by  the  optic  thalami  and 
cerebral  convolutions,  and  then  handed  over  to  consciousness. 
Therefore  the  instrumental  relation  between  brain  and  mind  is 
not  that  of  transmitting  a  peculiar  kind  of  motion  from  one  phase 
into  another,  or  from  one  being  to  another.  Nevertheless,  no  sen- 
sations will  arise  in  the  mind  unless  the  brain  be  affected  in  a  cer- 
tain way.  Looking  at  the  chain  of  sequences  as  it  runs  from  with- 
out inward,  we  might  say:  The  brain  is  the  organ,  or  instrument, 
through  which  the  stimuli  of  the  outside  world,  acting  on  the  end- 
organs  of  sense,  finally  reach  the  mind.  Or,  to  say  the  same  thing 
in  other  terms:  The  brain  is  the  last  and  most  important  physical 
antecedent  to  the  mind's  being  affected  with  the  different  sensations. 
§  11.  Still  another  class  of  attempts  to  generalize,  and  embody 
in  a  single  term,  the  various  essential  relations  of  the  brain  to  the 
mind  leads  to  the  inquiry  after  some  one  special  "connection"  or 
"bond"  between  the  two.  Here,  again,  any  too  literal  answer  to 
this  inquiry  leads  at  once  to  manifest  absurdity.  A  material  bond 
designed  to  unite  mind  and  brain  might  perhaps  be  conceived  of  as 
connected  with  the  latter,  and  yet  as  remaining  material;  but  in 
order  to  make  it  connect  with  the  former  (the  mind)  it  would  have 
to  become  non-material,  unless  we  are  ready  to  concede  that  the 
material  and  the  non-material  can  stand  connected  without  any 
special  bond.  In  case  this  concession  is  once  made,  however,  we 
cease  to  feel  the  need  of  a  special  bond  between  the  mind  and  the 
brain.  But  if  it  be  at  once  admitted  that  no  connection  is  to  be 
sought,  or  can  be  found,  between  the  mind  and  the  brain,  beyond 
the  fact  that  their  modes  of  behavior  are  mutually  dependent,  it  will 
not  be  necessary  to  appeal  to  any  special  mystery.  This  is  simply 
to  admit  that  general  fact  of  correlations  upon  which  every  form  of 
science  depends  for  its  conclusions.  What  bond  connects  together 
the  planets  of  the  solar  system  so  that  each  one  moves  invariably 
with  reference  to  the  position  of  all  the  others,  and  yet  in  a  path  pe- 
culiarly its  own  ?  We  can  only  respond  by  talking  of  the  force  and 
laws  of  gravitation.  These  "laws,"  however,  are  only  a  mathe- 
matical statement  of  the  uniform  modes  of  the  behavior  of  certain 
physical  beings;  this  "force"  is  no  entity  existing  between  the  indi- 
viduals, as  the  rods  of  the  orrery  bind  its  parts  to  a  common  centre. 
Cohesion  and  chemical  affinity  are  not  special  bonds;  they,  too, 


640       GENERAL  RELATIONS  OF  BODY  AND  MIND 

are  but  expressions  for  the  facts  that  the  elements  of  material  real- 
ity, under  certain  conditions  and  according  to  the  kind  to  which 
they  belong,  behave  as  though  bound.  That  is,  they  are  actually 
correlated.  The  behavior  of  the  so-called  atoms,  like  that  of  the 
stars,  is,  as  a  simple  matter  of  fact,  "relative,"  each  to  the  others. 

§  12.  It  will  scarcely  be  supposed  that  information  of  scientific 
value  concerning  the  nature  of  the  real  connection  between  the 
body  and  the  soul  can  be  obtained  from  terms  which  are  yet  more 
purely  figurative  and  poetic  than  those  which  have  already  been 
examined.  The  limited  and  defective  nature  of  our  sense-percep- 
tions, the  misery  of  much  of  life,  the  unrealized  longings  for  knowl- 
edge and  happiness,  and  the  work  of  imagination  in  framing  a  pict- 
ure of  some  state  of  existence  in  which  the  limitations  are  removed 
and  the  longings  realized,  have  led  men  in  all  ages  to  regard  the 
body  as  the  "prison"  of  the  soul.  Because  the  senses  are  not  more 
in  number  than  they  really  are,  or  more  far-reaching  and  accurate 
than  their  construction  permits  them  to  be,  they  are  regarded  as  re- 
straining the  soul,  rather  than  as  bringing  it  information  which  has 
the  character  of  satisfying  reality.  The  brevity  and  uncertainty  of 
life,  and  the  speed  with  which  accident  and  disease  impair  or  dis- 
solve the  bodily  functions,  together  with  the  persuasion  that  the 
thinking  principle  will  have  a  continued  existence,  suggest  the  re- 
flection: the  body  is  only  the  "tenement"  or  "tabernacle"  of  the 
soul. 

§  13.  It  appears,  then,  that  all  the  terms  in  popular  use  to  convey 
the  impressions  of  a  "  dualistic "  theory  of  the  relations  between  the 
body  and  the  mind  are  well  grounded  in  facts  of  experience.  They 
express  the  truth,  although  only  in  an  incomplete  and  figurative 
way,  which  we  have  tried  to  summarize  under  the  word  "correla- 
tions"; and  by  this  term  we  understand  series  of  changes  occurring 
in  the  physical  mechanism  that  are  dependently  related,  either  as 
antecedents  or  consequents,  to  series  of  phenomena  occurring  in 
consciousness.  So  unlike  in  their  most  essential  characteristics  are 
these  two  classes  of  phenomena  that  we  are  forced  to  assign  them 
to  different  species  of  beings — in  this  case,  minds  and  bodies;  and 
yet  so  intimate  and  regular  are  at  least  some  of  the  forms  of  this  re- 
ciprocal dependence,  that  we  are  entitled  to  speak  of  the  general 
facts  of  relation  as  constituting  so-called  laws.  All  the  researches 
of  physiological  and  experimental  psychology,  as  thus  far  conducted, 
do  not  contradict,  but  rather  confirm,  this  popular  and  naive  dualism. 

The  same  thing  cannot  be  said,  however,  of  certain  other  terms 
which  have  been  proposed  in  the  name  of  psycho-physical  science, 
and  for  use  by  all  its  various  subdivisions  and  branches,  taking  the 
words  in  their  widest  possible  application.  For  these  terms  at- 


CONSCIOUSNESS  AS  A  PRODUCT  OF  BRAIN        641 

tempt  in  some  manner,  or  to  some  degree  which  the  popular  im- 
pressions do  not  warrant,  to  identify  the  body  and  the  mind.  Now, 
although  this  particular  word  is  seldom  used,  there  are  theories  which 
have  not  a  few  defenders,  and  which  have,  indeed,  not  a  few  facts 
of  experience  in  their  favor,  that  would  bring  the  correlations  be- 
tween the  mechanism  and  the  mental  life  under  some  such  concep- 
tion as  that  of  "product,"  or  other  similar  term.  In  their  more 
customary  form  these  theories  regard  the  brain  as  in  some  sort  the 
producer  of  the  phenomena  of  the  conscious  mental  life. 

By  the  word  "product"  we  ordinarily  understand  the  result  of 
some  process  of  manufacture;  or  in  the  case  of  a  living  organism 
like  the  human  body,  the  results  of  the  secretory  or  metabolic  proc- 
esses of  these  organs  are  spoken  of  as  their  "products,"  after  the 
analogy  of  the  products  of  the  field  or  of  the  loom.  But  to  speak 
of  mental  states  and  processes  as  products  of  the  brain,  in  any  cor- 
responding meaning  of  the  words,  involves  us  in  the  grossest  ab- 
surdities. The  peculiar  secretory  product  of  the  brain  is  the  fluid 
found  in  certain  of  its  cavities;  and  its  metabolic  products  are  the 
worn-out  materials  which  it  discharges  into  the  venous  circulation, 
or  the  renewed  elements  of  its  own  substance.  In  case  it  is  prepar- 
ing abnormal  or  diseased  products,  these  take  the  form  of  abscesses 
or  tumors,  or  of  blood-vessels  and  nerve-cells  that  have  gone  wrong. 

A  more  plausible  use  of  the  word  product,  as  applying  to  certain 
correlations  between  the  brain  and  the  phenomena  of  consciousness, 
might  assume  the  following  form:  The  functional  activity  of  the 
nervous  centres  might  be  regarded  as  the  product  of  the  matter  con- 
stituting these  centres.  But  after  admitting  the  propriety  of  this 
manner  of  speech,  the  very  same  problem  remains  upon  our  hands; 
and  it  is  no  less  difficult  and  mysterious  than  it  was  before.  For 
the  problem  is:  How  shall  we  conceive  of  the  correlations  between 
the  different  forms  of  this  very  nervous  activity  and  the  antecedent, 
concomitant,  or  sequent  changes  in  the  conscious  mental  life  ?  If 
our  knowledge  of  these  relations  were  indefinitely  increased,  and 
even  if  it  became  perfect,  it  is  still  impossible  to  see  how  the  term 
"product,"  or  any  similar  term,  would  fitly  characterize  these  re- 
lations. 

§  14.  The  term  "  cause,"  or  necessary  precondition,  seems  of  all 
much  most  appropriate  to  describe  in  general  the  nature  of  the  cor- 
relations between  the  body  and  the  mind.  And,  indeed,  as  we  shall 
subsequently  show,  when  properly  understood,  the  essential  nature 
of  these  relations  may  be  described  as  causal.  But  here  again,  it  is 
customary  to  give  a  too  strictly  mechanical  and  physical  interpreta- 
tion to  the  words  employed,  and  to  assume  that  this  causal  relation 
works  in  only  one  way. 


642       GENERAL  RELATIONS  OF  BODY  AND  MIND 

Thus  we  are  treated  to  a  theory  of  the  correlations  between  the 
body  and  the  mind  which  renders  the  latter  absolutely  dependent 
upon  the  former,  not  only  for  the  precise  forms  of  its  characteris- 
tic functioning,  but  also  for  its  unity  and  claim  to  even  a  temporary 
existence.  This  theory  assumes  that  all  mental  phenomena,  what- 
ever their  varied  characteristic  shading,  have  their  exact  equivalents 
and  necessary  conditioning  causes,  only  in  specific  forms  of  the  nerve- 
commotion  of  the  living  human  brain.  We  may  give  a  more  definite 
statement  to  this  mechanical  theory  in  the  following  way:  With 
changes  in  the  substance  of  the  brain  which  may  be  designated 
A,  B,  C,  D,  etc.,  the  mental  processes  called  a,  6,  c,  d,  etc.,  are  uni- 
formly and  necessarily  joined ;  and  with  the  combination  of  molecu- 
lar changes  which  may  be  described  by  A  +B  +  C  +D,  etc.,  the  mental 
states  a+b+c+d,  are  as  uniformly  and  necessarily  joined.  When 
the  same  molecular  changes  recur  in  a  fainter  or  modified  form,  as 
A',  B't  Cr,  D',  then  there  must  be  a  recurrence  of  the  corresponding 
mental  states,  only  in  fainter  form,  as  a',  &',  c',  d'.  Finally,  it  is 
without  exception  true — so  this  theory  holds — that  nothing  happens 
in  the  mental  life,  by  way  of  conscious  sensation,  presentation  of 
objects  of  sense,  ideation,  reproduction  of  mental  images,  and  higher 
aesthetic  feeling,  or  processes  of  reasoning,  or  choice,  which  does 
not  find  its  only  real  explanation  in  the  equivalent  changing  states 
of  the  nervous  system.  How  stupendous  are  the  assumptions  in- 
volved in  such  a  theory,  and  how  far  it  outruns  all  our  knowledge 
of  the  facts,  has  been  made  obvious  by  the  entire  course  of  our 
previous  investigations.  Besides,  these  investigations  have  abund- 
antly shown  that  no  one-sided  view  of  the  nature  of  the  corre- 
lations between  body  and  mind  can  lay  claim  to  all  the  facts  in  its 
support. 

§  15.  We  have  now  to  examine  more  carefully  the  propriety  of 
applying  terms  which  imply  causation  (such  as  "  energy,"  "  action," 
"force,"  "impulse,"  "effective  agency,"  etc.)  to  the  case  of  mind 
and  brain.  Everything  which  has  been  said  has  involved  the  in- 
ference that  these  terms  may  be  so  understood  as  to  be  really  applica- 
ble. There  would  be  no  advantage  to  the  mind  in  being  "seated" 
in  the  brain — that  is,  in  being  under  any  special  relations  to  a  given 
extent  of  nervous  matter — unless  it  were  somehow  influenced  or 
acted  upon  by  this  nervous  matter,  and  could  in  turn  influence  and 
act  upon  it.  No  "organ"  or  instrument  is  of  any  use  whatever — 
that  is,  no  thing  can  become  an  organ  or  instrument — unless  it  can 
be  acted  upon  by  that  which  employs  it  as  an  organ,  and  can  in 
its  turn  act  upon  other  things.  Action  of  mind  on  brain  is  implied 
in  calling  the  latter  the  organ  of  the  mind's  volitions;  action  of  brain 
on  mind  is  implied  in  calling  it  the  organ  of  the  mind's  sensations. 


INFLUENCE  OF  BODY  ON  MIND  643 

In  general,  to  act  and  to  be  acted  upon  is  equivalent  to  standing  in 
the  relation  of  cause  and  effect. 

It  is  not  at  present  necessary  to  point  out  in  detail  how  much  of 
obscurity  and  contradiction  are  involved  in  all  the  more  popular 
ways  of  mentally  representing  the  foregoing  relation.  The  trans- 
mission of  energy  (or  force)  is  popularly  spoken  of  as  though  such 
energy  streamed  off  from  one  body  and  attached  itself  to  another; 
and  as  though  the  quantity  of  energy  thus  given  off  were  dependent 
upon  the  strength  of  the  blow  given  by  one  body  to  another.  Let  it 
be  supposed,  however,  that  the  application  of  the  law  of  causation  to 
the  case  of  brain  and  mind  is  made  in  the  most  approved  manner. 
It  is  simple  matter  of  fact,  as  tested  by  thousands  of  observations 
and  experiments,  that  changes  in  the  condition  and  functional  ac- 
tivity of  the  nervous  centres  are  followed  by  changes  in  states  of 
consciousness,  in  a  regular  way;  and  that,  conversely,  changes  of 
the  latter  sort  are  followed  by  changes  in  the  relations  of  the  masses 
of  the  body,  and  of  the  functional  activity  of  nervous  centres  and 
end-organs  of  sense.  Now,  unless  we  are  ready  to  be  satisfied  with 
simply  stating  the  facts,  without  making  the  attempt  to  find  any 
rational  account  for  them,  we  are  obliged  to  consider  these  cor- 
related changes  under  the  terms  of  cause  and  effect;  and  in  fact, 
were  it  not  for  the  influence  of  prejudice  derived  from  speculation 
upon  certain  philosophical,  ethical,  and  religious  questions,  no  one 
would  think  of  hesitating  to  apply  the  terms  of  causation  to  the  case 
of  mind  and  brain. 

How  impossible,  indeed,  it  is  to  avoid  speaking  of  the  connection 
of  mind  and  brain,  in  terms  of  causation,  may  be  illustrated  by  the 
relations  between  the  condition  of  the  intercranial  blood-supply  and 
the  states  of  consciousness.  A  slight  increase  of  this  circulation, 
resulting  from  a  small  quantity  of  alcohol  or  other  drugs,  or  from 
the  hearing  of  interesting  news,  produces  an  increased  speed  in  the 
mental  train.  Reaction-time  is  found  to  vary  with  changes  in  the 
circulation.  In  the  delirium  of  fever  the  wild  and  quickly  moving  con- 
dition of  the  thoughts,  fancies,  and  sensations  is  a  direct  expression 
of  the  kind  of  work  which  is  going  on,  because  of  the  accelerated 
heart-beat  and  the  disordered  character  of  the  blood,  within  the 
cerebral  arteries.  Schroeder  van  der  Kolk  tells  of  a  patient  who, 
when  his  pulse  was  reduced  by  digitalis  to  50  or  60  beats  per  minute, 
was  mentally  quiet  and  depressed;  when  it  was  allowed  to  rise  again 
to  90  beats,  his  mind  was  in  maniacal  confusion.  Cox  narrates  the 
case  of  a  sick  man  who,  at  40  pulsations  in  the  minute,  was  "  half- 
dead";  at  50,  melancholic;  at  70,  quite  "beside  himself";  at  90, 
raving  mad.  The  character  of  dreams  is  determined,  to  a  consid- 
erable extent,  by  the  position  of  the  head  and  the  way  in  which 


644       GENERAL  RELATIONS  OF  BODY  AND  MIND 

this  position  affects  the  cranial  circulation.  Hallucinations  not  in- 
frequently are  immediately  made  to  cease  when  the  person  having 
them  assumes  the  standing  posture,  or  has  leeches  applied  to  the 
head.  Indeed,  the  phenomena  which  illustrate  the  causal  influ- 
ence of  the  nervous  mechanism  on  the  mental  life  constitute  a  large 
part  of  th£  entire  body  of  the  science  of  psycho-physics  and  physio- 
logical psychology. 

§  16.  On  the  other  hand,  phenomena  which  indicate  that  mind 
operates  as  a  true  cause  within  the  structure  of  the  body  are  also  in- 
numerable. They  are  quite  as  numerous,  though  perhaps  not  so 
obvious  and  impressive,  as  those  which  indicate  the  reverse  rela- 
tion. The  chief  reason  why  these  phenomena  are  relatively  little 
regarded  in  psycho-physical  researches  is  that  the  real  causes  are 
in  this  case  not  readily  made  the  objects  of  observation  and  measure- 
ment. External  stimuli  constitute  those  causes  of  mental  changes 
which  we  can  most  easily  observe  and  estimate.  Ideas,  feelings, 
and  acts  of  will  arising  in  consciousness,  and  considered  as  causes  of 
the  resulting  bodily  changes,  cannot  be  treated  by  the  same  methods 
of  experimental  science  as  apply  to  physical  stimuli.  But  that  the 
mind  acts  on  the  body  is  one  of  the  most  familiar  of  experiences. 
Such  action  penetrates  and  modifies  all  the  life  of  the  body.  Hence 
the  material  mechanism  of  the  animal  structure  can  never  be  con- 
sidered, with  a  view  to  explain  what  is  going  on  within  it,  as  though 
it  were  disconnected  from  the  consciousness  of  the  animal.  The 
most  purely  vegetative  of  the  processes  of  the  human  body  are 
dependent  for  their  character  upon  the  states  of  the  human  mind. 
The  nutrition  of  the  tissues,  the  circulation  of  the  blood,  the  secre- 
tion of  different  kinds  of  fluids,  the  healthy  or  diseased  nature  of 
the  vital  processes,  are  greatly  influenced  by  conscious  processes. 
If  abnormal  digestion  produces  melancholy,  it  is  equally  true  that 
melancholy  causes  bad  digestion.  In  the  case  of  the  rise  of  strong 
emotions,  like  anger  or  grief,  the  increasing  affection  of  the  mind 
builds  itself  up  upon  a  physical  basis  of  increasing  disturbance  of 
the  organs;  but  it  is  equally  obvious  that  the  starting  of  the  emo- 
tion in  consciousness,  and  the  letting  of  it  slip  from  control,  are 
necessarily  followed  by  gathering  momentum  to  the  organic  dis- 
turbance. Irregular  action  of  the  heart,  caused  by  organic  defect 
or  weakness,  occasions  a  feeling  of  indescribable  alarm  in  the  soul; 
fear  is  followed,  through  the  action  of  the  mind  upon  the  nervous 
centres,  by  functional  incapacity  of  the  heart.  The  impure  condi- 
tion of  the  arterial  blood  which  is  characteristic  of  certain  diseases 
brings  about  a  chronic  state  of  mental  lassitude  or  anxiety;  care, 
chagrin,  and  ennui  poison  the  arterial  blood.  The  lesion  of  the 
cortical  substance  produced  by  a  growing  abscess  or  broken  blood- 


INFLUENCE  OF  MIND  ON  BODY  645 

vessel  impairs  the  mind's  powers  of  sensation  and  thought;  ex- 
cessive thought  and  over-excited  feeling  wear  away  the  brain. 

The  entire  class  of  phenomena  which  we  are  entitled  to  call 
"voluntary,"  in  the  widest  sense  of  the  word,  might  be  appealed 
to  in  proof  of  the  same  principle.  Whether  they  show  that  the 
mind  is  "free,"  in  the  highest  ethical  meaning  of  the  word,  or  not 
(and  upon  this  question  psycho-physical  science  cannot  pronounce), 
they  certainly  do  show  that  the  condition  of  the  bodily  organs  is 
made  dependent,  through  the  nervous  elements  of  the  brain,  upon 
the  states  of  the  mind.  And  here  are,  in  point,  the  phenomena  of 
the  voluntary  innervation  of  the  organ  by  fixing  the  attention,  of  the 
dependence  of  reaction-time  upon  the  exercise  of  the  will  through 
attention  of  the  person  reacting,  of  the  abstraction  of  regard  from 
the  images  of  sense  when  occupied  in  reflective  thought,  as  well 
as  all  the  more  marvellous  instances  of  self-control  in  determining 
the  results  of  disease,  etc. 

The  elevation  of  the  bodily  activities  to  the  most  astonishing 
precision,  under  the  influence  of  high  and  strong  artistic  feeling, 
or  sense  of  duty,  is  also  a  noteworthy  fact  of  the  same  order.  The 
mind  has  not  the  power  to  constitute,  in  opposition  to  fixed  chem- 
ical affinities,  a  single  molecule,  or  to  execute  the  slightest  move- 
ment of  a  single  muscle,  without  involving  the  nervous  system  in 
the  expenditure  of  the  requisite  energy.  Moreover,  this  energy 
must  be  started  in  the  appropriate  cortical  area  and  descend  along 
the  allotted  motor  tracts.  We  cannot  explain  how  it  is  that  mole- 
cules of  nervous  matter  can  be  acted  upon  in  view  of  states  of  con- 
sciousness. But  neither  can  we  explain  how  one  kind  of  atoms 
comes  to  act  as  it  does  in  view  of  the  presence  and  action  of  atoms 
of  another  kind.  Nevertheless,  we  can  just  as  little  assume  to  ex- 
plain away  the  fact  of  such  obvious  causal  connection,  because  we 
cannot  bring  the  measure  of  the  connection  under  the  same  law  as 
that  which  maintains  itself  among  certain  modes  of  physical  mo- 
tion. 

§  17.  It  would  scarcely  be  worth  while  to  consider  seriously 
either  the  older  or  the  more  modern  forms  of  the  denial  that  any 
real  causal  relation  exists  between  body  and  mind,  were  it  not  for 
the  fact  of  their  essential  agreement  upon  a  false  conception  of  the 
nature  of  causal  relations  in  general,  and  upon  a  false  theory  as  to 
the  particular  case  of  the  correlations  of  body  and  mind.  Two  re- 
marks bearing  upon  all  such  theories  are,  therefore,  necessary.  The 
assumption  that  matter  and  mind  are  separated  from  each  other 
"by  the  whole  diameter  of  being,"  if  it  be  held  to  mean  that  the 
two  forms  of  being  are  so  disparate  in  nature  as  to  be  unable  to  act 
on  each  other,  is  an  unverifiable  assumption.  It  even  goes  squarely 


646       GENERAL  RELATIONS  OF  BODY  AND  MIND 

in  the  face  of  many  of  the  most  important  psycho-physical  facts. 
We  know  nothing  about  what  kind  of  beings  can  or  cannot  act  on 
each  other,  except  through  our  experience  of  what  beings  do  actually 
act  upon  each  other.  The  mystery  involved  in  any  one  being  act- 
ing on  any  other  is  equally  deep  and  unfathomable^  in  whatever  di- 
rection we  attempt  to  explore  it.  Before  experience  with  the  facts, 
we  should  be  quite  at  a  loss  to  tell  whether  atoms  of  oxygen  could  act 
on  atoms  of  hydrogen,  under  the  laws  of  chemical  affinity,  or  not; 
whether  molecules  of  iron  could  act  on  other  molecules  of  iron, 
under  the  laws  of  cohesion,  or  not,  etc.  How  it  is  that  material  masses 
or  molecules  can  "influence"  each  other,  or  what  is  the  real  nature 
of  the  force  which  binds  them  together,  physical  science  is  quite 
unable  to  say.  So  that  even  if  we  were  entitled  to  regard  matter 
as  somewhat,  the  very  essence  of  which  it  is  to  be  spread  out,  and 
mind  as  somewhat,  the  very  essence  of  which  it  is  to  be  conscious 
and  not  to  be  spread  out,  we  should  still  be  quite  without  justification 
in  asserting  (a  priori,  as  it  were)  that  one  cannot  act  upon  the  other. 
But — just  the  contrary — if  we  are  to  accept,  unbiassed,  the  obvious 
witness  of  the  facts,  we  are  compelled  to  affirm:  The  phenomena  of 
mind  and  the  conditions  of  the  brain  are  related  so  constantly  and 
immediately  under  law,  that  we  are  warranted  in  believing  in  the 
action  of  each  upon  the  other. 

§  18.  But,  in  the  second  place,  most  of  the  modern  objections 
to  speaking  of  causal  relations  existing  between  the  nervous  mechan- 
ism and  the  processes  of  conscious  mental  life,  are  connected  with 
the  current  theory  of  the  conservation  and  correlation  of  physical 
energy.  And  here  it  is  interesting  to  notice  certain  relations,  both 
of  similarity  and  of  difference,  between  a  prominent  modern  theory 
as  to  the  mutual  action  of  mind  and  brain  and  the  now-abandoned 
views  of  Occasionalism  and  Pre-established  Harmony.  These  views 
had  regard  to  the  reality  and  integrity  of  the  soul,  and  respect  for 
the  ethical  character  of  the  Divine  relations  toward  its  activities  and 
development.  Modern  science,  on  the  contrary,  raises  most  of  its 
objections,  against  regarding  the  conditions  of  the  central  nervous 
system  and  the  states  of  consciousness  as  connected  by  a  real  causal 
tie,  out  of  a  profound  regard  for  matter  and  the  laws  of  physics. 
The  great  value  and  significance  of  physical  phenomena,  and  the 
regular  modes  of  their  recurrence,  if  not  the  independent  and  eternal 
existence  of  material  beings,  are  taken  for  granted  by  this  theory, 
whatever  difficulties,  fears,  or  hopes  to  the  contrary  may  arise  from 
the  sphere  of  mind.  Elements  of  material  reality  (called  "atoms") 
are  assumed  to  exist;  the  universal  form  of  their  relation  is  held  to 
be  the  law  of  the  conservation  and  correlation  of  energy.  By  "  en- 
ergy" we  are  to  understand  that  which  moves  or  tends  to  move  the 


ANALYSIS  OF  CONCEPTIONS  647 

elementary  atoms,  or  their  aggregations  into  molecules  and  masses. 
The  energy  which  is  regarded  as  causing  actual  motion  is  kinetic; 
that  which  is  to  be  regarded  as  tending  to  produce  motion  is  stored 
or  potential.  But  inasmuch  as  we  have  no  test  or  suggestion  of  the 
presence  of  energy  except  motion,  we  seem  compelled  to  consider 
the  so-called  "tendency"  to  move  (potential  energy)  as  motion  that 
is  beyond  the  sphere  of  the  senses,  because  distributed  over  so  vast 
a  number  of  minute  portions  of  matter  whose  amount  of  motion 
is  too  small  to  be  discoverable.  All  physical  elements  and  masses 
are,  accordingly,  always  in  motion,  and  the  total  quantum  of  this 
motion  is  invariable  throughout  the  entire  universe.  All  forms  of 
energy  must  be  classified,  as  respects  quality,  by  the  kind  of  their 
motion;  and  as  respects  degree,  by  the  amount  of  their  motion. 

§  19.  It  requires,  however,  only  a  brief  and  rather  superficial 
examination  of  the  conceptions  of  physical  science,  as  they  are  pop- 
ularly taken,  and  even  as  they  are  correctly  and  helpfully  employed 
for  purposes  of  scientific  research,  to  discover  how  full  they  are  of 
concealed  figures  of  speech  and  of  as  yet  unverifiable  hypotheses. 
The  known  facts  are  of  about  the  following  order.  Observed  events 
in  the  elements  and  masses  of  the  physical  world  make  upon  our 
minds  impressions  of  more  or  less  magnitude  and  intensity.  Our 
experience,  especially  with  respect  to  the  correlations  between  our 
own  mental  strivings,  feelings  of  effort  or  so-called  "  deeds  of  will," 
and  the  sequent  changes  in  the  bodily  organism,  and  through  this 
organism  in  material  objects,  begets  and  fosters  the  conceptions  of 
"power,"  "force,"  "energy,"  etc.  This  power,  force,  energy,  is 
conceived  of  as  residing  in  "these  objects,"  and  as  "passing  over" 
from  one  to  another  of  them;  and  so  as  being  the  cause  of  the  inter- 
dependent, or  mutually  related,  changes.  But  the  causal  concep- 
tion is  a  distinctly  metaphysical  conception.  For  this  vague  and 
indeterminate  idea  of  cause,  therefore,  the  physico-chemical  sciences 
very  properly  strive  to  substitute  mathematical  formulas  which  are 
designed  to  express  definite  relations  in  the  amounts  as  measured  by 
units  of  time  and  space,  of  these  changes.  Thus  arises  the  theory 
of  work,  actually  doing,  or  that  may  be  expected  to  be  done,  by  ma- 
terial masses  and  their  elements.  Between  certain  kinds  of  changes, 
either  directly  observed  or  capable  of  an  approximately  correct  cal- 
culation, science  has  already  established  formulas  which  serve  the 
purposes  both  of  explanation  and  of  prediction.  Such  are  the  for- 
mulas which  give  the  equivalents  of  the  kinetic  energy  of  masses  in 
terms  of  the  energy  called  heat,  or  light,  or  electricity.  Further,  as 
a  result  of  their  experience,  in  general,  with  the  quantitative  correla- 
tions existing  between  the  different  material  objects,  the  physico- 
chemical  sciences  have  evolved  an  hypothesis  of  a  more  universal 


648       GENERAL  RELATIONS  OF  BODY  AND  MIND 

character.  This  is  the  hypothesis  of  the  conservation  of  energy. 
It  holds  that  any  system  of  material  elements  or  masses,  so  long  as 
it  can  be  kept  "  closed,"  or  protected  from  dissipating  its  energy,  and 
from  receiving  accessions  of  energy  from  the  outside,  will  do  only  a 
fixed  amount  of  work; — although  this  gross  sum  of  energy  may  mani- 
fest itself  in  any  of  the  various  forms  known  to  physical  science,  and 
may  be  distributed  and  redistributed  among  the  different  elements 
and  masses  of  the  system.  Moreover,  in  the  process  of  distribu- 
tion and  redistribution,  the  equivalents  of  the  different  kinds  of 
energy  will  remain  the  same. 

This  hypothesis  of  the  conservation  of  energy  is  still  undergoing 
a  process  of  experimental  testing  at  the  hands  of  the  students  of  the 
physico-chemical  sciences.  Only  in  the  case  of  certain  pairs  of  the 
various  recognized  kinds  of  so-called  energy  have  any  definite,  ap- 
proximately exact  formulas  for  their  correlations  been  as  yet  dis- 
covered. In  the  world  of  man's  experience  there  is  no  such  thing 
as  a  "  closed  system."  And  wonderful  new  forms  of  energy,  and  of 
the  doing,  and  the  potentiality  of  work,  on  the  part  of  the  physical 
universe,  are  constantly  being  discovered.  But  slowly,  and  on  the 
whole  in  a  satisfactory  way,  some  hypothesis  corresponding  in  general 
to  that  called  by  the  title — "The  Conservation  and  Correlation  of 
Energy" — seems  to  be  gaining  support  by  the  sum-total  of  human 
experience  with  material  things. 

§  20.  How  far,  now,  can  such  an  hypothesis  as  the  foregoing  be 
applied  to  the  nervous  system  of  man  ?  That  this  system  is  a  mech- 
anism and  like  every  other  mechanism  dependent  for  the  amount 
of  work  which  it  can  do,  upon  resources  of  energy,  either  stored 
within  itself  or  derived  by  accessions  from  outside  itself,  there  can 
be  no  manner  of  doubt.  All  the  various  kinds  of  automatic  and 
reflex,  or  sensory-motor  activity  illustrate  and  enforce  this  truth. 
In  these  activities  the  various  kinds  of  energy  recognized  by  physi- 
cal science  as  kinetic  energy  of  masses,  quantity  of  heat,  electrical 
energy,  and  chemical  energy,  take  part.  And  so  far  as  the  laws  of 
their  conservation  within  the  nervous  system  are  known  they  ap- 
pear to  confirm  the  general  hypothesis  in  question.  It  is  true,  how- 
ever, that  the  work  done  by  the  nervous  mechanism  cannot  as  yet 
be  fully  explained — not  to  say,  even  correctly  stated — in  terms  of 
other  forms  of  energy.  And  thus  far,  the  need  of  assuming  a  very 
special  and,  indeed,  unique  form  of  energy,  to  be  called  "  nervous," 
does  not  seem  to  be  in  the  least  degree  diminished.  This  is  only 
to  say:  What  the  nervous  mechanism  does,  considered  as  having 
the  power  to  store,  to  transmit,  and  to  distribute,  its  own  resources 
for  doing  its  own  work,  cannot  as  yet  be  explained  in  terms  that  are 
mere  modifications  or  combinations  of  the  formulas  derived  from  a 


CONSERVATION  OF  CEREBRAL  ENERGY  649 

study  of  other  forms  of  energy,  employed  in  other  kinds  of  work. 
But  this  fact  by  no  means  disproves  the  assumption  that  the  general 
principle  implied  in  the  phrase  we  are  discussing  applies  somehow 
to  the  peculiar  functioning  of  the  nervous  mechanism. 

§  21.  So,  too,  if  we  consider  the  external  relations  of  man's  ner- 
vous system,  we  find  it  dependent  on  energies  outside  of  itself  for 
the  amounts,  whether  stored  or  kinetic,  of  its  own  peculiar  way  of 
doing  work.  This  dependence  is  of  two  principal  kinds.  The 
nervous  system  must  have  nutriment;  otherwise  it  cannot  build 
itself  up;  cannot  store  energy;  cannot  acquire  the  material  to  be 
transmuted  into  its  own  form  of  work.  In  its  metabolic  processes, 
therefore,  it  is  constantly  illustrating  a  species  of  the  law  of  the  con- 
servation of  energy.  It  is  true  that  we  cannot  at  present  give  the 
exact  equivalents,  in  the  potentiality  or  kinetic  energy  of  the  nervous 
mechanism,  for  the  different  food  products;  whether  they  are  con- 
sidered apart  from  the  whole  organism,  or  as  carried  to  the  nervous 
system  in  the  blood.  The  problem  of  food-supply  as  related  to 
neural  energy  is  immensely  complicated — not  only  as  a  chemical 
and  physiological  study,  but  also  as  having  a  large  mixture  of  ob- 
scure or  totally  unknown  psycho-physical  and  mental  factors.  It 
is  conceivable,  however,  that  mathematical  formulas  may  at  some 
time  be  discovered  for  stating  the  quantitative  relations  between  the 
energies  stored  in  the  blood  supplied  and  the  energies,  stored  or 
kinetic,  of  the  nervous  mechanism — including  its  own  peculiar 
form  of  so-called  "nervous  energy." 

The  second  form  of  the  more  obvious  correlations  between  ex- 
ternal forms  of  energy  and  the  energy  of  the  nervous  system  is 
realized  every  time  any  appropriate  stimulus  is  applied  to  the  end- 
organs  of  this  system.  How  far  we  know  of  formulas  that  state  in 
mathematical  terms  this  kind  of  correlation  has  already  been  suf- 
ficiently discussed.  Indeed,  the  whole  science  of  psycho-physics 
involves  the  attempt  to  deal  with  these  correlations. 

Let  us  suppose,  then,  that  all  the  functions  of  the  nervous  mechan- 
ism, and  all  their  relations  to  the  various  forms  of  energy  which  act 
upon  it,  and  upon  which  it  reacts,  have  become  so  well  known  as  to 
form  a  theory  of  the  conservation  of  energy  completely  adapted  for 
application  to  this  mechanism.  We  should  then  be  able  to  explain 
in  terms  of  quantity  the  successive  redistributions  of  work  actually 
done:  between  the  different  parts  of  the  nervous  system  of  man;  be- 
tween it  as  a  whole  and  the  other  organs  of  the  body;  and  between 
it  and  the  physical  world  as  it  affects  the  body.  But  the  explanation 
WTmld  be  couched  in  terms  of  quantity  only.  The  mystery  of  the 
quality,  or  kind,  of  the  energizing  of  the  nervous  mechanism  might 
remain  as  great  as  before.  And  the  mystery  of  its  correlations  with 


650       GENERAL  RELATIONS  OF  BODY  AND  MIND 

the  phenomena  of  the  mental  life  would  not  be  changed,  or  its  pro- 
spective better  solution  even  approximated.  For  the  popular  con- 
ception of  energy  as  a  sort  of  entity  that  can  actually  be  split  up, 
and  distributed,  or  passed  over,  is,  of  course,  especially  absurd  when 
considered  as  applied  to  the  relations  of  the  body  and  the  mind. 
That  varying  amounts  of  the  activity  of  the  central  nervous  system 
are  correlated  with  somewhat  comparable  changes  in  the  intensity 
of  our  consciously  felt  experiences,  is  a  statement  sufficiently  war- 
ranted by  a  numerous  class  of  observed  facts  and  of  reasonable 
inferences  from  facts.  But  all  this  scarcely  touches  the  point  at 
issue. 

§  22.  The  entire  conception  of  the  conservation  and  correlation 
of  energy,  as  it  is  made  fruitful  use  of  in  the  physical  and  chemical 
sciences,  and  even  as  it  hopes  to  be  introduced  into  the  biological 
sciences,  is,  therefore,  essentially  and  utterly  inappropriate  to  ex- 
press those  peculiar  relations  which  are  matters  of  experience  be- 
tween the  body  and  the  mind.  This  is  as  true  of  the  brain,  with  all 
its  complicated  structure  and  subtile  forms  of  functioning,  as  it  is 
of  any  of  the  more  massive  and  grosser  forms  of  the  bodily  organ- 
ism. The  so-called  "  mind"  is  not  the  kind  of  an  existence  in  which 
physical  energy  can  be  stored,  or  to  which  it  can  be  transferred,  or 
from  which  it  can  be  derived.  The  utmost  that  could  possibly  be 
claimed  on  the  valid  ground  of  experience,  would  be  statable  only 
in  terms  somewhat  like  the  following:  The  brain,  considered  as  that 
part  of  the  nervous  mechanism  which  has  the  most  intimate  and 
important  relations  with  the  phenomena  of  conscious  mental  life, 
is  itself  subject  to  the  principle  of  the  conservation  of  physical  energy. 
The  felt  intensity  of  some  of  the  mind's  experiences  varies  in  some 
sort  of  dependence  upon  the  amounts  of  the  energy  expended  by  the 
brain ;  and,  possibly,  upon  the  particular  portions  of  the  brain  in  which 
these  changes  of  stored  into  kinetic  energy  take  place.  But  if  this 
relation  is  to  be  looked  upon  as  causal — and  we  hold  that  it  is,  in  a 
defensible  and  rational  meaning  of  the  word  "  causal " — it  is  a  quite 
different  kind  of  causal  relation  from  that  which  maintains  itself 
between  material  objects.  And,  in  truth,  even  for  the  latter  kind  of 
relation,  all  that  the  principle  of  the  conservation  of  energy  at  its 
best  can  do,  is  to  give  formulas  applicable  to  the  quantitative  as- 
pect of  physical  changes  only.  But  this  kind  of  quantitative  formu- 
las, and  its  mathematics,  have  no  valid  applications  to  the  phenomena 
of  consciousness,  as  such. 

§  23.  To  go  over  the  ground  again  in  a  somewhat  different  way: 
On  attempting  to  account  for  the  whole  world  of  phenomena  in 
terms  of  motion,  kinetic  or  potential,  under  the  law  of  the  conser- 
vation of  energy,  we  are  met  with  insuperable  difficulty  as  soon  as 


CONSERVATION  OF  CEREBRAL  ENERGY     651 

we  enter  the  domain  of  consciousness.  States  of  consciousness 
are  not  modes  of  motion.  If  they  were,  the  general  theory  of  physics 
would  compel  us  at  once  to  attempt  a  strict  mathematical  correla- 
tion between  physical  and  mental  phenomena.  Just  as  the  kinetic 
energy  of  masses  can  be  expressed,  with  a  tolerable  approximation 
to  exactness,  in  terms  of  heat  as  a  mode  of  motion,  so  would  some 
formula  be  conceivable  for  indicating  what  amount  of  chemical 
changes,  or  nerve-commotion,  in  the  matter  of  the  brain,  is  the  mathe- 
matical equivalent  of  the  conception  of  home,  of  the  sense  of  obliga- 
tion, or  of  the  idea  of  God.  In  other  words,  it  seems  impossible  to 
regard  any  amount  of  physical  energy  as  abstracted  from  the  brain, 
so  to  speak,  and  expended  or  stored  up  in  consciousness.  Energy 
is  stored  by  the  process  of  nutrition  in  the  nervous  elements  of  the 
brain;  it  becomes  kinetic  in  connection  with  the  phenomena  of  con- 
sciousness. But  between  the  mind,  whether  regarded  as  merely 
the  formal  subject  of  consciousness  or  as  a  real  unit-being  whose 
faculty  or  power  it  is  to  be  conscious,  and  the  physical  basis  of  mind 
in  the  brain,  no  correlation,  in  the  sense  of  a  passing  back  and  forth 
of  physical  energy,  can  occur. 

The  entire  circuit  of  the  transmission  and  distribution  of  energy 
is,  therefore,  complete  within  the  brain  itself.  Not  a  single  atom  enters 
its  substance  that  does  not  come  forth  unchanged,  with  all  its  forces 
inherent  in  it.  No  atom  is  transferred  from  brain  to  mind,  as  all 
the  atoms  are  transferred  from  the  blood  to  the  nervous  substance 
of  the  brain.  Not  the  most  infinitesimal  amount  of  energy  exists, 
stored  in  the  constitution  of  the  molecules  of  this  substance,  which 
is  not  either  used  up  there  or  returned  to  external  nature  in  connec- 
tion with  the  constitution  of  the  molecules  separated  from  this  sub- 
stance. The  stricter  we  make  our  application  of  the  law  of  the 
conservation  of  energy  within  the  physical  realm,  the  more  impos- 
sible does  it  become  to  apply  it  at  all  to  the  relations  of  body  and 
mind. 

It  is  not  surprising  that,  in  the  estimate  of  one  who  is  unaccus- 
tomed to  regard  with  favor  any  explanation  of  phenomena  which 
does  not  come  under  the  most  general  law  of  all  physics,  the  case 
of  the  mind  and  the  brain  should  seem  to  demand  the  most  ex- 
traordinary treatment.  In  any  event,  the  facts  of  consciousness,  as 
facts,  cannot  be  denied.  Whether  we  can  explain  them  or  not,  they 
are  equally  plain  and  persistent.  Men  perceive,  and  imagine,  and 
remember,  and  reason,  and  believe  in  the  invisible,  and  choose,  etc. 
All  this  they  do,  as  possessed  of  a  body — and,  particularly,  of  a  ner- 
vous mechanism,  the  activities  of  whose  central  portion  are  related 
in  some  special  and  unique  way  with  the  doing  of  all  this.  And  yet 
sure,  beyond  doubt — it  is  argued — is  the  existence  of  the  atom,  with 


652       GENERAL  RELATIONS  OF  BODY  AND  MIND 

its  host  of  inherent  energies;  and  supreme  is  the  law  of  the  conserva- 
tion and  correlation  of  these  energies  regarded  as  modifications  of 
one  fundamental  form.  But  here  the  whole  conception  becomes  a 
figure  of  speech  so  vague  and  evanescent  that  it  loses  all  definable 
meaning  when  the  attempt  is  made  to  render  it  into  terms  of  actual, 
matter-of-fact  experience. 

§  24.  When  freed,  then,  from  a  physical  hypothesis  which,  of  its 
very  nature,  has  no  valid  application  to  the  subject  in  hand,  the  con- 
ception of  causal  influence,  and  of  reciprocally  dependent  relations 
under  terms  of  this  influence,  between  the  nervous  mechanism  and 
the  phenomena  of  mental  life,  meets  with  no  objection  to  itself  in 
a  scientific  study  of  the  facts.  In  a  word,  we  may  affirm  anew  that 
kind  of  dualism,  which  holds  that  some,  at  least,  of  our  conscious 
states  are  caused  to  be  such  and  no  other,  by  the  antecedent  or  accom- 
panying functions  of  the  nervous  system;  and,  on  the  other  hand, 
that  some,  at  least,  of  the  changes  which  occur  in  the  bodily  states  are 
caused  through  the  nervous  system,  by  the  antecedent  or  accompany- 
ing functions  of  the  mental  life.  This  is — as  has  already  been  re- 
peatedly said — the  popular  view.  Science  has  nothing  to  adduce 
against  it;  but  much  to  confirm  it.  So  far  may  we  carry  the  theory 
of  the  natural  correlations  between  body  and  mind,  by  a  direct  ap- 
peal to  the  experienced  facts. 

§  25.  But  the  facts  also  compel  us  to  admit  that  neither  one  of  these 
forms  of  functioning  wholly  explains  the  other.  What  goes  on  in 
the  nervous  mechanism  does  not  completely  account  for  what  goes 
on  in  consciousness;  what  goes  on  in  consciousness  does  not  com- 
pletely account  for  the  activities  in  the  nervous  mechanism.  In- 
deed, the  very  nature  of  the  two  series  of  occurrences  is  as  unlike 
as  it  is  conceivable  that  different  classes  of  phenomena  should  be. 
The  behavior  of  radium  under  the  influence  of  radio-active  energy, 
is  not  so  unlike  that  of  the  star  Sirius  under  the  influence  of  the  force 
of  gravity,  as  is  the  simplest  conscious  sensation  from  the  nerve-com- 
motion which  caused  it.  Moreover,  the  brain  has  always  something 
to  say  as  to  what  it  will  do,  when  under  the  most  determined  and 
strongest  influence  from  the  mind.  And  the  mind  has  always 
something  to  -say  as  to  what  its  own  behavior  shall  be,  even  when 
under  the  strongest  of  influences  from  the  brain.  There  is,  however, 
nothing  unique  about  all  this,  so  far  as  the  propriety  of  any  applica- 
tion of  the  conception  of  a  causal  relation  is  concerned.  For  there 
is  no  material  thing  so  mean  that  it  does  not  have  a  "nature"  of  its 
own;  and  that  nature  determines,  in  the  most  powerful  and  prac- 
tical way,  what  it  shall  be  caused  to  do  by  its  entering  into  all  sorts 
of  relations  with  other  things.  And  there  is  no  psychic  existence 
which  has  not  a  similar  claim  to  a  nature  of  its  own. 


THE  THREE  PROCESSES  INVOLVED  653 

In  what  sense  we  may  assert  any  reality  or  unity  for  the  subject 
of  the  mental  life  is  not  the  question  at  the  present  time.  But  all 
our  scientific  research,  as  directed  toward  this  particular  kind  of 
correlations,  assumes  that,  in  some  valid  meaning  of  the  word,  there 
is  a  being  to  be  called  the  Mind.  On  the  other  hand,  no  one  can  cross 
the  threshold  of  the  science  of  physiological  psychology,  or  psycho- 
physics,  without  assuming  the  existence  of  the  body;  and  of  the  ner- 
vous system  and  brain,  as  portions  of  this  body.  The  two  existences, 
body  and  mind,  may  not  be  identified  by  the  science  which  investi- 
gates their  correlations;  for  they  have  markedly  different  ways  of 
functioning;  the  phenomena  which  research  both  assumes  and  dis- 
covers, and  so  assigns  to  them,  are  characteristically  unlike.  They 
are,  however,  dependently  connected.  Each  stands  in  causal  re- 
lations to  the  other;  although  this  dependence  appears  to  be  by 
no  means  complete.  Body  and  Mind,  changes  in  the  nervous  proc- 
esses and  the  changing  processes  of  the  conscious  mental  life,  can- 
not be  identified.  Neither  can  be  stated  with  scientific  precision, 
or  in  any  other  than  a  convenient  figure  of  speech,  in  terms  of  the 
other. 

§  26.  In  the  more  particular  description  of  the  connection  between 
the  mind  and  the  brain,  it  may  be  said  that  all  intercourse  between 
material  objects  and  the  subject  of  consciousness  involves  three 
processes — a  physical,  a  physiological,  and  a  psychical.  In  these 
processes  the  perceived  object  and  the  perceiving  subject  mutually 
condition  each  other.  This  fact,  however,  does  not  destroy  the  ne- 
cessity, under  which  all  scientific  investigation  finds  itself,  of  assum- 
ing that  both  object  and  subject  exist  as  real  beings.  The  physical 
process  consists  in  the  action  of  the  appropriate  modes  of  physical 
energy  upon  the  nervous  end-apparatus  of  sense.  The  bringing  of 
such  modes  of  energy  to  bear  upon  the  apparatus  is  accomplished 
through  mechanical  contrivances — such  as  the  means  for  forming 
an  image  on  the  retina  in  the  eye,  and  for  conveying  the  modified 
acoustic  impulses  to  the  organ  of  Corti  in  the  ear. 

The  second  process  consists  in  transmuting  the  physical  ener- 
gies, in  part  at  least,  into  a  physiological  process,  a  nerve-commo- 
tion within  the  nervous  system;  and  in  propagating  such  nerve- 
commotion  along  the  proper  tracts  and  diffusing  it  over  the  various 
areas  of  this  system.  Inasmuch  as  the  physiological  process  is 
also  a  physical  process — that  is,  a  mode  of  the  motion  of  material 
molecules,  accompanied  by  chemical  and  electrical  and  other 
changes — it  must  be  conceived  of  as  standing  in  certain  relations 
of  quality  and  quantity  to  the  first,  or  more  distinctively  physical, 
process.  That  the  law  of  the  conservation  of  energy,  as  formulated 
for  much  simpler  cases  of  the  relations  of  forces  between  inorganic 


654       GENERAL  RELATIONS  OF  BODY  AND  MIND 

bodies,  applies  to  the  relations  of  the  nervous  system  and  its  stimuli 
or  within  the  different  parts  of  the  nervous  system  itself,  we  are  not 
yet  able  to  affirm  with  confidence.     But  we  have  valid  grounds  for 
the  belief  that  something  of  the  sort  is  true. 

The  third  process  is  psychical;  it  is  a  process  which  is  a  psychi- 
cal event,  a  forth-putting  of  the  energy  of  mind.  It  is  directly 
correlated  with  the  physiological  process  only  when  the  latter  has 
been  realized  in  certain  cerebral  areas.  It  is  not  to  be  explained 
as  a  resultant  of  the  cerebral  physiological  process,  but  as  an  ac- 
tion of  the  mind  which  is  conditioned  upon  that  process.  So,  also, 
are  we  entitled  to  say  that,  when  certain  psychical  processes,  by 
way  of  feeling,  ideation,  and  volition,  take  place,  then,  and  as  con- 
ditioned upon  these  processes,  certain  corresponding  physiological 
processes  occur  in  the  brain;  the  physiological  processes,  being 
propagated  from  the  central  nervous  system,  end  in  physical  proc- 
esses returning  energy  to  the  world  outside  of  the  body. 

When  the  mental  process  is  a  perception  of  some  object,  called 
an  "external"  object,  it  is  no  less  truly  a  psychical  process.  The 
mind  creates  its  own  objects;  presents  itself  with  its  own  presen- 
tations of  sense;  acts  to  bring  forth  that  which  it  knows  as  not 
itself.  But  it  does  all  this  as  dependent  upon  the  processes  which 
take  place  outside  of  itself,  and  with  the  assumption  of  extra-men- 
tal realities  as  existing,  to  which  it  stands  in  the  relation  of  cause 
and  effect. 

Finally,  then,  the  assumption  that  the  mind  is  a  real  being,  which 
can  be  acted  upon  by  the  brain,  and  which  can  act  on  the  body  through 
the  brain,  is  the  only  one  compatible  with  all  the  facts  of  experience. 
There  is  nothing  which  we  know  about  the  nature  of  material  be- 
ings and  the  laws  of  their  relation  to  each  other,  or  about  the  nature 
of  spiritual  beings  and  their  possible  relation  to  material  beings,  or 
about  the  nature  of  causal  efficiency  whether  in  the  form  of  so-called 
physical  energy  or  in  that  of  activity  in  consciousness,  which  for- 
bids the  aforesaid  assumption.  And  no  other  assumption,  sub- 
stantially different  from  this,  is  compatible  with  the  facts  of  experi- 
ence. 

§  27.  A  distinctive  feature  of  modern  science  is  its  endeavor  to 
satisfy  inquiry  into  the  nature  of  the  objects  of  its  investigation  by 
a  detailed  description  of  their  development.  In  answer  to  the  in- 
quiry what  a  thing  is,  we  are  invited  to  listen  to  an  account  of  how 
it  became  what  it  is.  The  history  of  the  egg  explains  the  bird  even 
more  than  the  nature  of  the  bird  explains  the  egg.  Indeed  the 
universal  process  of  "Becoming"  has  been  almost  personified  and 
deified  so  as  to  make  it  the  true  ground  of  all  finite  and  concrete 
existences.  There  can  be  no  doubt  as  to  the  great  fruitfulness  and 


THE  CONCEPTION  OF  DEVELOPMENT  655 

value  of  this  historical  and  genetic  way  of  studying  everything.  Let 
us,  then,  now  consider  the  most  general  relations,  or  correlations,  of 
body  and  mind,  from  the  point  of  view  of  their  dependent  develop- 
ment. 

Undoubtedly,  both  of  the  two  subjects,  with  whose  correlations 
Physiological  Psychology  deals,  require  for  their  most  satisfactory 
understanding  to  be  studied  by  the  genetic  method.  The  structure 
of  the  nervous  system,  as  we  saw  in  the  earliest  chapters  of  this 
treatise,  appears  in  a  new  light  wrhen  regarded  as  the  result  of  a 
process  of  evolution.  Beginning  with  the  impregnated  ovum, 
by  propagation  of  cells  of  living  protoplasm,  by  segmentation  of 
larger  sections  of  these  cells,  by  proliferation  of  cells  and  separation 
into  layers,  the  one  portion  of  the  germ  from  which  the  mechanism 
of  nerve-fibres  and  nerve-cells  is  to  unfold  itself  becomes  differenti- 
ated from  the  other  portions.  By  vital  processes  kept  up  through  nu- 
trition and  resulting  in  the  growth  of  some  areas  beyond  others,  and 
by  mechanical  influences  at  work  to  crowd  forward  here  or  push 
back  there,  to  fold  and  tuck  and  cause  to  dip  or  curve,  etc.,  this 
epiblastic  portion  develops  the  system  of  end-organs,  central  or- 
gans, and  connecting  tracts  of  nerves. 

Psychology,  also,  has  felt  strongly  the  same  impulse.  It  has 
been  forced  to  confess  that  its  real  task  is  but  begun  when  it  has, 
by  introspection,  examined  and  classified  the  phenomena  of  adult 
conscious  life.  All  the  mental  phenomena  undoubtedly  have,  as 
respects  their  genesis  and  order,  a  truly  vital  connection.  Those  of 
the  present  have  their  roots  in  those  of  the  past.  The  so-called 
faculties  of  the  mind  are  neither  hard  and  fixed  lines  drawn  to  ex- 
clude from  internal  relation  the  various  modes  of  its  behavior  in 
consciousness,  nor  are  they  kinds  of  activities  that  spring  up,  full- 
formed  at  once,  at  different  intervals  in  its  entire  history.  Percep- 
tion itself  is  a  result  of  development;  for  "  things"  are  not  ready-made 
products  existing,  as  they  appear,  outside  of  the  mind,  but  resultants 
of  mental  activities  that  have  to  be  performed  anew  so  often  as  the 
things  appear.  It  is  in  the  evolution  of  the  mind  that  we  find  our 
means  for  understanding  its  true  nature.  Moreover,  the  character- 
istics which  distinguish  one  mind  from  another  are  to  be  under- 
stood as  largely  resulting  from  the  order  and  relative  prominence 
of  different  activities  in  the  development  of  each. 

So  far  as  the  connection  of  mental  phenomena  with  the  increasing 
complexity  of  the  nervous  activities,  and  with  the  stored  energies 
and  hardening  habitus  of  the  nervous  elements,  affords  any  explana- 
tion of  the  development  of  the  mind,  we  have  already  said  all  that 
is  necessary.  The  growth  of  the  mental  life  in  the  acquirement 
and  arrangement  of  sensations,  in  the  recalling  of  ideas,  in  the  form- 


656       GENERAL  RELATIONS  OF  BODY  AND  MIND 

ing  of  judgments  about  objects  of  sense,  etc.,  is  plainly  dependent 
upon  the  evolution  of  the  bodily  members.  But  the  real  nature 
of  the  relation  which  exists  between  the  mental  phenomena  and 
the  nervous  mechanism,  so  far  as  this  can  be  learned  by  studying 
the  development  of  both,  furnishes  us  with  another  question.  Upon 
this  question,  also,  the  same  conflict  of  view  as  that  to  which  we  have 
already  drawn  attention  may  arise. 

§  28.  There  can  be  no  doubt  of  a  general  correspondence  be- 
tween the  two  developments,  of  the  body  and  of  the  mind.  Nervous 
system  and  mental  condition  are  both  immature  in  infancy;  both  de- 
velop with  great  rapidity  in  early  childhood,  and  then  more  slowly  on 
into  adult  life;  both — it  is  claimed — remain  comparatively  stationary 
through  the  period  of  man's  highest  maturity;  and  as  old  age  ad- 
vances, both,  in  some  respects  at  least,  customarily  keep  pace  in 
their  decline.  Moreover,  cases  of  arrested  development  of  brain  are 
cases  of  arrested  development  of  mental  capacity.  Idiots  are  fre- 
quently microcephalic;  many  of  them  have  brains  weighing  less 
than  thirty  ounces.  Degeneracy  of  the  tissues  of  the  cerebral  hemis- 
pheres is  commonly  connected  with  increasing  degeneracy  of  the 
mind.  As  the  tides  of  molecular  nerve-commotion  rise  and  fall  in 
the  nervous  mass,  so  rise  and  fall  the  tides  of  mental  vigor. 

On  the  other  hand,  attempts  to  account  for  the  orderly  increase 
in  complexity  and  comprehensiveness  of  all  the  mental  phenomena 
by  tracing  the  physical  evolution  of  the  brain  are  wholly  unsatis- 
factory to  many  minds.  We  have  no  hesitation  in  classing  ourselves 
among  this  number.  That  something  more  than  an  absolutely  de- 
pendent and  physically  conditioned  development  is  implied  in  the 
history  of  each  individual  mind  may  be  argued  on  two  principal 
grounds.  In  the  first  place,  it  may  be  shown  that  the  stages  and  laws 
of  mental  development  do  not  fully  correspond  to  those  which  are  ob- 
served on  tracing  the  evolution  of  the  nervous  system.  It  may  also  be 
shown  that  certain  elements  necessarily  enter  into  the  development 
of  mind,  which  have  nothing  like  them,  or  strictly  correlated  with 
them,  in  the  evolution  of  the  material  mechanism.  Any  being  may 
be  dependent  on  other  beings  for  its  starting,  as  it  were,  and  for  cer- 
tain factors  that  enter  into  its  growth  or  furnish  the  indispensable 
conditions  of  its  growth ;  and  yet  this  fact  gives  us  no  right  whatever 
to  refuse  to  such  a  being  all  title  to  take  rank  among  other  real  ex- 
istences as  having  a  complex  nature  of  its  own.  No  existence  loses 
or  impairs  its  claim  to  reality  by  being  dependent  on  other  existences 
for  its  development.  The  mind,  on  the  contrary,  most  indubitably 
establishes  such  a  claim,  because  the  stages  and  laws  of  its  unfold- 
ing, and  some  of  the  factors  which  necessarily  enter  into  this  unfold- 
ing, are  peculiar  to  itself  (sui  generis). 


THE  CONCEPTION  OF  DEVELOPMENT  657 

§  29.  That  the  words,  "development  of  the  mind,"  stand  for  some- 
thing real  and  verifiable,  there  can  be  no  reasonable  doubt.  The 
sum-total  of  the  conscious  experience  of  each  individual  is  far  more 
than  a  mere  series  of  states  of  consciousness.  No  difference  in  de- 
grees under  the  same  kind  can  be  conceived  of  which  is  greater 
than  the  difference  between  the  most  mature  and  highly  developed 
mental  performances  and  those  inconceivably  simple  activities  with 
which  the  mental  life  begins.  So  far  as  the  character  of  the  phe- 
nomena of  consciousness  is  concerned,  the  mind  of  the  adult  New- 
ton or  Kant  is  much  farther  removed  from  the  mind  of  the  infant 
Newton  or  Kant  than  the  latter  is  from  the  mind  of  one  of  the 
lower  animals.  There  is  no  doubt,  also,  that  the  incomparable  im- 
provement of  the  mental  processes  which  distinguishes  the  adult 
from  the  infantile  human  being  is  a  true  development.  Each  stage 
of  this  improvement  is  dependent  upon  preceding  stages.  The 
changes  are  all  in  some  sort  according  to  a  plan.  Thus  the  life  of 
every  individual's  mental  experiences  is  capable  of  being  made  into 
a  history.  A  certain  tolerably  uniform  order  in  the  relative  devel- 
opment of  the  different  faculties  is  discernible.  At  first  the  senses 
are  awakened  to  a  lively  and  varied  activity;  then  memory  and  imag- 
ination become  more  prominent;  and,  finally,  judgment  and  the 
reasoning  powers  assert  their  sway.  Gradually,  things  become 
known  and  conduct  shaped  under  principles  which  are  assumed  to 
have  a  universal  validity  as  so-called  general  laws.  The  history  of 
the  mental  life  of  every  human  being,  from  the  cradle  (or  even  from 
its  embryonic  existence)  to  the  grave,  has  all  these  marks  of  un- 
folding itself  in  a  regular  order,  in  which  every  characteristic  event 
happens  in  due  sequence  and  in  dependence  upon  what  has  pre- 
ceded. This  is  the  very  essence  of  a  true  development. 

Can  this  mental  development  be  explained  as  merely  the  result- 
ant or  expression  of  the  physical  evolution  of  the  nervous  system — 
the  latter  being  regarded  as  situated  in  the  rest  of  the  bodily  en- 
vironment, and  surrounded  by  the  more  extended  environment  of 
the  world  of  active  physical  energies  outside  ?  Against  an  affirma- 
tive answer  to  this  inquiry  stand  many  facts  and  laws  of  all  such 
mental  development.  In  spite  of  certain  striking  correspondences 
between  the  evolution  of  the  bodily  organism  and  the  development 
of  the  mental  powers,  it  must  be  held  that  there  are  marked  di- 
vergences as  well.  At  certain  epochs  of  life  the  evolution  of  the 
brain  seems  to  stand  far  in  advance  of  the  mind;  at  others,  the  mind 
appears  to  have  overtaken  and  passed  by  the  stage  reached  by  its 
physical  substratum.  During  a  long  period  of  life  the  growth  of 
mental  powers  is  constant  and  solid,  while  the  growth  of  the  physi- 
cal basis  has  nearly  ceased,  and  such  changes  as  are  taking  place 


658       GENERAL  RELATIONS  OF  BODY  AND  MIND 

in  it  appear  quite  inadequate  to  serve  as  correlates  for  the  mental 
growth.  Moreover,  the  most  distinctly  typical  features  in  the  de- 
velopment of  the  mind  remain  the  same  when  malformation  or  dis- 
ease or  accident  have  largely  changed  the  physical  evolution  of  the 
brain. 

§  30.  We  have  no  sufficient  means  for  deciding  how  far  the  mental 
life  of  the  human  embryo  keeps  pace  with  its  organic  evolution. 
We  do  not  even  know  beyond  doubt  that  the  embryo  has  a  mental 
life,  in  the  only  tenable  meaning  of  the  words — that  is,  a  life  of  con- 
scious states.  It  is  probable,  however,  that  its  antenatal  movements 
are  not  all  purely  reflex,  but  are  accompanied  and  directed  by  con- 
scious sensation,  feeling,  and  volition.  But  the  mental  life  of  the 
embryo,  if  it  exist  at  all,  can  hardly  be  more  than  an  irregular  and 
fitful  succession  of  the  lowest  and  least  complex  of  conscious  proc- 
esses. Taste,  smell,  hearing,  and  sight  are,  of  course,  not  to  be 
thought  of  as  entering  into  such  a  mental  life.  Touch,  as  we  under- 
stand the  word  to  express  the  localized  sensations  of  pressure  which 
arise  through  the  practised  organ  of  the  skin,  is  scarcely  more  likely 
to  belong  to  the  human  embryo.  Obscure  feelings  arising  from 
changes  in  its  relation  to  the  surrounding  tissues  and  fluids  of  the 
mother,  or  from  disturbances  in  its  own  internal  organs,  and  result- 
ing equally  obscure  feelings  of  position  and  motion,  as  its  limbs  are 
moved,  must  constitute  the  greater  part,  if  not  the  whole,  of  its  ex- 
periences. As  yet  there  is  no  experience,  properly  so  called;  no 
perception  of  things,  no  feelings  of  self,  no  discrimination  of  ego  and 
state.  Yet  long  before  the  child  is  born  it  possesses  a  wonderfully 
elaborate  nervous  mechanism,  far  surpassing  in  its  grade  of  evolu- 
tion the  nervous  system  of  the  most  intelligent  adult  animals.  Pre- 
vious to  birth  this  nervous  mechanism  must  also  be  constantly  in 
action  in  a  highly  complicated  way;  it  is  engaged  in  supervising  the 
processes  of  nutrition,  and  in  the  reflex  and  automatic  activities 
which  are  expressed  by  the  changes  of  the  child's  position  within 
the  womb  of  the  mother.  The  mind,  however,  is  as  yet  unawakened; 
this  is  not  because  the  nervous  mechanism  is  not  complex  and  active 
enough  to  serve  as  the  physical  basis  of  a  rich  mental  development, 
but  because  the  kinds  of  sensation — visual,  tactual,  auditory,  etc. — 
which  start  and  furnish  and  direct  this  development  have  not  yet 
been  supplied.  The  mental  life  cannot  then  be  said  to  have  kept 
pace  before  birth  with  the  evolution  of  the  brain,  or  with  its  dis- 
tinctive activities.  On  the  contrary,  it  is  far  behind  the  stage  al- 
ready reached  by  its  physical  support.  It  waits  to  be  aroused  and 
set  to  its  own  work  of  combining  and  interpreting  those  sensations 
which  are  to  serve  as  its  chief  means  of  early  culture. 

For  the  first  few  weeks  of  infancy  the  embryonic  relation  between 


MENTAL  CONDITION  OF  THE  EMBRYO  659 

the  relative  developments  of  the  body  and  soul  of  the  child  seems 
to  be  maintained.  Both  are  subjects  of  a  rapid  growth,  but  the 
former  is  still  much  in  advance  of  the  latter.  The  newly  born  infant 
is,  in  respect  to  the  condition  of  its  nervous  system,  much  the  most 
highly  organized  and  fully  equipped  of  all  young  animals;  but  as 
judged  by  the  number  and  quality  of  its  volitions  and  perceptions, 
many  other  young  animals  are  less  stupid  and  insensate.  If  we 
may  represent  its  mental  condition  by  anything  conceivable  through 
the  adult  imagination,  the  human  infant  is  in  a  dreamless  sleep  oc- 
casionally interrupted  by  instants  of  unlocalized  and  unmeaning 
sensations. 

The  cavity  of  the  infant's  tympanum  is  filled  with  a  fluid,  the 
place  of  which  is  only  gradually  taken  by  the  air.  Sensations  of 
sound,  if  they  arise  at  all,  must  be  at  first  only  occasional  and  faint. 
Binocular  movements  of  the  eyes  in  the  direction  of  bright  objects 
take  place  early;  and  it  is  through  sensations  of  light  and  color  that 
the  first  activities  of  the  mind  in  perception  are  chiefly  aroused  and 
controlled.  But  for  some  weeks  there  are  only  sensations  and  im- 
pressions, without  true  perceptions;  there  is  as  yet  no  knowledge  of 
any  "Thing."  This  earliest  relation  of  mind  and  brain,  with  re- 
spect to  the  degree  and  rate  of  their  development,  is  not  favorable 
to  any  form  of  the  materialistic  theory.  It  rather  favors  the  view 
that  the  mental  phenomena  belong  to  another  principle  than  any 
material  substratum.  The  dependence  of  the  mind  on  the  brain 
is  indirect  and  through  the  sensations  (chiefly  of  sight  and  touch) 
which  must  somehow  be  furnished  as  the  primary  factors  in  its  de- 
velopment. The  halt  in  the  development  of  mind  at  first,  and  its 
distinct  backwardness  with  respect  to  the  relative  stage  it  has 
reached,  are  due  to  a  lack  of  such  sensations  as  have  the  charac- 
teristics of  spatial  series,  and  so  are  able  to  stimulate  the  mind,  and 
to  afford  it  the  requisite  material  for  the  construction  of  true  pres- 
entations of  sense. 

§  31.  Within  a  few  months  after  birth  the  child  has  undergone 
an  enormous  mental  development;  it  has  become  a  mind,  in  some 
inchoate  way  recognizing  itself  as  the  subject  of  states,  and  per- 
ceiving a  surrounding  world  of  objects  of  sense.  It  has  also  be- 
gun to  attend  to  the  objects  presented  in  consciousness,  and  to 
direct  its  attention  by  voluntary  choice.  The  mind's  relating  ac- 
tivity has  been  aroused;  and  acts  of  memory,  discrimination,  and 
judgment,  as  the  basis  for  those  concepts  which  require  articulate 
language  to  express  them,  are  repeatedly  taking  place.  And  soon, 
the  assumptions  of  reason,  as  involved  in  all  human  experience  of 
things,  and  of  their  action  and  reaction  upon  each  other,  are  found 
to  be  shaping  the  growth  of  the  mental  powers. 


660       GENERAL  RELATIONS  OF  BODY  AND  MIND 

As  accompanying  and  forming  the  ground  for  this  sudden  blos- 
soming of  the  mind  in  the  use  of  its  conscious  powers,  there  is  a 
continuous  and  yet  diminishing  monthly  increase  of  the  substance 
of  the  brain.  No  new  organs  are  formed  within  the  cranial  cavity; 
but  those  which  have  been  formed  previous  to  birth  are  further 
developed  under  the  changed  conditions  of  nutrition.  In  respect  to 
the  quantity  and  arrangement  of  its  molecules,  the  nervous  mech- 
anism certainly  undergoes  no  development  during  the  first  year  of 
the  child's  life  which  at  all  corresponds  to,  or  accounts  for,  the  de- 
velopment of  the  child's  mind. 

It  may  be  claimed,  however,  that  the  most  important  develop- 
ment of  the  nervous  mechanism  has  been  overlooked  in  the  fore- 
going description.  This  development  does  not  consist  so  much  in 
the  increased  quantity  of  the  brain's  substance,  or  in  the  more 
intricate  arrangement  of  its  elements  with  relation  to  each  other; 
but,  the  rather,  in  the  forming  of  what  have  already  been  referred 
to  as  "dynamical  associations"  among  the  existing  elements.  The 
statement  that  such  is  the  nature  of  the  developing  activities  of  the 
nervous  mechanism,  and  the  assumption  that  such  activities  are  an 
indispensable  physical  condition  for  the  growth  of  the  mind,  must 
be  taken  for  granted.  But  even  then  the  argument  is  far  from  com- 
plete upon  which  the  development  of  mind  as  a  real  being,  with  a 
nature  of  its  own,  and  with  a  history  controlled  by  its  own  laws, 
can  be  denied.  The  formation  of  so-called  "dynamical  associa- 
tions" among  the  molecules  of  the  nervous  mass  furnishes  no  ade- 
quate account  of  the  development  of  mind.  This  development  is 
not  in  the  direction  simply  of  associating  together  states  of  feeling, 
each  one  of  which  has  an  exact  physical  correlate  in  a  physical  as- 
sociation among  the  minute  parts  of  the  nervous  substance.  It  is 
rather  a  development  which  for  its  very  existence  requires  something 
different  from  such  associations.  The  child  might  go  on  forever 
merely  associating  together  affections  of  its  own  mind  in  correspond- 
ence to  dynamical  associations  among  the  nervous  molecules,  and 
yet  have  no  growth  of  experience  such  as  it  actually  attains.  The 
fact  is  that  within  a  single  year,  or  within  two  years,  the  child  has 
learned  to  know  "  Things,"  to  attend  to  some  in  preference  to  others, 
to  refer  its  states  in  some  crude  way  to  itself,  to  form  concepts  and 
judgments  by  the  mind's  relating  activity,  and  to  underlay  the  world 
of  its  sensuous  experience  with  another  world  of  assumption  re- 
specting certain  non-sensuous  realities.  To  account  for  this  bound- 
less expansion  of  the  activities  of  consciousness,  with  its  surprising 
new  factors  and  mysterious  grounds  of  synthesis  and  assumption, 
by  proposing  an  hypothesis  of  "dynamical  associations"  among 
the  particles  of  nervous  substance  in  the  brain,  is  a  deification  of 


DEVELOPMENT  OF  THE  ADULT  661 

impotency.  So  far  as  we  really  know  anything  about  the  develop- 
ment of  both  brain  and  mind,  we  are  compelled  to  say  that  the  latter, 
when  once  started  by  the  sensations  furnished  through  excitation 
of  the  former,  proceeds  to  unfold  its  activities  with  a  rapidity  and 
in  an  order  for  which  no  adequate  physical  causes  can  be  assigned. 

§  32.  During  the  period  of  young  manhood,  or  young  womanhood, 
the  dependence  of  the  development  of  the  mind  on  that  of  the  body 
is  most  strikingly  seen  in  the  influence  over  the  emotions  and  imag- 
ination from  the  sudden  unfolding  of  certain  bodily  organs  and 
powers.  The  indirect  influence  of  these  acts  of  feeling  and  imag- 
ination upon  the  more  purely  intellectual  progress  of  the  mind  is, 
of  course,  correspondingly  great.  But  the  dependence  of  mind  on 
body  is  by  no  means  such  as  to  favor  the  view  that  there  is  no  ground 
in  a  real  being,  other  than  the  brain,  for  the  order  and  rate  of  the 
mental  development. 

This  same  statement  is  emphatically  true  of  the  long  period  of 
maturity  which  constitutes  what  we  call  the  "middle  life"  of  man. 
During  this  time  the  nervous  matter  undergoes  scarcely  any  dis- 
cernible development.  Nothing  that  microscope  or  electrometer  can 
detect  distinguishes  the  brain  characteristic  of  the  man  of  twenty- 
five  from  that  of  the  man  of  fifty.  A  few  grams  of  weight  have 
perhaps  been  added  to  it  during  this  long  period  of  years.  Any  one 
is  at  liberty  to  speculate  as  to  the  immense  development  of  so-called 
"dynamical  associations"  which  has  taken  place  during  the  same 
period.  We  are  far  from  denying  the  possibility  of  such  develop- 
ment. But  the  fact  that  a  large  development  of  mind  may  have 
taken  place  during  the  same  period  cannot  be  denied.  If  it  be 
true  that  large  numbers  of  mankind  remain  mentally  stationary  for 
most  of  their  adult  life,  this  truth  in  no  way  favors  a  materialistic 
view  of  the  development  of  mind.  Most  observing  persons  will 
rightly  find  the  chief  account  of  the  failure  of  mental  growth  in 
precisely  those  kinds  of  mental  activity  which  least  admit  of  being 
explained  by  physical  analogies.  It  is  from  want  of  mental  curi- 
osity, attention,  careful  and  comprehensive  judgment,  sound  moral 
purpose,  etc.,  that  most  men  fail  to  develop  during  adult  life  in 
their  mental  powers.  And  these  are  mental  activities  for  explain- 
ing which  no  one  as  yet  has  been  able  to  conjecture  any  analogous 
or  corresponding  class  of  cerebral  changes. 

Many  minds,  however,  not  only  make  vast  acquisitions,  but  also 
experience  a  large  unfolding  of  mental  capacities  during  the  period 
of  middle  life.  How  mature  and  wide-reaching  do  the  judgments 
of  some  men  then  become!  How  profound  the  insight  into  the 
most  abstract  and  difficult  speculations  comes  to  be!  What  cere- 
bral evolution  shall  be  conceived  of  as  being  the  only  true  cause, 


662       GENERAL  RELATIONS  OF  BODY  AND  MIND 

and  the  exact  physical  correlate,  of  the  mental  development  of  Kant 
during  the  years  preceding  the  appearance  of  the  "  Critique  of  Pure 
Reason,"  or  of  Newton  while  he  was  unfolding  the  calculations  and 
conjectures  of  the  "Principia"?  To  hold  that  the  changing  mole- 
cules of  the  brain  substance  of  these  thinkers  were  the  sole  subjects, 
really  being  and  acting  in  the  unrolling  of  these  great  dramas  of 
human  speculation,  involves  an  astonishing  credulity.  On  the 
contrary,  we  seem  compelled  to  affirm  that  no  important  activity, 
or  law,  or  fact,  in  the  order  of  such  mental  development,  fails  to 
demand  the  assumption  of  a  real  and  non-material  unit-being,  un- 
folding its  powers  according  to  its  own  nature,  although  in  de- 
pendence upon  certain  elements  and  conditions  furnished  through 
the  brain. 

§  33.  Advancing  old  age  is  doubtless,  as  a  rule,  characterized  by 
a  simultaneous  decline  both  of  certain  mental  and  of  certain  bodily 
powers.  In  this  period  of  life,  however,  the  correspondence  be- 
tween the  changes  in  the  character  of  the  phenomena  of  conscious- 
ness and  the  altered  vigor  and  quality  of  the  nervous  mechanism  is 
not  such  as  to  suggest  that  the  two  have  an  altogether  common 
basis.  In  healthy  normal  old  age  the  course  of  the  organic  life  is 
distinguished  chiefly  by  the  dropping  out  or  diminished  action  of 
certain  factors  that  are  relatively  prominent  in  youth.  The  circu- 
lation is  slower;  the  vital  energy  is  declining;  the  muscles  are  less 
promptly  and  completely  under  the  control  of  the  volitions;  the 
end-organs  of  sense  are  less  sensitive  under  impressions;  and  cer- 
tain emotions  and  passions  whose  physical  basis  is  of  the  most  ob- 
vious sort  become  greatly  modified  or  disappear.  As  to  the  marked 
effect  of  these  bodily  changes  upon  the  mental  development  there 
can  be  no  doubt;  and  if  the  previous  mental  development  has  been 
chiefly  along  lines  indicated  by  organic  activities  the  apparent  de- 
cay of  mental  vigor  when  the  physical  basis  begins  to  fail  is,  of 
course,  also  most  plainly  marked. 

On  the  other  hand,  there  are  many  other  cases,  where  no  notable 
difference  can  be  detected,  or  even  fairly  assumed,  in  the  course  of 
the  psychical  evolution  down  to  the  "feebleness"  of  old  age;  where 
the  course  of  mental  development  continues  substantially  undis- 
turbed in  all  its  most  important  features.  The  mind  of  the  culti- 
vated old  man,  with  calm  and  broad  judgment,  with  refined  kind- 
liness and  fixed  moral  principles,  is  not  to  be  spoken  of  as  suffering 
a  decline  which  keeps  pace  with  the  failing  of  his  physical  powers. 
It  may  justly  be  claimed  that  the  final  period  of  human  life,  on  the 
whole,  favors  that  theory  which  regards  the  mind  as  by  no  means 
wholly  conditioned  upon  the  brain  for  the  character,  order,  and 
laws  of  its  development. 


PHENOMENA  OF  DECAY  663 

§  34.  The  same  general  view  of  the  development  of  mind,  which 
is  most  consistent  with  the  facts  of  the  different  stages  of  life,  is 
also  favored  by  considering  those  sudden  checks  or  changes  in  the 
course  of  this  development  that  are  caused  by  disturbing  or  de- 
stroying considerable  portions  of  the  nervous  matter.  The  phe- 
nomena which  follow  experimental  extirpation  of  the  substance  of 
the  brain  in  the  lower  animals,  and  loss  of  it  by  serious  lesions  in 
the  case  of  man,  do  not  favor  a  theory  which  completely  identifies 
the  two  developments,  or  one  which  makes  the  unfolding  of  the  men- 
tal life  nothing  more  significant  than  a  dependent  expression  of  the 
evolution,  maturing,  and  decay  of  the  nervous  mechanism.  Ex- 
tensive losses  in  certain  areas  of  the  cerebral  hemispheres  are  often 
followed  by  no  appreciable  disturbance  even  of  any  sensory  or  motor 
activity.  When  lesions  are  followed  by  such  disturbance,  their  ef- 
fects may  in  time  wholly  or  partially  disappear.  When  such  dis- 
turbance is  permanent  it  is  not  necessarily  connected  with  loss  in  the 
power  of  judgment,  in  the  higher  intellectual,  aesthetic,  and  ethical 
activities  of  feeling,  intellect,  or  will.  Even  where  aphasia  is  so 
severe  as  to  include  the  loss  of  all  power  to  utter  or  understand 
articulate  language,  the  patient  may  still  show  a  good  degree  of 
mental  acuteness  by  ability  to  make  calculations  or  play  games  of 
skill. 

On  the  other  hand,  a  much  more  serious  interruption  or  complete 
loss  of  mental  development  may  occur  when  no  adequate  explana- 
tion can  be  detected  in  the  disturbance  or  arrest  of  cerebral  develop- 
ment. It  is,  of  course,  natural  to  conjecture  that,  in  all  this  latter 
class  of  cases,  more  accurate  information  would  show  us  some  dis- 
eased condition  of  the  brain  as  the  physical  antecedent  of  the  mental 
defects.  We  know  that  subtle  changes  in  the  character  of  the 
blood-supply,  such  as  we  have  no  physical  means  whatever  for  de- 
tecting, are  often  the  causes  of  most  profound  changes — either 
temporary  or  more  permanent — in  the  train  of  ideas.  None  the 
less,  however,  do  both  classes  of  cases  above  mentioned  favor  the 
theory  we  are  advocating,  rather  than  the  so-called  materialistic 
theory  of  mind. 

§  35.  Several  references  to  the  second  argument  for  our  view  of 
the  development  of  mind  have  already  been  made.  This  argument 
is  based  upon  the  fact  that  certain  indispensable  elements  enter  into 
the  development  of  mental  life  which  have  nothing  similar  to  them, 
or  strictly  correlated  with  them,  in  the  evolution  of  the  material 
mechanism.  The  mind  can,  indeed,  undergo  no  development  ex- 
cept as  conditioned  upon  these  elements.  But  the  elements  them- 
selves are  of  such  a  nature  that  they  cannot  be  regarded  as  the  ex- 
pression in  consciousness  of  merely  physical  causes,  or  as  flowing 


664       GENERAL  RELATIONS  OF  BODY  AND  MIND 

necessarily  from  more  primitive  activities  of  the  mind  which  may 
possibly  be  regarded  as  the  expression  of  such  causes. 

If  we  accept  for  the  moment  the  customary  classification  of  modern 
psychology,  we  may  say:  All  of  those  fundamental  forms  of  activity 
which  are  recognized  in  the  threefold  division  of  conscious  processes 
— namely,  acts  of  feeling,  acts  of  knowledge,  and  acts  of  will — neces- 
sarily enter  into  the  development  of  the  mind.  Its  development 
consists  in  increased  capacity  for  these  three  classes  of  acts,  in  their 
mutual  dependence  and  according  to  the  laws  which  belong  to  them. 
Among  each  of  these  three  great  classes  of  acts  there  are  certain 
kinds  that  defy  all  attempts  whatever  to  correlate  them  with  changes 
in  the  nervous  mechanism,  or  to  explain  them  as  necessarily  or 
actually  arising  out  of  such  physical  changes.  Such  are  the  feel- 
ing of  moral  obligation,  the  sentiment  of  justice,  the  love  of  truth, 
and  certain  of  the  higher  sesthetic  feelings.  Among  the  acts  of 
knowledge,  such  are  the  mind's  relating  activity,  its  use  of  the  prin- 
ciple of  reason  and  consequent  in  drawing  deductions,  its  confident 
assumption  that  similar  phenomena  are  signs  of  like  realities,  and 
that  the  world  of  sensuous  individual  experience  is  but  the  manifes- 
tation of  an  invisible  world  of  real  beings,  with  permanent  proper- 
ties and  forces,  acting  and  reacting  under  law.  Such,  also,  are  the 
acts  of  deliberate  choice  among  courses  of  conduct,  under  the  in- 
fluence of  moral  considerations — the  so-called  acts  of  "free  will" 
in  the  highest  sense  of  the  term. 

Not  one  of  the  higher  acts  of  feeling,  knowing,  or  willing,  so  far 
as  its  sui  generis  character  is  concerned,  admits  of  being  correlated 
with,  or  represented  under,  any  of  the  conceivable  modes  of  the 
motion  and  relation  of  molecules  of  nervous  substance.  Certain 
sensations  and  perceptions  connected  with  the  rise  and  growth  of 
the  higher  forms  of  feeling  have,  undoubtedly,  a  physical  basis; 
but  such  basis  is  not  assignable  to  the  feelings  themselves.  Sen- 
sations and  perceptions,  which  are  the  resultants  (in  some  meaning 
of  the  word)  of  physical  processes,  are  discriminated  by  judgment 
and  made  the  basis  of  deductions  and  inductions.  But  admitting 
this  does  not  one  whit  the  better  enable  us  to  conceive  of  a  physical 

Erocess  which  can  account  for  the  sui  generis  character  of  the  re- 
tting activity  itself.  Acts  of  "free  will,"  so  called,  always  take 
place  under  certain  conditions  of  sensation  and  perception,  as  well 
as  of  desire;  but  the  physical  correlates  of  these  conditions  can  in 
no  respect  be  conceived  of  as  being  also  correlates  of  the  conviction 
that  the  choice  is  responsible  and  free. 

Now,  if  such  activities  as  the  foregoing  do  actually  constitute  in- 
dispensable elements  of  mental  development — and  it  is  obvious  that 
they  do — this  development  cannot  properly  be  accounted  for  by 


PECULIARLY  MENTAL  OPERATIONS  665 

assigning  it  to  a  mass  of  nervous  matter  undergoing  a  physical 
process  of  evolution,  after  the  manner  of  the  growing  human  brain. 
Such  development  rather  implies  a  being  of  another  than  the  phys- 
ical order.  And  this  other  being  must  be  thought  of  as  stimulated 
by  the  rise  and  recurrence  of  sensations  and  images  of  past  sensa- 
tions to  unfold  its  own  activities  as  conditioned  by  its  own  inherent 
powers.  Like  every  other  real  being,  the  history  of  its  unfolding 
is  dependent  upon  the  relations  in  which  it  is  placed  to  other  real 
beings;  but  it  is  nevertheless  a  history  determined  also  by  what 
the  being  is. 

§  36.  More  particularly,  it  may  be  said  that  perceptions  are  not 
merely  developed  forms  of  sensations  which  latter  are  products  of 
the  brain;  they  are  rather  advanced  forms  of  a  mental  life  developing 
under  the  experience  of  sensation — elaborate  products  of  the  syn- 
thetic activity  of  mind.  Moreover,  the  knowledge  of  things  by  per- 
ception involves  the  development  of  mental  life  in  the  forms  of 
memory  and  judgment.  But  acts  of  memory  and  judgment  are  not 
developments  from  perception;  they  are  not  merely  modified  forms 
of  sensations  as  recurring  or  combined  under  the  action  of  physical 
antecedents.  All  talk  about  the  "image"  of  memory  as  though  it 
were  merely  a  faint  or  faded-out  impression  of  sense  is  quite  un- 
availing; it  does  not  hit  the  real  point  of  inquiry,  and  consequently 
does  nothing  to  explain  the  mystery.  The  vital  element  in  memory, 
that  which  makes  it  to  be  memory,  is  neither  a  sensation,  nor  a  modi- 
fied form  of  sensation,  nor  a  development  of  sensation.  It  is  the 
subject  Ego's  recognition  of  its  own  past  history  as  belonging  to  the 
life  of  a  mind. 

The  same  statement  is  true  of  judgment.  The  relating  activity 
of  mind,  the  power  to  bring  two  objects  together  in  the  unity  of  con- 
sciousness, and,  while  keeping  their  ideas  distinctly  separate,  to 
bind  them  into  one  under  the  mental  affirmation  of  their  likeness  or 
unlikeness — this  is  a  new  and  startling  mode  of  the  activity  of  mind 
as  contrasted  with  merely  being  affected  in  sensation.  Minimize 
it  as  we  may,  we  cannot  look  upon  this  activity  as  a  mere  "  re- 
sultant" of  two  sensations  or  images  of  sensations  arising  simul- 
taneously in  consciousness.  We  cannot  consider  judgment  under  the 
principle  of  the  conservation  of  energy.  To  treat  it  as  such  involves 
the  grossest  misapplication  of  the  laws  which  control  the  coinci- 
dence or  conflict  of  physical  forces.  Nor  are  the  different  forms  of 
the  relating  activity  of  the  mind — concept,  judgment,  deduction, 
induction — to  be  regarded,  strictly  speaking,  as  developments  from 
each  other  or  from  any  one  mental  activity  simpler  than  any  of  them. 
They  may  all,  indeed,  be  considered  as  modes  of  the  relating  ac- 
tivity, because  they  involve  discrimination,  the  discernment  of  like- 


666       GENERAL  RELATIONS  OF  BODY  AND  MIND 

nesses  and  unlikenesses.  But  each  one  of  them  involves  somewhat 
more  than  simple  discrimination;  each  one  involves  other  elements 
peculiar  to  itself.  That  a  sentient  being  should  simply  judge,  or 
affirm  this  or  that,  is  not  of  itself  a  sufficient  reason  why  it  should 
also  make  inferences  by  syllogistic  processes  or  arrive  at  general 
laws  by  induction.  Indeed,  the  former  may  belong  to  many  animals 
which  are  incapable  of  the  latter. 

We  might,  properly  and  almost  indefinitely,  continue  the  fore- 
going line  of  remarks  into  the  consideration  of  the  mind's  most  gen- 
eral activities.  Modern  psychology,  we  have  seen,  is  accustomed  to 
distinguish  faculties  of  knowing,  feeling,  and  willing  as  belonging 
to  the  mind.  But  if  we  adopt  this  division,  it  becomes  emphatically 
true  that  no  one  of  these  three  faculties  can  be  regarded  as  devel- 
oped from  any  other  one,  or  from  any  two  combined.  That  a  being 
feels — that  is,  is  affected  with  a  state  of  consciousness  more  or  less 
pleasurable  or  painful,  and  having  a  characteristic  quality — is  in 
itself  no  ground  for  explanation  of  its  knowing  "Things"  through 
sense-perception  and  inference.  Conversely,  a  being  is  conceivable 
with  the  knowledge  of  an  archangel,  but  without  experience  of  de- 
sire, emotion,  or  sentiment  of  attraction  or  repulsion.  Such  a  being 
would,  indeed,  have  to  attain  its  knowledge  in  other  ways  than  those 
open  to  us,  and  we  find  it  difficult  or  impossible  to  imagine  precisely 
what  such  knowledge  could  be  like.  But  growth  in  knowledge  is 
a  different  thing  from  the  unfolding  of  mere  feeling;  and  the  former 
cannot  be  explained  as  arising  out  of  the  latter.  Acts  of  will  are, 
indeed,  always  actually  dependent  upon  knowledge  and  feeling, 
and  cannot  even  be  conceived  of  as  taking  place  without  this  de- 
pendence. But  acts  of  will  are  not  mere  developments  of  those  acts 
of  knowledge  and  feeling  on  which  they  undoubtedly  depend.  The 
act  of  choice  involves  a  new  element,  an  element  not  to  be  neces- 
sarily evolved  from  the  other  activities  of  mind. 

§  37.  The  development  of  mind,  therefore,  cannot  be  explained 
after  the  analogy  of  the  accretion  of  molecules  within  a  germ,  and 
the  resulting  division,  multiplication,  and  advancing  arrangement  of 
the  living  cells  into  separate  organs  of  the  entire  system.  No  real 
elements  of  the  mind  exist  which  can  aggregate  to  themselves  other 
elements  by  absorbing  them  as  pabulum,  or  can  grow  by  arranging 
the  new  material  thus  gained  according  to  the  energies  inherent  in 
the  material  already  organized.  The  life  of  consciousness  is  a  never- 
ceasing  change  of  states.  Yet  the  result  of  this  change  of  states  is 
an  orderly  history,  a  true  development.  Such  development  is  not 
merely  the  expression  of  the  evolution  of  the  material  basis  of  some 
of  these  mental  states.  For  it  does  not  follow  precisely  the  same 
order  or  the  same  laws  as  govern  the  material  evolution;  and  some 


THE  CONCLUSION  DRAWN  667 

of  its  most  important  factors  cannot  be  regarded  as  having  any 
physical  correlate,  or  as  evolved  from  other  factors  which  have  such 
a  correlate.  The  development  of  Mind  can  only  be  regarded  as  the 
progressive  manifestation  in  consciousness  of  the  life  of  a  real  being 
which,  although  taking  its  start  and  direction  from  the  action  of  the 
physical  elements  of  the  body,  proceeds  to  unfold  powers  that  are  sui 
generis,  according  to  laws  of  its  own. 


CHAPTER  II 
REALITY  AND  UNITY  OF  THE  MIND 

§  1.  In  all  the  discussions  of  the  previous  chapter  it  was  implied 
that  we  were  dealing  with  two  different  existences — separable  at 
least  in  thought,  and  apparently  belonging  to  widely  divergent  species 
of  existence.  And  yet  these  two  existences  are  in  every  case  related 
to  each  other  in  a  peculiarly  and  even  uniquely  intimate  way:  they 
are  so  correlated  that  each  furnishes  to  the  other  the  conditionating 
antecedent,  or  cause,  of  its  habitual  behavior  and  characteristic  de- 
velopment. For  example:  I  have,  or  rather  am,  a  body  and  also  a 
mind;  and  you  have,  or  rather  are,  another  and  different  body  and 
also  a  mind.  My  body  and  my  mind  are  bound  together  with  these 
peculiar  ties  of  correlation ;  and  the  same  thing  is  true  of  your  body 
and  your  mind.  The  individuality  of  each  of  us  as  human  beings 
is  the  joint  product  of  the  nature  and  development  of  the  two,  in 
this  their  naturally  appointed  form  of  being  correlated.  In  some 
such  way  as  this,  the  popular  conceptions  relating  to  the  subject 
would  have  to  be  expressed;  and  as  yet  we  have  found  nothing  in 
psychological  science,  as  pursued  from  the  physiological  and  experi- 
mental points  of  view,  to  contradict  or  substantially  to  modify  the 
popular  conception. 

Now,  only  an  extreme  form  of  scholastic  idealism  (sometimes 
called  "subjective  idealism"  or  "solipsism"),  of  which  science  can 
take  no  account,  ventures  to  deny  the  reality  of  the  human  body, 
in  the  fullest  meaning  of  that  word  which  can  apply  to  any  material 
entity.  A  piece  of  quartz  rock  and  the  gold  extracted  from  it,  or  a 
ton  of  steel,  is  not  a  bit  more  real,  and  truly  a  part  of  physical  nature, 
than  is  the  most  delicately  organized  human  body,  including  the 
wonderfully  intricate  and  tenuous  network  of  nerve-cells  in  the 
brain  attached  to  that  body.  Nor  would  any  one  think  of  denying 
that  the  different  parts  of  the  body  constitute  a  certain  kind  of 
organic  whole;  or  that  the  brain,  considered  by  itself,  shows  mani- 
fest signs  of  both  an  architectural  and  a  functional  unity.  All  this, 
and  much  more  of  the  same  sort,  enters  into  all  the  sciences  of  bi- 
ology, anatomy,  histology,  and  physiology,  in  their  several  ways  of 
dealing  with  the  same  material  substance.  All  these  assumptions, 

668 


NATURE  OF  CLAIMS  DISCUSSED  669 

however,  are  a  naive  and  uncritical  but  legitimate  and  scientifically 
productive  form  of  metaphysics. 

§  2.  When  we  come  to  deal  with  the  claims  of  the  subject  of  the 
conscious  mental  life  to  a  reality  and  a  unity  of  its  own,  similar  as- 
sumptions are  apt  to  be  ruled  out,  on  the  ground  that  they  are  meta- 
physical, and  that  science  ought  not  to  have  anything  to  do  with 
metaphysics.  But  why  this  prejudice  against  claiming  for  the  sub- 
ject of  the  mental  life  and  mental  development  a  title  to  some  sort 
of  reality,  and  to  some  sort  of  unity;  when  both  these  claims  are 
admitted  without  contest  for  the  bodily  organism  with  which  this 
mental  life  and  mental  development  are  correlated  ?  In  answer  to 
this  inquiry,  four  considerations  may  be  alleged.  One  of  the  chief 
reasons  for  such  discrimination  against  the  claims  of  the  mind  is 
just  that  lack  of  the  critical  analysis  of  metaphysical  conceptions,  to 
which  reference  has  already  been  made.  What,  indeed,  do  we  mean 
by  calling  things  "real,"  and  by  speaking  of  them  as  having  a  cer- 
tain kind  of  "unity"?  Until  we  have  fixed  upon  some  at  least 
provisional  answer  to  this  inquiry,  how  can  we  say  whether  any  par- 
ticular thing  is  entitled  to  be  called  real,  or  to  be  spoken  of  as  one, 
two,  or  any  particular  number?  Another  reason  is  to  be  found  in 
the  peculiar  characteristics  which  affirm  the  existence  in  reality  of  the 
so-called  mind;  and  which  are  relied  upon  to  demonstrate  its  unity. 
A  third  reason  has  operated  to  produce  scepticism  ending  in  a  vir- 
tually materialistic  conception,  in  a  yet  more  powerful  way.  This 
is  to  be  found  in  the  vagueness  and  extravagance  of  those  who,  in 
the  supposed  interests  of  morals  and  religion,  have  presented  what 
they  were  pleased  to  regard  as  a  "  spiritual "  conception  of  the  human 
soul.  And,  finally,  a  reason  is  to  be  found,  in  many  individual  cases, 
in  a  hardened  and  highly  prejudiced  attitude  on  the  part  of  the  ob- 
server, against  admitting  anything  that  does  not  easily  accord  with 
a  purely  mechanical  and  materialistic  conception  of  nature  and  her 
processes.  In  the  opinion  of  such  minds,  nothing  can  be  real  that 
cannot  be  perceived  by  the  senses,  that  cannot  be  weighed  and  meas- 
ured in  the  laboratory  or  in  the  field;  and  nothing  can  have  a  real 
unity  that  does  not  admit  of  having  its  elements  physically  or  chem- 
ically combined  and  separated  for  future  combination  with  other 
elements.  Each  one  of  these  four  considerations  will  now  be  ex- 
amined in  the  order  just  given. 

§  3.  As  to  its  conceptions  of  "reality"  and  "unity,"  modern  psy- 
chology may  properly  claim  to  be  following  the  lead  of  the  chemico- 
physical  sciences.  These  conceptions  are  now,  more  than  formerly, 
of  the  "dynamical"  and  less  of  the  statical  order.  No  physical 
reality — not  even  the  ancient  atom — is  to  be  fitly  conceived  of  as 
an  unchanging  mass  or  unalterable  tiny  bit  of  material  substance. 


670     „        REALITY  AND  UNITY  OF  THE  MIND 

To  have  its  claim  entered  in  the  world  of  real  beings,  every  indi- 
vidual thing  must  prove  this  claim  by  continually  doing  something 
and  by  having  something  done  to  it.  What  kind  of  a  reality  any 
particular  thing  is  to  be  proved  to  be,  depends  upon  what  it  is  able 
to  do  to  other  things  and  to  have  done  to  it  by  other  things.  In  a 
word,  it  is  action  which  demonstrates  reality — both  that  it  is,  and  what 
it  is. 

In  somewhat  similar  manner,  what  gives  unity  to  any  particular 
thing  is  not  the  fact  that  it  is  a  solid  and  indissoluble  mass,  or  an  un- 
analyzable,  tiny  bit  of  substance;  but  its  unity  is  gained  for  it  by  the 
co-operation  of  its  various  elements,  in  their  action  and  in  their  de- 
velopment. Or,  to  say  the  same  thing  in  other  words:  Every  re- 
ality becomes  "one"  through  the  continuous  action  and  reaction 
of  its  elements  in  definite  ways  upon  one  another  and  upon  all  the 
other  things  which  constitute  its  particular  environment.  This  is  true 
even  of  so  loosely  constituted  a  unity  as  a  heap  of  sand;  the  differ- 
ent grains  form  one  heap,  only  so  long  as  they  are  held  together  in 
one  place  by  the  forces  of  gravity  and  cohesion.  But  there  are  differ- 
ent kinds  of  unity,  and  different  degrees  of  being  united,  correspond- 
ing to  these  different  kinds.  In  general,  the  more  complex  and  un- 
stable the  unity,  the  higher  it  is  in  kind.  According  to  modern 
physics,  as  respects  its  elements  there  is  no  more  rapidly  and  subtly 
alterable  unity  than  is  the  atom;  and  yet  the  atom,  as  the  very  word 
signifies,  was  formerly  regarded  as  the  most  stable  and  unalterable 
of  all  conceivable  unities.  In  the  case  of  all  living  forms,  the  unity 
which  they  possess  is  the  oneness  that  is  attained  by  a  process  of  de- 
velopment. And  here,  in  general,  the  original  elements  are  com- 
paratively few  in  number  and  simple  in  arrangement;  but  they  be- 
come more  and  more  numerous  and  highly  complex  in  arrangement, 
as  this  process  goes  forward.  Indeed,  by  certain  theories  it  would 
seem  to  be  held,  that  to  give  a  simple  descriptive  history  of  this  pro- 
gressive complication  of  elements  is  a  sufficient  explanation  of  the 
process  itself.  Such  theories  would  think  to  account  for  the  devel- 
opment by  describing  it  as  an  increase  in  "aggregation,"  "differ- 
entiation," "  integration,"  etc. ;  and  would  regard  the  unity  resulting 
from  the  process  as  having  no  more  reality  than  that  which  belongs 
to  the  process;  and  this  is  a  never-ceasing,  but  ever  restless,  activity 
by  way  of  action  and  reaction. 

§  4.  If  now  we  insist  upon  having  some  further  and  more  satis- 
factory explanation  of  a  unity  which  is  achieved  by  a  process  of 
development  that  combines,  even  into  a  temporary  oneness,  a  num- 
ber of  elements  through  the  co-operation  of  a  number  of  kinds  of 
energy,  we  are  obliged  to  introduce  the  conception  of  "plan."  No 
unity  of  any  kind  or  degree  of  complexity  can  be  realized  without 


UNITY  AS  INVOLVING  PLAN  671 

giving  evidence  of  some  "indwelling"  or  "overruling"  idea.  In 
the  case  of  such  unitary  beings  as  appear  to  our  rninds  ready-made, 
as  it  were,  the  plan  is  thought  of  as  already  realized;  while  in  the  case 
of  such  beings  as  are  found  to  be  still  in  a  process  of  development, 
the  plan  is  thought  of  as  in  the  way  of  being  realized.  Whether 
this  manner  of  explaining  the  unitary  nature  of  any  physical  thing 
is  only  a  sign  of  the  sort  of  necessity  under  which  the  human  mind 
finds  itself,  and  gives  us  no  clue  to  the  Nature  of  Reality,  or  whether 
it  is  also  a  trustworthy  revelation  of  the  essential  character  of  the 
Being  of  the  World,  is  a  metaphysical  inquiry  which  need  not  con- 
cern us  at  the  present  time. 

It  would  seem,  then,  that  if  the  phenomena  of  man's  mental  life 
give  unmistakable  signs  of  forms  of  energy,  which  act  upon  other 
real  beings  and  are  acted  upon  by  them,  and  which  arrange  them- 
selves according  to  some  plan,  then  it  is  certainly  permissible  to 
claim  reality  and  unity  for  the  Subject  of  these  phenomena — i.  e., 
for  the  so-called  soul  or  mind.  At  once,  however,  we  are  met  by 
objections  based  upon  the  peculiar  nature  of  these  phenomena;  and 
especially  upon  the  well-known  fact  of  their  dependence — even  to 
the  point  of  not  appearing  at  all,  or  of  total  disappearance — on  the 
constitution  and  functioning  of  the  bodily  organism. 

Now  a  certain  very  intimate,  and  in  some  respects  seemingly  ab- 
solute, dependence  of  all  mental  phenomena  upon  the  integrity,  con- 
dition as  respects  the  character  of  its  blood-supply,  habitual  ways 
of  reacting,  established  "dynamical  associations,"  "preoccupa- 
tions," and  concurrent  sensory  and  motor  nerve-commotions,  etc., 
etc.,  of  the  brain,  must  be  admitted.  How  intimate  and  far-reach- 
ing this  dependence  actually  is,  has  occupied  with  its  statement  more 
than  one-half  of  this  entire  treatise.  It  has  been  shown  that  the 
beginnings,  the  varying  phases  of  intensity,  the  rise  and  fall  below 
the  so-called  "  threshold,"  and  even  the  complete  temporary  cessa- 
tion of  the  conscious  states,  are  involved  in  this  dependence.  Here 
it  is  not  necessary  to  repeat,  however,  what  has  been  sufficiently 
insisted  upon  in  the  last  chapter,  and  in  numerous  other  places, — 
namely,  that  the  phenomena  which  are  ascribed  to  the  nervous  mech- 
anism are  also  dependent  for  their  existence  and  their  character 
upon  the  reactions  of  the  conscious  mental  life.  But  it  is  especially 
necessary  at  this  point  to  remind  ourselves  that  no  kind  of  physical 
reality  succeeds  in  escaping  a  similar  condition  of  dependence.  All 
the  phenomena  which  material  substances  exhibit  as  signs  of  their 
reality  are  liable  to  vary  in  degree,  in  characteristic  quality,  and  even 
to  disappear  and  reappear,  or  cease  entirely,  in  dependence  upon 
internal  changes  and  upon  their  relations  to  other  material  sub- 
stances. The  higher  in  the  scale  of  complexity,  expecially  as  a  re- 


672  REALITY  AND  UNITY  OF  THE  MIND 

suit  of  biological  development,  the  living  unity  has  come  to  be,  the 
more  significant,  and  yet,  generally,  the  more  unstable,  is  its  unity 
apt  to  become. 

§  5.  If  now  in  the  light  of  even  so  little  criticism  of  the  metaphysi- 
cal conceptions  involved,  we  examine  anew  the  claims  of  the  mind  to 
reality  and  unity,  we  shall  find  it  impossible  not  to  accept  them  in  a 
certain  meaning  of  the  words  employed.  In  some  meaning  of  these 
words,  the  mind  undoubtedly  has  an  actual  existence  as  a  unitary 
being;  it  is  real  and  it  is  one.  It  has  at  least  what  Kant  called  a 
"phenomenal"  reality;  and  it  develops  in  time  a  "phenomenal" 
unity.  To  establish  so  much  of  a  claim  we  may  appeal  with  con- 
fidence to  every  one's  indisputable  experience.  Both  the  reality 
and  the  unity  of  this  sort  are,  indeed,  a  matter  of  degrees  and  of  de- 
velopment. But  why  should  this  fact  diminish  the  trustworthiness 
or  the  value  of  the  experience  on  which  the  claim  is  based  ?  Indeed, 
this  kind  of  reality  and  unity  is  involved  in  the  very  conception  of  a 
development.  As  a  bare  conception — not  to  speak  of  any  corre- 
sponding facts  in  reality — no  development  can  exclude  the  neces- 
sity of  a  process  which  is  itself  characterized  by  a  certain  kind  of 
unity,  and  which  has  its  result  in  a  series  of  existences  possessing 
a  unitary  character.  All  this  is  conspicuously  true  of  that  organism 
with  which  the  development  of  the  mental  life  is  especially  corre- 
lated. The  human  nervous  mechanism,  as  was  shown  in  detail 
in  the  opening  chapters  of  the  book,  has  resulted  from  a  process 
which,  in  the  case  of  each  individual  man,  has  followed  a  somewhat 
peculiar  plan;  and  at  every  stage  in  this  complex  process,  from  the 
beginning  of  the  faintest  discernible  traces  of  nerve-fibres  and  nerve- 
cells  to  the  maturing  of  the  adult  brain,  spinal  cord,  peripheral 
ganglia,  end-organs  of  sense,  etc.,  etc.,  this  mechanism  has  exhibited 
all  the  signs  of  a  "unitary  being."  Indeed,  were  this  not  true,  it 
could  not  properly  be  called  either  a  "system,"  or  a  "mechanism/* 

§  6.  But  to  examine  further  the  claims  of  the  subject  of  the  phe- 
nomena of  the  mental  life,  the  so-called  mind,  to  the  titles  of  reality 
and  unity:  It  was  just  now  said  that  this  claim  can  scarcely  be  com- 
bated, if  we  qualify  the  nouns  by  the  adjective  "phenomenal." 
By  this  it  was  meant  that  the  phenomena  present  themselves  to  our 
inspection,  whether  we  study  them  by  the  method  of  self-conscious- 
ness solely  or  also  and  chiefly  by  the  methods  of  physiological  and 
experimental  psychology,  as  matter-of-fact  experiences  of  a  uni- 
tary character.  Many  ill-advised  attempts  have  recently  been  made 
to  destroy  or  to  minimize  what  used  to  be  called  "  the  authority  of 
consciousness,"  and  even  to  prove  that  no  such  experience  as  self- 
consciousness,  properly  so  called,  can  possibly  be  had.  This,  to- 
gether with  the  loose  use  of  such  terms  as  "unconscious"  and  "sub- 


THE  ACTIVITY  OF  APPERCEPTION  673 

conscious"  in  application  to  the  phenomena  of  mental  life,  has  been 
the  source  of  much  obscuration  of  the  entire  problem.  And  when 
we  add  to  this  result  the  effect  of  the  loose  employment  of  such  terms 
from  the  mathematics  of  physics  as  "double,"  "triple,"  etc.,  to 
human  selfhood  or  personality,  we  have  a  state  of  affairs  in  which  the 
plainest  testimony  of  the  most  indubitable  experiences  is  tolerably 
sure  to  be  either  sophisticated  or  totally  overlooked.  Surely,  the 
consistent,  tenable,  and  hopeful  course,  upon  which  to  enter  for  the 
scientific  definition  of  the  existence  and  the  nature  of  any  thing, 
physical  or  otherwise,  is  not  through  the  door  of  a  denial  of  the 
actual  facts  of  our  experience  with  that  particular  thing. 

§  7.  Now  in  the  special  case  of  man's  mental  life,  its  very  es- 
sence consists  in  conscious  activity,  culminating  in  the  achievement 
of  perceptive  consciousness  (sometimes  called  "apperception"), 
or  the  knowledge  of  things,  on  the  one  hand;  and  in  self-conscious- 
ness or  the  knowledge  of  Self,  on  the  other  hand.  In  this  very 
process  of  development  the  mind  not  only  constructs  and  affirms  its 
own  unitary  being,  but  also  constructs  all  the  unities  which  it  affirms 
of  other  selves  and  of  material  things.  But  when  we  speak  of  this 
reality  and  this  unity  as  an  achievement,  we  in  no  wise  invalidate  or 
diminish  the  truthfulness  or  the  value  of  the  claim.  In  fact  nothing 
that  is  alive,  especially  no  product  of  an  elaborate  process  of  devel- 
opment— and  probably  no  inorganic  physical  reality  even — has 
come  into  existence  "ready  made,"  as  it  were.  On  the  contrary, 
each  unity  in  nature  has  become  one,  through  a  process  of  self- 
making.  Looked  at  from  the  purely  objective  and  scientific  point 
of  view,  Nature  is  constantly  making  and  unmaking  all  those  prod- 
ucts, which  appear  to  our  minds  to  have  a  certain  temporary  unity, 
but  which  are  in  fact  never  wholly  delivered  from  the  necessity 
of  ceaseless  change.  The  rather  must  we  say  that  the  reality  of 
their  being  at  all  consists  in  this  actual  process  of  change;  and  that 
their  individual  natures  are  defined  to  us  by  the  character  of  the 
changes  which  take  place  in  them  and  which  they  contribute  to 
produce  in  other  natural  objects.  But  subjectively  regarded,  as  the 
theory  of  perception  already  advanced  has  sufficiently  made  clear, 
all  these  same  natural  products  are  made  to  be  one  to  us,  through 
no  process  of  merely  copying-off,  but  by  the  constructive  activity  of 
our  own  minds. 

§  8.  Now  one  essential  thing  about  the  activity  of  human  con- 
sciousness, as  the  truth  has  all  along  been  implied,  is  to  affirm  the 
actuality  of  the  existence  of  the  Subject  of  all  the  conscious  states. 
In  a  word,  the  reality,  or  actuality  for  the  time  being  of  this  Subject 
of  all  the  mental  life  is  an  indispensable  implicate  of  all  the  phenomena 
of  the  same  mental  life.  The  mind  must  do  duty  here;  and  there  is 


674  REALITY  AND  UNITY  OF  THE  MIND 

nothing  else  which  can  by  any  possibility  be  supposed  to  take  its 
place.  Let  the  case  be  tried  by  making  a  beginning  with  that  sort 
of  testimony  with  which  every  one  is  most  familiar.  I  know  that 
I  think,  feel,  will;  that  is  to  say,  phenomena  take  place  in  my  con- 
sciousness which  there  is  no  conceivable  way  of  describing  except 
by  attributing  them  to  the  subject  of  all  my  consciousness — to  the 
self-conscious  "me"  called  mind.  But  because  I  cannot  perceive 
this  subject  of  all  consciousness  as  an  extended  and  external  some- 
what— a  "Thing"  so  large,  and  shaped  and  colored  in  just  such  a 
manner,  with  a  definitely  hard  or  soft  feel — that  is  to  say,  because 
I  do  not  appear  to  myself  in  consciousness  to  be  just  such  a  kind  of 
being  as  are  some  of  the  objects  of  my  perception,  I  begin  to  raise 
the  question  whether  this  subject  (the  "I"  that  thinks,  etc.)  has  any 
real  being  at  all.  May  it  not  in  fact  be,  I  ask  myself,  that  some 
"  thing,"  or  collection  of  things,  like  those  which  I  have  often  seen 
and  felt,  is  the  subject  to  which  the  thoughts  and  feelings  and  acts 
of  will  that  I  have  called  "mine"  should  be  attributed?  Of  course, 
if  this  question  is  to  be  answered  in  the  light  of  modern  physiology 
with  even  a  provisional  affirmative,  the  particular  "  thing,"to  which 
such  activities  as  those  I  am  conscious  of  are  to  be  attributed,  is  my 
brain.  Nothing,  surely,  but  my  brain  can  think,  and  feel,  and  will 
— so  to  speak — for  me. 

But  the  inquiry  may  now  be  raised:  How  do  we  know  that  there 
is  actually  a  brain,  which  may  serve  as  the  real  substratum  of  the 
phenomena  of  consciousness?  In  view  of  the  science  of  body  and 
mind,  it  is  as  fair  to  ask  this  question  in  a  sceptical  way,  as  to  ask 
the  corresponding  question  about  the  mind.  It  scarcely  need  be 
said  that  no  one  has  any  evidence  presented  directly  to  the  senses 
that  such  organ  exists  within  his  own  cranial  cavity.  To  be  con- 
scious, and  at  the  same  time  to  observe  the  substratum  of  one's 
consciousness,  is  an  unattainable  opportunity.1  It  may  even  be 
that  the  particular  ego  (the  "  I "  of  consciousness)  which  is  engaged 
in  the  search  for  its  own  real  being  in  a  material  substratum  has 
never  seen  so  much  as  a  single  human  brain.  Since  there  is  such 
scarcity  of  direct  ocular  and  tangible  demonstration  of  a  special  re- 
lation between  the  brain  and  mental  phenomena,  it  is  plain  that  the 
testimony  of  experts  must  be  summoned. 

On  the  other  hand,  it  must  be  confessed  that  no  expert  has  any 
more  direct  evidence  than  every  self-conscious  ego  has  of  the  ex- 

1  After  all,  perhaps  this  opportunity  is  not  forever  in  all  respects  "  unattain- 
able," for  the  most  recent  experiences  with  cerebral  surgery  without  anaesthetics, 
leave  it  possible  that  an  arrangement  of  mirrors  might  give  one  the  rare  sight 
of  seeing  his  own  brain  exposed  and  operated  upon  to  an  hitherto  inconceivable 
extent. 


THE  ACTIVITY  OF  APPERCEPTION  675 

istence  of  a  real  material  structure  called  brain,  which  may  account, 
by  its  presence  and  activities,  for  his  own  mental  phenomena.  Nor 
can  he  offer  any  evidence  peculiar  to  himself  for  his  belief  that  the 
particular  ego  which  he  calls  "himself"  is  connected  with  his  brain. 
And  how  many  soever  other  brains  he  may  have  seen,  he  only  knows 
by  a  series  of  very  indirect  and  complicated  inferences  that  any 
individual  whose  brain  he  has  not  seen  really  possesses  one.  But 
whence  these  inferences  ?  and,  What  are  the  grounds  on  which  the 
confidence  attached  to  them  is  based  ?  To  these  questions  only 
one  answer  is  possible.  The  inferences  themselves  are  acts  of 
knowledge,  modes  of  consciousness,  phenomena  of  mind.  The 
only  possible  grounds  of  confidence  in  them,  as  valid  inferences, 
must  be  referred  back  to  our  inherent  faith  in  the  power  of  the 
mind  rightly  to  infer,  from  its  own  phenomena,  the  real  existence 
of  beings  the  phenomena  of  which  it  has  never  perceived.  More- 
over, if  the  mind  had  perceived  the  phenomena  of  its  own  brain, 
there  could  be  nothing  in  the  phenomena  themselves  to  account  for 
the  power  to  make  inferences  which  belong  to  it  as  mind.  On  the 
ground,  then,  of  an  inferred  reality  called  the  brain,  I  am  asked 
to  dispense  with  my  confidence  in  the  reality  of  the  being  which 
makes  the  inference,  and  which,  at  the  same  time,  makes  a  much 
more  irresistible  inference  as  to  its  own  reality  as  an  active  infer- 
ring force. 

§  9.  The  case  is,  however,  by  no  means  so  favorable  as  the  state- 
ment just  made  would  imply,  for  that  phase  of  scientific  material- 
ism which  refers  the  phenomena  of  consciousness  to  the  brain  as 
their  sole  cause.  For  it  is  not  in  the  brain,  as  a  mere  mass  of  mat- 
ter whose  structure  and  mechanical  functions  can  be  made  obvious 
to  any  intelligent  observer,  that  the  real  substratum  of  mental  phe- 
nomena must  be  sought.  Considered  as  such  a  mass,  this  organ 
is  no  better  than  any  other  similar  soft  and  pulp-like  bulk.  It  is 
the  wonderful  molecular  constitution,  atomic  play,  and  changing 
dynamic  relations  of  the  invisible  particles  of  this  mass,  which  are 
responsible  for  its  unique  functions.  But  the  very  existence  of  the 
atoms  as  real  beings,  capable  of  acting  on  each  other  and  of  being 
acted  on — how  shall  this  remote  and  obscure  fact  be  ascertained? 
and  how  shall  we  learn  what  is  the  nature  of  these  beings,  so  as  to 
determine  whether  or  not  they  are  capable  of  performing  the  stu- 
pendous task  of  bringing  forth  the  various  mental  phenomena  ? 

In  attempting  to  answer  the  last  two  questions  we  are  in  great 
danger  of  losing  completely  all  that  we  have  taken  most  pains  to 
gain.  It  is  to  the  all-powerful  "atoms,"  with  their  potent  forces, 
that  we  are  now  looking  as  the  real  subjects  at  once  of  the  molec- 
ular changes  in  the  brain-mass  and  of  the  phenomena  of  conscious- 


676  REALITY  AND  UNITY  OF  THE  MIND 

ness.  From  these  real  beings  and  their  relations  there  must  be 
derived,  not  only  the  activities  which  physiology  ascribes  to  nervous 
matter,  but  also  those  which  psychology  is  constrained  to  ascribe 
to  conscious  mind.  And  yet,  how  do  we  know  that  any  real  beings 
whatever  called  atoms  exist  ?  Certainly  not  by  direct  evidence  of  any 
of  the  senses.  Not  even  the  most  pronounced  advocate  of  the  reality 
of  physical  things,  and  of  the  unreality  of  mind,  would  venture  to 
affirm  that  he  has  seen  or  touched  an  atom,  or  can  demonstrate  its 
existence  and  nature  to  ordinary  observation  through  the  human 
senses.  Atoms  are  supersensible  beings.  Moreover,  they  are  hypo- 
thetical existences,  or  beings  whose  existence  is  inferred  in  an  ex- 
tremely roundabout  way  in  order  that  we  may  be  able  to  give  to 
ourselves  a  rational  account  of  the  grounds  on  which  certain  classes 
of  phenomena  rest. 

§  10.  Moreover,  the  best  efforts  of  modern  investigation  to  de- 
scribe the  nature  of  the  atom  appear,  not  only  incomplete,  but  also, 
in  certain  particulars,  self-contradictory.  It  is  certain  that  the 
atom  cannot  be  regarded  as  an  independent  reality.  What  it  is  can 
only  be  described  by  telling  what  it  does;  but  in  telling  what  it  does 
we  always  find  ourselves  implying  certain  relations  to  other  atoms. 
That  is  to  say,  we  know  nothing  about  the  nature  of  any  of  the 
atoms  which  does  not  involve  also  complicated  hypotheses  concern- 
ing its  mode  of  behavior  as  caused  by  the  presence  and  mode  of 
behavior  of  other  hypothetical  beings.  In  this  way  the  reality  of 
the  atoms  is  made  ultimately  to  depend  on  the  reality  of  some  law, 
or  ideal  force,  that  binds  different  atoms  together,  as  it  were,  and 
makes  them  work  to  a  unity  of  plan.  But  here,  again,  we  are  re- 
minded that  we  can  form  no  conception  of  a  "  plan "  which  is  not  a 
phenomenon  of  mind,  and  no  conception  of  a  "unity"  that  does  not 
depend  upon  the  unifying  actus  of  the  mind.  Moreover,  all  ideas 
of  "relation"  are  dependent  upon  mental  activities  that  are  quite 
without  physical  analogy.  All  "Things"  are  made  into  the  units 
which  they  appear  to  be  by  the  unifying  action  of  the  mind.  Such 
action  is  implied  in  the  most  elementary  and  naive  perceiving  of 
things;  for  the  study  of  perception,  from  the  physiological  point  of 
view  even,  has  enabled  us  to  show  that  no  so-called  "thing"  is  a 
ready-made  material  product,  apprehended  by  mind  in  a  form  which 
is  a  copy  of  some  extra-mental  existence. 

§  11.  Accordingly,  the  whole  course  of  argument  and  the  whole 
weight  of  conviction  appear  to  be  the  reverse  of  what  is  assumed 
by  the  objector  to  the  reality  of  mind.  The  material  molecules  of 
the  brain  are  not  beings  about  the  reality  and  exact  nature  of  which 
we  have  the  most  indubitable  evidence — evidence  so  indubitable 
that  we  may  venture  to  press  it  into  the  contradiction  of  the  more 


DISTINCTION  OF  EGO  AND  NON-EGO  677 

immediate  data  of  consciousness.  If  these  elements  of  all  physical 
being  are  real,  they  come  to  us  as  inferences  and  hypotheses;  they 
involve  a  vast  amount  of  conjecture,  indirect  inference,  and  unsolved 
difficulties,  or  even  contradictions.  And  if  we  ask,  On  what  au- 
thority are  these  inferences  made?  Whence  comes  the  demand  for 
any  rational  explanation  whatever?  Where  do  the  conjecture,  hy- 
pothesis, and  sense  of  difficulty  and  seeming  contradiction  exist? 
then  the  only  answer  to  be  given  to  all  these  questions  refers  them 
to  the  Mind.  What  atoms  and  forces  and  laws  can  be,  or  mean, 
without  the  being  and  activity  of  self-conscious  mind,  is  even  harder 
to  conjecture  than  what  a  color  can  be  which  is  not  seen,  a  sound 
which  is  not  heard,  an  odor  that  is  not  smelled. 

Shall  this,  then,  be  the  last  word  of  science  on  the  subject? — 
namely,  that  the  one  being  in  whose  active  energizing  all  conceptions 
of  all  real  being  arise,  feels  justified  in  denying  its  own  reality  in  the 
supposed  favor  of  certain  of  its  most  remote  and  doubtful  conceptions. 

§  12.  What  is  meant  by  affirming  the  reality  of  mind  may  now 
be  made  somewhat  clearer  by  pursuing  the  following  train  of  re- 
flections: In  the  development  of  the  mental  life  its  phenomena  come 
inevitably  to  divide  themselves  into  two  great  classes.  As  it  ap- 
pears to  adult  experience,  not  only  the  unfolding,  but  even  the  very 
existence  of  self-consciousness  seems  to  involve  the  distinction  be- 
tween the  ego  and  the  non-ego — between  the  "I"  with  its  states, 
and  the  "Things"  which  it  knows  with  their  manifold  properties 
or  attributes.  Each  of  these  two  classes  of  phenomena — the  so- 
called  subjective  and  the  so-called  objective — is  inevitably  attributed 
in  consciousness  to  a  different  subject;  the  one  to  the  "I"  as  its  own 
states,  the  other  to  somewhat  left  undefined,  except  that  it  is  not 
the  "I,"  and  is  called  "matter,"  "material  substance,"  etc.  (the  un- 
known X  which  is  not  I).  It  is  only  as  involving  all  this  mental 
process  that  any  real  being  whatever,  whether  Mind  or  Thing,  is 
known  or  believed  to  exist;  but  the  mind  in  the  development  of  ex- 
perience inevitably  completes  the  process,  which  involves  the  as- 
sumption that  real  beings  do  exist,  and  that  all  these  real  beings  are 
either  "  things,"  such  as  I  know,  or  myself  and  other  conscious  be- 
ings, such  as  I  am. 

§  13.  Peculiar  and  cogent  reasons  may  be  given,  however,  which 
further  enforce  and  verify  the  assumption  of  a  real  existence  for 
the  Mind.  Repeated  attention  has  been  called  to  the  fact  that  there 
is  a  class  of  so-called  mental  faculties,  most  important  and  distinc- 
tive, for  the  distinguishing  characteristic  of  which  no  physical  anal- 
ogies or  correspondences  whatever  can  be  discovered  or  imagined. 
This  is  true  of  memory  as  active  reminiscence,  of  the  unity  of  con- 
sciousness, of  voluntary  attention,  and  of  the  relating  activity.  The 


678  REALITY  AND  UNITY  OF  THE  MIND 

existence  of  these  modes  of  mental  behavior  requires  the  assump- 
tion of  a  characteristic  real  being,  other  than  the  molecules  of  the 
brain,  to  which  they  may  be  referred.  Some  of  these  modes  of  be- 
havior are  conspicuously  unintelligible  and  meaningless  without 
granting  such  an  assumption.  For  example,  an  act  of  recollection 
involves  the  presence  in  consciousness  of  a  state  the  very  essence  of 
which  is  that  it  claims  to  represent  (or  stand  for)  an  absent  past 
state  of  consciousness.  No  way  of  verifying  this  claim  in  any  par- 
ticular instance,  which  does  not  involve  its  acceptance  in  general, 
can  possibly  be  devised.  For  all  argument  is  valid,  only  if  we  accept 
the  validity  of  some  memory.  But  every  present  act  of  memory 
on  my  part  is  a  process  in  my  consciousness,  and  that  which  it 
claims  to  represent  was  also  a  process  in  my  consciousness.  To 
recollect  the  past  state  of  another  consciousness  than  my  own  in- 
volves an  absurdity;  to  recollect  a  past  state  otherwise  than  as  rep- 
resented in  a  present  state  of  my  own  consciousness  also  involves 
an  absurdity.  Of  course,  such  reflection  upon  the  nature  of  the 
act  of  memory  affords  no  demonstration  of  the  claim  that  the  sub- 
ject of  the  present  state  is  one  and  the  same  real  being  with  the 
subject  of  the  past  state.  On  the  contrary,  all  demonstration  itself 
rests  on  this  assumption;  for  without  accepting  it  as  valid  we  could 
not  reach  the  conclusion  of  any  demonstration.  The  premises  of 
every  syllogism  are  connected  with  one  another  and  with  their 
conclusion  in  a  living  unity  of  thought,  only  on  the  assumption  that 
one  real  being  is  the  subject  of  each  of  the  thoughts  which  consti- 
tute the  syllogism.  To  "be  really,"  and  to  be  the  one  subject  of 
changing  states,  are,  therefore,  but  different  ways  of  expressing  the 
same  truth.  In  this  meaning  of  the  words:  the  soul  exists  in  reality, 
above  all  other  kinds  of  being;  because  it  alone,  so  far  as  we  know 
on  good  evidence,  knows  itself  as  the  subject  of  its  own  states;  or, 
indeed,  knows  the  states  of  which  it  is  the  subject  as  states  belong- 
ing to  itself. 

§  14.  A  similar  train  of  argument  leads  us  at  once  and  irresisti- 
bly to  an  affirmation  of  the  at  least  "phenomenal"  unity  of  the  de- 
veloped human  mind.  In  every  act  of  self-consciousness  its  unity 
is  achieved,  as  it  were,  by  the  very  form  of  its  own  activity.  Every 
act  of  memory  is  a  further  affirmation  of  the  fact  of  some  sort  of 
unity  between  the  Ego  of  the  present  and  the  Ego  of  the  past.  Mem- 
ory, of  the  "  recognitive "  type,  recognizes  the  oneness  of  the  Sub- 
ject of  all  the  conscious  states.  All  human  business  and  social 
intercourse,  all  attribution  of  conduct  and  the  moral  ideas  and  senti- 
ments of  responsibility,  all  assignment  and  distribution  of  property 
rights,  depend  upon  the  significance  and  the  value  of  the  mind's 
unity.  Even  the  much  overworked  and  misunderstood  phenomena 


MIND  AS  UNIFYING  ACTUS  679 

of  "double  consciousness"  confirm  rather  than  confute  the  same 
view.  Where  the  phenomena  are  not  proofs  of  the  unsuspected  versa- 
tility, wealth  of  ideas  and  resources,  and  unexplored  capacities, 
of  the  one  mind,  they  are  proofs  of  a  mind  disordered,  disorganized, 
or  even  degenerate.  The  very  conception  of  the  mind  at  all  is  de- 
pendent upon  this  self-unifying  actus  of  man's  conscious  mental 
life.  In  a  word,  the  experience  upon  which  the  conception  of  oneness 
is  applied  to  the  human  mind  is  that  of  our  actual  unification,  ac- 
complished and  recognized  by  the  activity  of  the  very  subject,  whose 
otherwise  disparate  states  of  existence,  and  phases  of  development,  are, 
in  fact,  united.  A  unifying  of  energies,  that  takes  place  more  and 
more  perfectly  as  the  development  of  mind  proceeds  toward  matu- 
rity and  ascertain  characteristic  type  of  the  development  of  each  indi- 
vidual mind — all  this  affords  the  indisputable  basis  for  affirming 
the  reality  of  the  mind's  unity. 

§  15.  Of  course,  most  of  the  terms  applied  to  the  unity  of  con- 
sciousness, when  carefully  examined,  turn  out  to  be  figurative,  and 
to  have  no  meaning  except  as  interpreted  over  from  entities  and  re- 
lations of  a  material  sort  into  terms  of  consciousness.  By  the 
"unity  of  consciousness"  it  cannot  be  meant  that  consciousness  is 
some  kind  of  an  entity  which  remains  one  and  unchangeable  through- 
out, like  those  atoms  which  physical  science  formerly  supposed  to 
constitute  the  whole  world  of  material  reality.  It  will  be  found, 
however,  that  no  conception  can  be  formed  of  the  unity  which  is 
supposed  to  belong  to  the  atom  without  involving  in  it  the  unity  of 
consciousness.  We  can,  indeed,  picture  to  ourselves  a  very  little 
bit  of  extended  matter,  barely  visible  under  the  highest  powers  of 
the  microscope,  which  never  changes  its  shape  or  color,  etc.,  and 
which  always  behaves  itself  in  exactly  the  same  way  under  precisely 
similar  circumstances.  But  this  mental  picture  would  itself  have 
any  unity  belonging  to  it  only  as  it  existed  in  the  unity  of  conscious- 
ness. It  is  this  unifying  mental  process  which  makes  each  "Thing" 
to  be  one  thing;  it  is  this  unification  which  imparts  to  all  else  that 
is  one  whatever  unity  it  may  have. 

When,  then,  we  speak  of  the  unity  of  consciousness  we  mean, 
first  of  all  and  chiefly,  to  call  attention  to  the  following  primary 
fact  of  experience:  All  states  of  consciousness  involve  a  reference 
of  the  state  to  an  "I,"  as  the  subject  of  the  state;  and,  in  spite  of 
the  constant  change  of  states  which  goes  on,  so  that  in  reality  the 
same  state  never  recurs,  and  even  the  same  thing  is  never  twice 
known,  all  the  states  are  somehow  understood  to  be  states  of  one 
and  the  same  subject.  This  reference  and  this  understanding  enter 
into  all  our  experience;  they  give  conditions  to  experience  and  make 
it  possible.  Whatever  changes  experience  may  be  conceived  of  as 


680  REALITY  AND  UNITY  OF  THE  MIND 

undergoing,  they,  as  conditions  of  all  possible  experience,  must  be 
conceived  of  as  remaining.  To  ask  us  to  try  to  imagine  a  mental 
state  or  act  not  involving  this  reference  and  this  understanding, 
with  respect  to  the  unit-subject  of  consciousness,  is  to  ask  us  to  try 
to  be  conscious  and  unconscious  at  the  same  time.  The  "I"  may 
become  unconscious;  that  is,  the  phenomena  of  consciousness  in 
that  connected  development  which  characterizes  the  individual  may 
cease  to  exist.  But  phenomena  of  self-consciousness  cannot  be  con- 
ceived of  without  implying  the  actual  and  accomplished  unifying 
of  the  self-conscious  Subject — the  Ego,  the  Mind. 

§  16.  This  kind  of  reality,  and  this  kind  of  unity  (call  it  "phe- 
nomenal," if  you  will),  for  the  mind  and  for  all  things,  is  all  that 
science  can  recognize,  and  all  that  it  can  prove.  But  this  is  a  kind 
of  reality,  and  a  kind  of  unity,  which  science  cannot  refuse  to 
recognize;  or  of  which  it  cannot  fairly  deny  the  sufficient  proof. 
Metaphysics,  presuming  upon  its  intimate  relations  to  the  "old 
psychology,"  has  doubtless  often  made  an  unwarrantable  use  of  the 
facts  above  mentioned.  It  has  often  declared  that  we  have  an  im- 
mediate and  indubitable  knowledge  of  the  mind  as  one  and  the 
same  real  being  in  all  acts  of  consciousness.  The  facts  have  been 
interpreted  as  though  the  case  stood  as  follows:  I  have  the  power 
to  loot  within  myself,  and  by  thus  looking  I  can  discern  what  I 
really  am.  I  immediately  know  (that  is,  know  by  the  introspective 
act  of  self-consciousness)  that  "I"  am  always,  however  my  states 
may  change,  one  and  the  same  real  being.  I  am  a  real,  self-identi- 
cal entity;  and  if  asked  how  I  know  that  I  am  all  this,  my  appeal 
is  to  the  indubitable  evidence  of  the  act  of  self-consciousness. 

The  foregoing  metaphysical  statement  of  the  case  is  by  no  means 
obviously  correct;  we  believe  it,  on  the  contrary,  to  be  exaggerated 
and  incorrect.  In  thus  overstating  the  case,  there  is  liability  that 
the  case  itself  will  be  lost.  Consciousness  carries  with  it  no  immediate 
knowledge  of  any  real  and  self-identical  being — not  even  of  that  real 
being  which  we  call  Mind  and,  with  good  reason,  assume  to  exist 
as  the  ground  or  permanent  subject  of  mental  phenomena.  Meta- 
physics is  the  science  which  treats  of  those  assumptions  that  under- 
lie all  of  our  experience  with  what  we  call  "reality."  But  it  treats 
of  assumptions  or  beliefs  such  as  we  find  do  actually  and  inevita- 
bly enter  into  all  our  experience.  The  real  existence  of  "  Things," 
whether  of  the  masses  of  matter  we  daily  test  by  the  senses,  or  of 
those  hypothetical  beings  called  atoms  which  physical  science  re- 
quires in  order  to  account  for  the  phenomena,  depends  upon  such 
assumptions.  If  it  be  admitted  that  we  cannot  be  immediately 
conscious  of  ourselves  as  real  unit-beings,  we  are  no  worse  off  than 
we  are  with  respect  to  our  belief  in  the  existence  of  any  of  the  so- 


THE  SPIRITUALITY  OF  MIND  681 

called  real  beings  of  which  all  men  suppose  the  world  to  be  com- 
posed. 

It  can  also  be  shown  that  the  case  of  the  mind  or  soul,  with  re- 
spect to  its  unity  as  a  real  being,  is  made  no  better  by  admitting 
that  an  immediate  consciousness  of  ourselves  as  such  unit-beings  is 
possible.  For  let  it  be  supposed  that  by  concentrating  all  my  at- 
tention upon  the  present  state  of  consciousness  I  most  clearly  and 
indisputably  discern  myself  as  one  real  being,  forming  the  ground 
of  that  state.  Let  it  be  supposed  that  every  half-hour  in  the  day  I 
repeat  this  mental  act.  It  would  still  have  to  be  assumed,  as  some- 
what altogether  out  of  consciousness,  that  the  real  being  discerned 
in  any  one  of  these  acts  of  introspection  is  one  and  the  same  real 
being  as  that  discerned  in  all  the  rest.  A  real  unit-being  that 
should  last  only  while  the  difficult  act  of  concentrated  introspec- 
tion was  taking  place  would  be  of  no  value  to  serve  as  a  self-con- 
scious mind.  In  fact,  such  a  unit-subject  of  the  individual  state 
would  have  no  claim  to  be  considered  as  a  real  being  at  all. 

§  17.  The  question  whether  the  mind  is  to  be  spoken  of  as  non- 
material  or  "  spiritual "  scarcely  merits  at  the  hands  of  psychological 
science  the  grave  and  lengthy  discussion  to  which  it  has  often  been 
carried.  Materiality,  as  predicated  of  any  real  being,  is  only  a  com- 
plex term  including  a  number  of  so-called  attributes,  which  are  all 
the  subjects  of  experience  only  as  belonging  to  individual  things. 
All  real  things  are  to  be  called  material  which  have  these  attributes, 
so  called.  Primarily,  as  has  been  frequently  shown  already,  the 
attributes  are  simply  modes  of  the  affection  of  the  mind  which  we 
have  learned  to  localize  and  objectify  as  belonging  to  extra-mental 
reality.  But  if  we  raise  the  question  whether  the  Mind,  too,  is 
known  to  itself  as  having  those  attributes  which  make  up  our  com- 
plex, general  notion  of  "materiality,"  no  one  would  find  it  easy  to 
think  of  giving  this  question  an  affirmative  answer.  The  mind  at- 
tributes to  "  things  "  the  qualities  of  extension,  impenetrability,  and 
all  the  various  subordinate  modifications  of  these  qualities.  It  per- 
ceives these  things  as  colored,  cold,  hot,  rough,  smooth,  etc.  But 
it  does  not  attribute  such  qualities  to  itself;  it  can  find  nothing  in 
the  modes  in  which  it  manifests  itself  to  itself  which  would  warrant 
the  application  of  similar  terms  to  these  modes  of  its  own  behavior. 

Indeed,  all  the  terms  which  do  apply  to  the  recognized  qualities 
of  mind  have  to  be  understood  as  figurative  when,  having  been 
borrowed  from  physical  relations,  they  are  made  to  apply  to  psychi- 
cal states.  Even  in  those  cases  where  the  analogy  seems  almost 
to  amount  to  an  identity,  closer  inspection  shows  that  this  seem- 
ing does  not  correspond  to  the  actual  fact.  For  example,  we  do 
attribute  quantity  to  sensations  and  feelings.  But  when  the  suffer- 


682  REALITY  AND  UNITY  OF  THE  MIND 

ing  from  pressure  becomes  more  intense,  we  do  not  regard  the 
mind  as  actually  passing,  like  some  material  thing,  under  a  heav- 
ier load  (sub-jero),  against  which  it  must  either  bear  up  or  break, 
through  the  physical  strain.  Just  so,  movements  of  the  mind  are 
not  to  be  defined  as  changes  of  its  position  with  relation  to  other 
things  in  space.  We  are,  then,  surely  warranted  in  affirming  that, 
so  far  as  the  mind  has  any  immediate  information  as  to  what  quali- 
ties should  be  assigned  to  itself  and  what  to  "things" — which  it 
always  looks  upon  as  not-itself — it  is  compelled  to  regard  itself  as 
non-material. 

As  has  already  been  pointed  out,  there  is  no  way  of  telling  what 
is  the  real  nature  of  any  existence  except  by  enumerating  its  quali- 
ties, or  those  modes  of  behavior  which  we  attribute  to  it  on  account 
of  its  affecting  our  consciousness  in  certain  definite  ways.  To  at- 
tempt to  regard  the  mind  as  material,  when  it  manifests  itself  co 
itself  as  non-material,  compels  us  either  to  use  the  word  "  material " 
in  an  unwonted  and  unauthorized  way,  or  else  to  attribute  to  matter 
in  general  certain  occult  powers  which  it  never  manifests  itself  to 
the  mind  as  possessing,  and  which  make  it  really  to  be  quite  differ- 
ent from  what  its  manifestation  of  itself  would  indicate. 

The  only  way  of  maintaining  the  materiality  of  mind  would  then 
appear  to  be  that  of  denying  its  real  existence  at  all,  and  of  attrib- 
uting its  phenomena  to  the  material  molecules  of  the  brain  as  their 
real  and  material  substratum  or  basis.  But  the  untenable  nature 
of  this  view  has  already  been  sufficiently  indicated.  Or  perhaps  a 
strong  temptation  may  be  again  felt,  at  this  point,  to  recur  to  the 
hypothesis  of  a  third  somewhat,  a  "  two-faced  unity,"  which  is  the 
ground  of  the  phenomena  of  both  body  and  mind.  But  such  hypoth- 
esis can  throw  no  light  whatever  on  the  inquiry  whether  the  mind 
is  material  or  non-material.  The  phenomena  we  call  "mental," 
and  attribute  to  the  subject  of  consciousness,  would  remain  just  as 
radically  unlike  those  which  we  call  "physical,"  and  attribute  to 
matter,  after  making  the  hypothesis  as  before.  And  to  the  hypoth- 
esis itself  the  same  objections  would  remain  opposed. 

The  negative  conclusion  that  mind  is  non-material  is  quite  in- 
evitable for  every  one  who  admits  that  mind  is  a  real  being  with 
any  nature  whatever. 

§  18.  It  is  not  difficult,  also,  to  show  that  we  must  make  the  cor- 
responding positive  statement,  and  affirm  the  spirituality  of  mind. 
This  we  can  do  with  confidence,  however,  only  so  long  as  we  mean 
by  the  term  "  spirituality"  simply  to  sum  up  and  express  in  one  word 
the  list  of  attributes  which  describe  the  known  activities  of  mind. 
To  perceive,  feel,  think,  will — in  brief,  to  be  conscious  in  some  one 
of  the  various  forms  of  conscious  life — this  is  to  be  positively  spirit- 


MIND  AS  A  UNITY  OF  GROWTH  683 

ual,  in  the  only  sense  in  which  science  is  entitled  to  affirm  spiritual- 
ity of  mind  as  such.  As  soon  as  we  conceive  of  spirituality  as  some 
ethereal  extension  of  thinking  substance,  we  enter  upon  the  vain 
effort  to  conceive  of  mind  under  terms  of  matter,  and  at  the  same 
time  escape  the  consequences  of  so  conceiving  of  it.  Nor  can  we 
hope  to  vindicate  for  the  mind  such  spirituality  as  would  be  implied 
in  its  being  freed  from  all  relations  to  material  things,  or  from  de- 
pendence for  the  modes  of  its  being  upon  the  material  substratum 
of  the  brain.  How  spirit,  in  the  sense  of  disembodied  or  unem- 
bodied  mind,  would  perceive,  and  feel,  and  think,  and  will,  is  a  ques- 
tion toward  the  answer  to  which  we  can  make  no  beginning.  But 
to  control  the  mental  train  as  distinguished  from  being  a  passive 
member  of  a  psycho-physical  mechanism,  to  reason  so  as  to  deduce 
conclusions  and  make  inductions  to  general  laws,  to  recognize  the 
call  of  duty,  and  to  call  up  and  classify  in  the  consciousness  the 
lofty  and  complex  ideas  of  art  and  religion — these  and  other  similar 
operations  of  the  mind  pre-eminently  emphasize  its  spirituality. 

§  19.  In  somewhat  the  same  way  must  it  be  admitted  that  the 
question  of  the  unity  of  mind  has  given  rise  to  much  fruitless  and 
by  no  means  altogether  pertinent  debate.  The  attempt  to  conceive 
of  the  mind  as  a  unit-being,  constituted  after  the  analogy  of  those 
physical  structures  which  we  are  accustomed  to  regard  as  unities, 
inevitably  leads  to  confusion  and  error.  The  important  psycho- 
logical fact  is,  that  there  is  no  one  of  these  physical  unities  which 
does  not  derive  its  unity  from  the  unifying  actus  of  the  mind.  This 
statement  is  true  of  each  such  so-called  unity,  whether  it  is  per- 
ceived as  one  or  is  conceived  of  as  one.  The  unity  which  belongs 
to  the  percept  finds  its  source  in  the  synthetic  activity  of  the  per- 
ceiving mind;  the  unity  of  the  conception,  in  the  unifying  activity 
of  the  mind's  relating  faculty.  It  is  sometimes  supposed,  however, 
that  an  atom  which  should  have  no  parts,  be  perfectly  homogeneous 
throughout,  and  so  incapable  of  changes  of  its  interior  states,  would 
be  the  highest  possible  type  of  a  unity  of  real  being.  Nothing 
could  ever  happen  to  disturb  or  destroy  such  a  unity.  The  tempta- 
tion has  therefore  arisen  to  conceive  of  the  mind  after  the  analogy 
of  a  thinking  atom. 

§  20.  Now,  it  must  be  admitted  that  such  a  thinking  atom  would 
be  in  far  less  danger  of  suffering  from  the  death  of  the  physical  basis 
of  its  thought  than  is  the  thinking  man.  But  two  considerations  of 
great  importance  are  likely  to  be  overlooked  in  the  mere  making  of 
the  hypothesis  of  such  an  atom.  Surely  such  an  atom  could  hardly 
have  any  experience  corresponding  to  what  we  call  the  unity  of 
our  consciousness;  and  if  it  had  any  unity  of  consciousness  what- 
ever, such  unity  could  no  more  be  explained  as  arising  out  of,  or 


684  REALITY  AND  UNITY  OF  THE  MIND 

conditioned  upon,  the  simplicity  of  the  physical  being  of  the  atom 
than  the  unity  of  our  consciousness  can  be  explained  as  arising  out 
of,  or  conditioned  upon,  the  complexity  of  our  physical  being. 

It  is  impossible  to  see  how  a  unity  of  consciousness  at  all  resem- 
bling what  we  understand  by  the  term  could  find  an  adequate  ma- 
terial substratum  in  a  single  rigid  atom.  In  other  words,  if  a  spirit- 
ual being  having  a  unity  of  consciousness  were  brought  into  special 
psycho-physical  relations  with  a  material  being  incapable  of  any 
interior  changes,  because  possessed  of  no  parts  to  undergo  change, 
these  relations  would  have  to  be  totally  different  from  any  which 
we  can  conceive  of  as  holding  between  the  body  and  mind  of  man. 
For  the  very  nature  of  the  mind's  unity  is  dependent  upon  that 
variety  of  experiences  which  is  occasioned  in  the  mind  through  the 
changing  states  of  the  brain.  The  physical  basis  of  the  human 
mind  is  undoubtedly  an  extremely  complex  system  of  interacting 
molecules.  Certain  relations  can  be  traced  between  the  character 
of  these  physical  interactions  and  the  character  of  the  states  arising 
in  the  mind.  These  states  depend  for  their  character,  and  even  for 
their  very  existence,  upon  the  occurrence  of  the  corresponding 
material  changes.  A  brain  that  is  not  in  a  ceaseless  change  of  ac- 
tivities of  the  peculiar  sort  called  "neural"  is  a  dead  brain,  so  far 
as  its  influence  on  the  mind  is  concerned;  such  a  brain  could  not 
serve  as  the  substratum  or  physical  cause  of  mental  phenomena. 

Moreover,  comparative  anatomy  shows  us  that  the  greater  the 
number  of  molecules,  and  the  larger  the  variety  and  the  size  of  the 
organs  specially  related  to  the  mental  processes,  the  richer  in  vari- 
ety and  nobler  in  quality  the  mental  processes  themselves  become. 
So  far  as  we  can  ascertain,  the  highest  unity  of  consciousness  be- 
longs in  connection  with  the  greatest  complexity  of  the  material 
substratum.  The  animals  which  have  the  largest  cerebral  develop- 
ment appear  to  have,  too,  not  only  the  most  manifold  and  extensive 
mental  life,  but  also,  in  the  highest  degree,  the  capacity  for  attrib- 
uting the  phenomena  of  that  life  to  one  subject.  Those  psychical 
activities  which  are  connected  with  the  physical  interaction  of  the 
greatest  number  of  material  elements  are  the  most  numerous  and 
significant;  and  they  are,  also,  actually  most  perfectly  harmonized 
into  a  higher  unity  of  spiritual  self-conscious  being. 

No  information  derived  from  the  study  of  Physiological  Psy- 
chology warrants  us  in  affirming  that  a  highly  developed  self-con- 
scious existence  must,  from  the  universal  necessities  of  the  case,  be 
united  with  a  vastly  complex  material  structure  like  the  human 
brain.  Such  study  does,  however,  compel  us  to  affirm  that  such 
a  unity  in  variety  as  is  the  human  mind  cannot  be  conceived  of 
in  dependence  upon  the  movements  in  space  of  a  single  perfectly 


MIND  AS  A  UNITY  OF  GROWTH  685 

rigid  and  unchanging  atom.  The  development  of  human  experi- 
ence is  conditioned  upon  the  arising  in  consciousness  of  many 
sensations  of  varied  quantities,  qualities,  and  orders  in  time;  upon 
the  synthesis  of  these  sensations  into  presentations  of  sense;  and 
upon  the  recall  of  the  presentations  in  the  form  of  representative 
ideas.  What  experience  would  be,  if  its  basis  were  not  laid  in 
such  rise  and  combination  and  recurrence  of  sensations,  we  cannot 
even  conjecture.  In  the  highest  flights  of  imagination,  in  the  pro- 
foundest  explorations  of  reflection,  we  never  wholly  escape  from  the 
influences  arising  in  this  basis.  The  nature  of  this  psychical  basis 
of  sensation  and  perception  depends  upon  the  nature  of  the  physi- 
cal basis  of  the  living  and  acting  brain.  In  other  words,  what  sen- 
sations and  perceptions  constitute,  at  least  in  part,  the  "stuff"  of 
all  consciousness,  depends  upon  what  the  molecules  of  the  central 
nervous  system  are  doing.  We  cannot  even  conceive  of  any  other 
relations  as  possible  between  the  mind,  on  the  one  hand,  and  the 
brain,  on  the  other,  than  relations  between  a.  system  of  moving 
molecules  and  a  corresponding  change  of  conscious  states. 

Furthermore,  the  unity  of  a  single  indestructible  and  eternally 
unchanging  atom  would  afford  no  explanation  of  a  mental  unity. 
In  the  case  of  man's  mind  and  brain,  the  variety  of  the  nervous 
changes  in  part  explains  the  variety  of  the  mental  states;  but  nothing 
in  the  changing  relations  of  the  innumerable  moving  molecules 
throws  any  clear  light  on  the  origin  of  the  unity  of  mind  in  con- 
sciousness. A  material  being  absolutely  without  distinction  of  parts 
would  be,  for  that  fact,  no  better  fitted  to  become  conscious  of 
itself  as  one.  A  series  of  states  of  consciousness  can  indeed  be 
attributed  by  our  imagination  to  such  a  being.  From  the  purely  psy- 
chological point  of  view  we  can  conceive  of  the  unit-atom  as  having 
an  experience  resembling  our  own.  We,  in  our  consciousness,  can 
imagine  such  a  being  as  the  subject  of  states,  and  as  attributing 
each  of  these  states  to  one  and  the  same  subject — namely  the  "I" 
of  the  unit-atom — after  the  fashion  of  our  customary  mental  be- 
havior. But  this  is  quite  a  different  thing  from  explaining  the  con- 
sciousness of  such  an  atom  as  arising,  with  respect  to  its  unity,  out 
of  the  material  nature  of  the  atom.  By  the  very  hypothesis,  the 
material  nature  of  this  particular  kind  of  atom  can  have  no  states; 
it  never  changes;  it  is  always  the  same.  But  consciousness  is  al- 
ways some  particular  definite  state;  and  self-consciousness  is  always 
the  being  aware  of  some  particular  definite  state.  There  is  no  con- 
sciousness in  general;  there  is  no  consciousness  which  does  not  in- 
volve change  of  state.  Indeed,  change  is  a  reality  in  human  con- 
sciousness, if  nowhere  else  in  the  universe  of  being.  No  particular 
state  of  consciousness,  whether  considered  as  involving  an  attribu- 


686  REALITY  AND  UNITY  OF  THE  MIND 

tion  of  that  state  to  a  subject  or  not,  could  be  explained  by  reference 
to  the  material  nature  or  condition  of  such  a  unit-atom. 

§  21.  The  foregoing  remarks  have  their  value  chiefly  as  a  warn- 
ing against  supposing  that  the  unity  of  the  soul  suffers  any  preju- 
dice because  it  is  not  to  be  regarded  or  explained  from  a  point  of 
view  furnished  by  physical  analogies.  To  be  one,  as  a  rigid  mate- 
rial atom  may  possibly  be  regarded  as  one,  would  be  of  no  advantage 
to  the  soul.  Or  if  it  be  admitted  that,  in  case  it  had  such  unity,  it 
could  never  cease  to  exist,  it  must  also  be  admitted  that  we  are  un- 
able to  see  how  it  could  ever  begin  to  exist  as  a  self-conscious  mind. 
If  the  unit-atom  could  never  die,  it  could  also  never  live — as  a  con- 
scious psychical  existence.  And  it  is  the  unity  which  the  mind 
plainly  has  in  self-consciousness  that  is  alone  worth  contending  for. 
If  the  mind  were  really — that  is,  regarded  as  out  of  its  own  con- 
sciousness— one,  and  yet  two  or  more  in  consciousness,  it  would 
be  no  better,  but  rather  the  worse  off.  If  it  were  really  one,  but 
were  obliged  not  to  know  itself  as  one,  and  could  never  be  aware 
of  its  own  states,  or  attribute  them  to  the  one  "I"  which  is  the 
subject  of  them  all,  it  would  surely  be  the  worse  off.  To  be  one,  in 
the  only  meaning  of  the  word  that  is  of  real  value,  is  to  have  and 
to  keep  the  unity  of  consciousness.  If  this  unity  were  really  a  mere 
seeming — a  trick  of  nature  to  cheat  the  mind — the  seeming  would 
forever  seem  real,  would,  indeed,  be  the  ground  of  all  reality;  the 
trick  would  be  the  kindest  of  all  illusions,  and  one  from  which  we 
should  crave  never  to  be  set  free.  When,  then,  we  have  recognized 
the  fact  that  all  ordering  and  development  of  human  consciousness 
implies  this  kind  of  unit-being  as  belonging  to  the  mind,  we  have 
gone  as  far  in  vindication  of  the  mind's  rights  as  we  have  any  psy- 
chological interest  in  going. 

§  22.  It  would  seem,  finally,  as  though  our  excursion  thus  far 
into  the  debatable  field  of  metaphysical  discussion,  might  commend 
itself  to  any  unprejudiced  and  critical  student  of  those  phenomena, 
which  we  have  called  "  correlations,"  existing  as  matter-of-fact  ex- 
periences, between  the  human  nervous  mechanism  and  the  human 
mental  life.  The  conclusions  warranted,  in  our  judgment,  by  the 
phenomena,  are  by  no  means  a  final  settlement  of  the  metaphysical 
problems  involved.  As  to  the  "ground"  of  these  correlations,  and 
as  to  how  it  is  related  to  that  Being  of  the  World,  which  science  is 
accustomed  to  speak  of  as  Nature,  and  which  the  different  schools 
of  philosophy  have  conceived  of  in  various  ways,  as  to  the  first  and 
last  things  of  mind,  psychology  as  studied  from  the  physiological 
and  experimental  points  of  view  finds  itself  unable  to  pronounce. 
It  cannot,  indeed,  explain  the  entire  being  of  the  mind  as  arising 
out  of  the  development  of  the  physical  germ  from  which  the  bodily 


LIMITS  OF  PHYSIOLOGICAL  PSYCHOLOGY         687 

members  unfold  themselves.  It  knows  no  decisive  reason  against 
the  belief  that  such  a  non-material  and  real  unit-being,  as  the  mind 
is,  should  exist  in  other  relations  than  those  which  it  sustains  at 
present  to  the  structure  of  the  brain.  On  the  contrary,  it  discloses 
certain  phenomena  which  at  least  suggest,  and  perhaps  confirm, 
the  possibility  of  such  existence  for  the  Mind.  But,  in  general, 
if  it  remain  faithful  to  its  own  mission,  within  its  own  limits,  it  en- 
trusts the  full  consideration  of  these  questions,  after  it  has  cleared 
the  way  from  barriers  of  ignorance  and  prejudice,  to  Rational 
Psychology,  to  Ethics,  to  Metaphysics,  and  to  Theology. 


INDEX 


INDEX    OF   AUTHORS 


ACH,  482,  484,  486  ff.,  496 

Alcmaeon,  213 

Aliotta,  379,  498 

Allen,  505 

d'Allonnes,  526 

Alrutz,  344 

Alsberg,  366 

Angell,  F.,  569 

Angell,  J.  R.,  396,  486,  569 

Angler,  409 

Apathy,  103,  106,  113 

Aristotle,  214 

d'Arsonval,  131 

Atwater,  291 

Aubert,  325,  331,  365,  370 

Auerbach,  476,  491  ff. 

BAILLARGER,  269 

Bain,  505,  515 

Bair,  568  f.,  580 

Baird,  335 

Baldwin,  486,  501 

Bang,  120 

Barker,  100 

von  Bayer,  137 

Bayliss,  151 

Beaunis,  478 

von  Bechterew,  59,  88 

Beer,  188 

Beevor,  236 

Bentley,  601 

Benussi,  442,  451 

Berger,  G.  O.,  479 

Berger,  H.,  509 

BergstrOm,  580 

Bernhardt,  118 

Bernstein,  403 

Bethe,  18  f.,  21,  103,  106,  113,  288,  290 

Beyerman,  262 

Bidder,  475 

Biedermann,  131,  133,  137,  363 

Bielschowsky,  106,  265 

Billings,  508 

Binet,  401,  535,  577 

Bingham,  530 

Birge,  75 

Blix,  344 

Boll,  116,195 

Bolton,  262 

Bonnet,  597 

Book,  556,  558  f.,  561  f.,  564,  569,  577 


Boruttau,  140,  142 
Bouguer,  369 
Bouillaud,  255 
Bourdon,  427,  429 
Bourgery,  66 

Bowditch,  136,  171  f.,  453 
Breese,  599 
Breitwieser,  480,  486 
Breuer,  210  f. 
Broca,  227,  253  ff. 
Brodhun,  370 
Brodie,  136 

Brodmann,  266,  270,  272 
Brown,  C.,  210 
Brown,  E.  M.,  565 
BrQcke,  392 
BrUckner,  400 
Bruner,  315,  368 
Bryan,  556  ff.,  561  f. 
Buccola,  478 
Budge.  227 
Buhler,  601 
Burch,  131 
Burnham,  574 
Byasson,  215 

CAJAL,  41,  43  f.,  47,  98,  102,  106,  109,  112, 

116,  193,  265  ff. 
Calkins,  573,  583,  592 
Camerer,  372 
Cameron,  318 

Campbell,  240,  245,  261,  270  ff. 
Cannon,  525 
Caton,  230 
Cattell,  132,  359,  371,  376,  478  f.,  484,  489. 

494,  496,  498,  537,  591,  597 
Charpentier,  524 
Chevreul,  310 
Chodin,  432 
Ciaccio,  183 
Cline,  459  f. 
Cole,  547 
Conty,  524 
Cook,  440 
Coover,  569 
Corti,  203  ff. 
Cowling,  595 
Cox,  643 
Cramer,  124 
von  Cyon,  209,  524 
Czermak,  402,  406 


INDEX  OF  AUTHORS 


D ANILE  WSKI,  118,  230 

Davies,  349 

Davis,  565 

Dax,  255 

Dearborn,  461,  568 

Debrou,  177 

Deiters,  83,  95,  97 

Dejerine,  84,  539 

Delabarre,  409,  459 

Delboeuf,  370,  378,  437 

Descartes,  634 

Despretz,  315 

Dewey,  147,  603,  610 

Diaconow,  123 

Dobrowolsky,  329 

Dodge,  457,  459  f. 

Dogiel,  180 

Dolley,  132,  478 

Donaldson,  58,  60,  62,  102,  344,  404 

Donders,  331,  432,  467,  470,  472,  476 

Dubois,  35 

Du  Bois-Reymond,  130  f.,  141  f. 

Durig,  136 

Duval,  289 

EBBINGHAUS,  378,  438  f.,  502,  510,  572  ff.f 

579,  588,  604 

Ebert,  566,  568,  574,  578,  582 
Ecker,  27,  176 
Eckhard,  116,  176,  227 
Edelmann,  315 
Edes,  136 
Edinger,  26,  29,  31,  32,  59,  62,  63,  85,  108, 

221  f. 

Ehrenfels,  601 
Einthoven,  131 
Engelmann,  149,  178  f. 
Ephrussi,  574,  578 
Erb,  73 
Eternod,  39 
Eulenberg,  365 
Ewald,  119,  150,  153,  207  ff. 
Ewing,  105,  539 
Exner,  167,  170,  214,  472  ff.,  478  f.,  483  f., 

524 

FECHNER,  9, 356,  358, 360  ff.,  366, 370  f.,  373, 

375,  377  f.,  504,  565 
Fe're',  509,  533 

Ferrier,  158,  228,  235  ff.,  246,  251 
Fick,  325,  327,  329  ff.,  339,  342,  474 
Filehne,  449 
Fite,  396 
Flateau,  539 

Flechsig,  59,  224,  ff.,  248,  250  f. 
Flourens,  208,  227  ff. 
Flournoy,  486 
Fowler,  305 
Fracker,  567 
Frankel,  118,  121,  124 
Franz,  263,  349 
Fraser,  446  f. 
Fraunhofer,  329 
Freud,  585  f. 

von  Frey,  181,  344,  365,  400,  402 
Fritsch,  228  f.,  235,  255 
Froeberg,  479  f. 
Fullerton,  359,  371,  376 
Funke,  399,  402,  517 


GALEN,  215 

Gall,  227,  232 

Galton,  315,  583 

Gamble,  592 

Gamgee,  123 

Garten,  136 

Gaskell,  149 

Gaupp,  27 

Gegenbaur,  184,  197,  203,*223 

Gennari,  269,  271 

Gerlach,  112 

Gieke,  116 

Gies,  121,  124 

Gley,  508 

Goethe,  521 

Goldscheider,  344  ff.,  349,  364,  366  f.,  398  ft, 

406,  477,  539 
Golgi,  97,  103,  116 
Goltz,  150,  152  f.,  155 
Gotch,  131,  133 
Gothlin,  143 
Griesbach,  401 
Grunbaum,  A.  A.,  601 
Griinbaum,  A.  F.,  228,  236  ff.,  272 

HAGGERTY,  547 

Haines,  403 

Hall,  404  f.,  408,  453 

Haller,  228 

Halliburton,  118,  124,  136 

Hamilton,  F.  M.,  595 

Hamilton,  G.  Van  T.,  554 

Hamilton,  W.,  515,  597 

Hammersten,  124 

Hankel,  476 

Hardesty,  75,  207 

Barter,  556  ff.,  561  f. 

Haycraft,  308 

Head,  148,  348  f.,  401 

Heidenhain,  524 

Held,  59 

Helmholtz,  132,  188  f.,  194,  197,  199,  206  ff., 

312  ff.,  319,  322,  326,  328,  332,  339,  341, 

370,  421,  423,  428  f.,  434,  456,  462,  467, 

540,  598 
Henderson,  582 

Henle,  71,  75,  114,  194,  197  f.,  201  f. 
Henmon,  360,  490 
Henschen,  248  f. 
Hensen,  205,  312  f.,  317 
Herbart,  511  f. 
Hering,  E.,  285,  339,  341  ff.,  346,  352,  363, 

416,  421,  426,  432,  434,  445  f.,  452 
Hering,  H.  E.,  148,  241 
Hermann,,  140  ff. 
Herophilus,  215 
Herschel,  331,  369 
Hiilebrand,  427 
Hippocrates,  214 
Hirsch,  472,  476 

His,  36,  40,  52  ff.,  97  f.,  112,  116 
Hitzig,  228  f.,  235,  255 
Hober,  312 
Hodge,  539 
Hoffmann,  410 
Hollingworth,  409 
Holm,  347 
Holmes,  240 
Hooke,  326 


INDEX  OF  AUTHORS 


691 


Horsley,  236 
Horwicz,  511,  513 
Hough,  537 
Howell,  129 
Huey,  459,  461 
Humboldt,  305 

INGBERT,  75 

JACOBS,  574 

James,  211,  403,  525,  566,  569,  618 

Janet,  535 

Janssens,  240 

Jastrow,  446,  448,  467,  480,  488 

Jennings,  217,  544  f. 

Jost,  573,  581 

Joteyko,  538 

Judd,  400, 402, 442, 445, 451,459, 480, 569, 595 

Jung,  509 

KAES,  62 

Kafka,  475 

Kammler,  365 

Kampfe,  367 

Kant,  524,  662,  672 

Kastle,  312 

Kennedy,  574 

Keppler,  372  f. 

Kiesow,  132,  312,  344,  346,  365,  477  f. 

Kinnaman,  547,  549,  555 

Kirkpatrick,  544,  574,  581 

Klug,  406  f. 

Koch,  119,  122 

Kolliker,  102,  110,  116,  204,  265,  326 

Konig,  370 

Kopsch,  81 

Kraepelin,  538 

Kreidl,  208 

von  Kries,  196,  302,  331,  340,  476,  491  ff. 

Krueger,  323 

Kuhne,  119,  195,  212 

Kttlpe,  592,  601 

Kundt,  438 

Kunkel,  475 

Kussmaul,  257 

LADD,  243,  339  f.,  410,  464,  466,  523,  525, 

527,  531,  542,  579,  584,  597 
Lange,  C.,  525 
Lange,  L.f  484,  486 
Lange,  N.,  600 
Langelaan,  262 
Langley,  J.  N.,  150 
Langley,  S.  P.,  371 
Learning,  101,  266 
Le  Conte,  434 
Lee,  538 

Lehmann,  508,  510,  600 
von  Lenhossgk,  98,  115 
Leuba,  560,  569,  581 
Lewis,  B.,  269 
Lewis,  E.  O.  451 
Liebreich,  123 
Liepmann,  254,  259  f.,  263 
Lindemann,  366 
Lindley,  534 
Linnaeus,  307 
Lipps,  449 
Listing,  421 


Loeb,  409 

Lombard,  171,  215,  532 

Longet,  227 

Lotze,  302,  317,  384,  398,  412,  432,  436,  456, 

511 

Lowit,  363 
Luciani,  156 

MACDONALD,  142 

Mach,  210,  312,  475 

Magendie,  227 

Mandelstamm,  329 

Marburg,  46,  83  f. 

Margo,  212 

Marie,  258  ff. 

Marshall,  510 

Martin,  602 

Matteuci,  227 

Mauss,  270 

May,  240 

McAllister,  460  f.,  480 

McDougall,  338,  475,  598,  611,  618  ff. 

McMurrich,  78,  220 

Mendelejeff,  308 

Merkel,  489 

Messenger,  402 

Metzner,  400,  402 

Meumann,  566,  568  f.,  571,  574,  578,  582 

Meyer,  A.,  258,  260 

Meyer,  M.,  207,  510 

Meynert,  227,  510 

Mihalcovics,  49 

Mill,  J.  S.,  384 

Mobius,  548 

Moldenhauer,  478 

von  Monakow,  159,  242  f.,  245  f.,  248  f.,  258, 

260  f. 

Montague,  610 
Moore,  A.  W.,  486 
Moore,  J.  M.,  537 
Moore,  T.  V.,  481 
Morgan,  548 
Mosso,  536 
Moutier,  258 
Muller,  G.  E.,  317,  358,  361,  379,  573,  578  f., 

582,  584  ff.,  591,  602 
MQller,  J.,  132,  284 
Muller,  J.  J.,  330 
Mttller-Lyer,  439,  441 
Munk,  242,  246,  252 
Munsterberg,  486,  532,  580,  610 
Myers,  395  f.,  403,  574 

NAGEL,  208,  305,  307,  310 
Nahlowsky,  522 
Nelson,  510 
Newton,  331,  340,  662 
Nissl,  102  f. 
Nothnagel,  366 

OETTINGEN,  312 
Ogden,  578 
Ohms,  582,  584 
Oppel,  437 
Osthoff,  517 

PARKER,  17 
Pearce,  439  f.,  442 
Peckham,  545 


692 


INDEX  OF  AUTHORS 


Pentschew,  578 

Peters,  588 

Peterson,  509 

Pfaff,  305 

Pfluger,  139,  216 

Pick,  254 

Pierce,  445  f. 

Pie*ron,  544 

Pillsbury,  349,  569,  596,  603,  606 

Pilzecker,  573,  584,  ff.,  591 

Piper,  473 

Plato,  214 

Plutarch,  213 

Pohlmann,  574 

Porter,  547 

Posner,  121,  124 

Preyer,  312,  315,  317,  474 

Purkinje,  194 

Pythagoreans,  320  f. 

RADOSSAWLJEWITSCH,  572,  576 

Ranvier,  180 

Rauber,  81 

Rayleigh,  368,  395 

Reid,  515 

Remak,  97 

Retzius,  23,  77,  182,  203  ff. 

Reuther,  574 

Richards,  312 

Ritter,  305 

Rivers,  348 

Robertson,  440 

Rosenheim,  124 

Rosenthal,  148 

Rouget,  212 

Royce,  501 

Ruediger,  461 

Ruger,  554  f.,  569,  605 

SANDER,  475 

Sappey,  87 

Schaefer,  396 

Schafer,  63,  98  f.,  107,  249,  479 

Schenck,  187,  537 

Schiff,  215,  227,  517 

Schrader,  152,  158 

Schroder,  240 

Schroeder  van  der  Kolk,  643 

Schultze,  176  f.,  190  ff.,  194 

Schumann,  573,  578  f.,  582,  601  f. 

Schuster,  262 

Schwalbe,  68,  71,  79,  80,  82 

Scripture,  537,  565 

Sergi,  525 

Shepard,  508,  526 

Sherren,  348,  401 

Sherrington,  147,  152,  154,  156,  159,  162  ff., 
166  ff.,  170,  181  f.,  228,  236  ff.,  241,  272, 
287,  289,  344,  365,  477, 524  f.,  552,  599,  612 

Sidis,  510 

Slaughter,  600 

Small,  547 

Smith,  S.,  545 

Smith,  T.  L.,  565 

Smith,  W.  G.,  442,  480,  677 

Sobotta,  78,  220 

Solomons,  402 

Sowton,  136,  442 

Spalding,  157 


Spaulding,  546 

Spearman,  401 

von  Spee,  38 

Spitzka,  61 

Spurzheim,  227 

Starch,  393,  395,  509 

Starling,  151 

Starr,  72,  101,  224,  266 

Steele,  480 

Steffens,  Laura,  602 

Steffens,  Lottie,  578 

Stein,  535 

Steinach,  477 

Stern,  591 

Storey,  539 

Starring,  606 

Stout,  543 

Stratton,  365,  455,  459,  461 

Streeter,  50 

Strieker,  317 

Strong,  36,  63,  101,  266 

Strttmpell,  88 

Stumpf,  312,  315  ff.,  321  f.,  378,  416,  510 

Swift,  561 

Symington,  63 

TANZI,  477 

Tarchanoff,  509 

Tawney,  402 

Tebb,  124 

Tesla,  131 

Theophrastus,  213 

Thie-ry,  449 

Thorndike,  33,  637,  546  ff.,  569,  573 

Thudicum,  120,  122  ff. 

Thunberg,  181,  344,  347,  403,  479 

Titchener,  350,  358,  378,  471,  480,  502,  510, 

601 

von  Tobel,  537 
Trautscholdt,  494 
Treves,  537  f. 
Triplett,  633,  547  f. 
Trotter,  349 
Turnbull,  315 
Tyndall,  328 

URBANTSCHITSCH,  600 
Urstein,  591 

VALENTIN,  305,  373  f.,  392,  474 

Valsava,  200 

Van  Biervliet,  574 

Van  Deen,  227 

Van  Gehuchten,  76,  103  f. 

Veraguth,  509 

Veratti,  108 

Verworn,  15,  290 

Vierordt,  329,  398  f. 

Vigual,  116 

von  Vintschgau,  392,  475,  477 

Voeste,  337 

Vogt,  C.,  236 

Vogt,  O.,  236,  248,  250,  267,  270  f. 

Volkmann,  369  ff.,  401,  565 

Volkmann  von  Volkmar,  413,  470,  511 

Volta,  305 

WAGNER,  97,  523 
Waldeyer,  112 


INDEX  OF  AUTHORS 


693 


Waller,  137 

Ward,  584,  594 

Warnecke,  38 

Warren,  171  f. 

Washburn,  544 

Watson,  59,  62,  547 

Watt,  494  f. 

Weber,  E.,  509 

Weber,  E.  H.,  304,  326,  346  f.,  360,  363, 

366,  371  f.,  394,  396  ff.,  404  f.,  474 
Weigert,  264 
Wernlcke,  257  ff. 
Whipple,  601 
Wien,  368  f.,  371 
Wilson,  395 
Winch,  571 
Wirth,  596 


Witasek,  582 

von  Wittich,  474,  476,  479 

Woodworth,  157,  350,  535,  537,  552,  569,  594 

Wright,  533 

Wundt,  10,  274,  290;  338,  376,  384,  397, 
410,  415,  421  ff.,  425,  427,  432,  448,  456, 
462,  476,  478  f.,  481  ff.,  501  f.,  506  f.,  516, 
526,  594,  601,  610 

YERKES,  547  f. 
Yoakum,  541 
Young,  341 

ZlEHEN,   511 

Zinn,  228 
Zoth,  421,  429 
Zwaardemaker,  307  f.,  374 


INDEX   OF  SUBJECTS 


ABSOLUTE  PITCH,  317 

Abstraction,   595  ff.;    in  perception,   596; 

in  recall,  600  f. 
Accommodation  of  the  eye,  188,  325,  414, 

417,  423,  427;   to  pitch,  200 
Achromatopsia,  253 
Action  theory  of  consciousness,  610  f. 
Action  time,  333,  474  f. 
Acuity,  357;   of  hearing,  368  f.;   of  muscle 

sense,  364,  409;  of  smell,  373  f.;  of  taste, 

372  f.;  of  temperature  sense,  365  f .,  406  f . ; 

of  touch,  365,  397,  408;    of  vision,  326, 

371  f.,  427  ff.,  461 
Adaptation,  545  f.,  570;  of  the  retina,  195  f., 

333,  337,  371,  378,  540;    of  other  sense 

organs,  308,  346  f.,  378,  540;    of  central 

organs,  540 
Adjustment,  482  ff.,  489,  491,  494,  551,  555, 

569,  582,  589  f.,  602,  605  f.,  621  f.;  neural 

mechanism  of,  624 
Adolescence,  661 
^Esthetics,  504 
Affection,  500  ff. 
After-brain,  48  ff. 
After-discharge,  166 
After-image,  460,  475,  587;    negative,  337, 

343;   positive,  338 
Ageusia,  311 
Agraphia,  254  f.,  261 
Alexia,  252 

Alliance  of  reflexes,  169,  172  f. 
Ambiguous  figures,  594,  620 
Amnesia,  253,  260,  544 
Amo3ba,  behavior  of,  14  f.,  64,  544,  612 
Amphibia,  brain  of,  27,  29  ff.;   intelligence 

of,  547 

Ampulla,  201,  203  f.,  210 
Amputation,  218 
Amusia,  250,  253 
Analgia,  517 
Analysis,  186,  302,  307,  310,  312  f.,  319,  326, 

330,  341,  343  f.,  350  ff.,  353,  380,  440, 

450  ff.,  458,  500,  551  ff.,  569,  605  ff.;  de- 
pendent on  association  by  similarity,  589; 

in  perception,  596  ff.;  neural  mechanism 

of,  624  f. 
Anastomosis  of  branches  of  nerve-cells,  18, 

21,  112  ff.;   of  nerves,  243,  285  f. 
Anesthesia,  244  f.,  464,  517,  526 
Anesthetics,  120,  287,  289  ff. 
Angle  illusion,  442  ff. 
Annelids,  nervous  system  of,  20  ff. 
Anosmia,  306,  308 
Anthropoid  apes,  brain  of,  33  ff.,  228  f., 

236  ff.;  intelligence  of,  552  f. 
Anvil,  197  fit. 


Aphasia,  253,  255  ff.,  544,  663;  varieties  of, 
256  ff. 

Apoplexy,  93 

Apperception,  380,  452,  483  ff.,  487,  497 
673 

Apprehension,  376,  436,  442,  444,  488,  491, 
597 

Apraxia,  253  f.,  263 

Aqueduct,  49,  78,  83  f. 

Aqueous  humor,  183  f.,  186  f. 

Arachnoid,  70 

Archipallial  system,  95 

Archipallium,  31  f.,  86,  219,  223,  250 

Area,  judgments  and  illusions  of,  447  f. 

Area  of  the  cortex,  functional,  235  ff.;  his- 
tological,  268  ff.;  auditory,  249  f.,  272; 
of  Broca,  255,  257  ff.;  gustatory,  251;  in- 
tellectual, 251,  262;  motor,  228,  232, 
235  ff.,  254,  267  ff.,  272;  olfactory,  250  f.; 
prefrontal,  270  ff.;  sensory,  244  ff.,  267  ff; 
silent,  229,  244,  251;  somesthetic,  244  ff., 
403  f.;  of  speech,  227,  233,  255  ff.;  stri- 
ate,  271  f.;  visual,  246  ff.,  271  f.;  of 
Wernicke,  257  ff. 

Association,  402,  427,  429  f.,  454  f.,  466  ff., 
493  ff.,  502;  by  contiguity,  584,  617  ff.; 
by  similarity,  588  f.;  convergent,  624; 
controlled,  495,  586,  589,  623  f.;  dy- 
namical, 562,  579,  584,  616,  660  f.;  forma- 
tion of,  542,  544,  546,  549,  551  f.;  554,  570, 
575  f.,  577  ff.,  617  ff.;  free,  495,  586; 
neural  mechanism  of,  617  ff.;  operation  of, 
583  ff.;  principal  and  subordinate,  578  f.; 
remote,  579,  581,  619;  serial,  578  f.,  581, 
620  ff.;  subliminal,  584,  612 

Association  fibres,  58,  96,  223  f.,  233,  246, 
251  f.,  269 

Association  time,  493  ff.,  573,  «85 

Astereognosis,  253 

Astigmatism,  187 

Asymbolia,  252  f. 

Atom,  concept  of,  676;  reality  of,  680; 
unity  of,  679;  the  unity  of  the  mind  not 
atomic,  683  ff.  \ 

Attention,  401,  4Q7\  435,  440,  450,  458,  483, 
532,  541,  550,  556,  561  ff.,  571  f.,  582  f., 
587,  596  ff.;  centre  for,  262,  274;  cerebral 
conditions  of,  609  ft,  618  ff.;  definition  of, 
596  f.;  determination  of,  600;  expectant, 
see  Expectancy;  feelings  of,  214,  507; 
field  of,  597,  609;  fluctuation  of,  599  f.; 
motor  adjustments  in,  249  ff.,  507  f.,  531; 
shifting  of,  598  f.,  611, 618,  620  ff.,  span  of, 
560,  597 

Attraction,  442  f. 

Auditory  area,  249  f. 


INDEX  OF  SUBJECTS 


695 


Auditory  perception  of  space,  392  ff. 

Auditory  sensations,  312  ff.,  367  ff.,  475, 
529  f.,  659 

Automatism,  64  f.,  147  ff.,  158,  485,  496. 
533  ff.,  559,  564,  610 

Autonomic  system  of  nerves,  150 

Average  error,  358  f. 

Axis  cylinder  process,  see  Axon. 

Axon,  74,  97,  100  ff.;  central.  44j  classes  of, 
44;  conduction  in,  111;  function  of,  109  f.; 
growth  of,  41  ff.,  98;  motor,  42,  212;  sen- 
sory, 42,  48,  177  ff.;  sheaths  of,  99,  142; 
structure  of,  100  f.;  types  of,  104  f.,  265; 
see  Nerve-Fibre. 

BAHNUNG,  170 

"  Baskets,"  108  ff. 

Beats,  322  f. 

Beauty,  529  ff. 

Behavior,  of  Amoeba,  14  f. ;  of  earthworm, 
22;  of  jelly-fish,  17  ff.;  of  sponge,  17 

Binocular  vision,  414  f.,  427  ff.,  436,  458, 
465;  contrast,  452;  rivalry,  452  f.,  598  f.; 
mixture,  452 

Birds,  brain  of,  29  ff.;    intelligence  of,  547 

Blind,  tactile  sensitivity  of  the,  402  f.,  411 

Blind  spot,  193  f. 

Blocking  of  nerve  impulses,  287,  289  f. 

Bodily  resonance  of  emotion,  525 

Body,  relations  of,  to  mind,  629  ff. 

Bone-conduction  to  the  ear,  197,  393  f. 

Brain,  66,  75;  action  of,  on  mind,  643  f.; 
chemistry  of,  117  ff.,  291;  circulation  in, 
68,  70,  125,  215,  291,  643,  663;  develop- 
ment of,  48  ff.;  functions  of,  213  ff.;  of 
invertebrates,  24  ff.;  of  man,  33  ff.;  as 
organ  of  mind,  636  ff.;  as  seat  of  mind, 
213  ff.,  632  ff.;  a  tube,  39,  48  f.;  of  ver- 
tebrates, 25  ff.;  weight  of,  see  Brain- 
weight 

Brain-stem,  27  f.,  77  ff.,  155,  241;  develop- 
ment of,  55  ff. 

Brain  vesicles,  48 

Brain-weight,  of  idiots,  656;  of  mammals, 
33  f.;  of  primates,  34  f.;  of  races  of  man, 
35;  as  related  to  age,  59  ff.,  660  f.;  to 
intelligence,  33,  60  f.,  216  f.;  to  sex,  60; 
to  stature,  33,  60 

Bridgman,  Laura,  brain  of,  58 

Brightness,  327,  329,  331,  333,  354  f.,  370  f. 

Bulb  or  medulla,  48,  66,  69,  75  ff.;  develop- 
ment of,  56  f.;  function  of,  157  f. 

CALCAKINE  region,  223,  248  f.,  252,  268  f., 

271 

Callosum,  33,  54,  77  f.;  85  f.,  222  ff.,  254,  263 
Calm,  501  f.,  506,  508,  531 
Carbon  dioxide  in  nervous  activity,  135,  137, 

291;   in  respiration,  148;   in  fatigue,  538 
Catabolism  in  nerve,  135  ff.;   in  brain,  215, 

290  f. 
Causal  relations  between  brain  and  mind, 

641  ff. 

Cause,  concept  of,  647,  650 
Cell,  definition,  13  f.;    chemistry  of,  119  f.; 

gustatory,  178  f.;    nerve,  see  Nerve-Cell, 

olfactory,  176  f. 
Cell-body,  286  ff. 
Cell  membrane,  14,  119  f. 


Central  canal  of  the  cord,  39,  45,  72 

Central  factors  in  illusions,  444,  447,  449  ff.; 
in  reaction  time,  481  f. 

Centralization  in  the  nervous  system,  20  ff. 

Cerebellar  system,  95 

Cerebellum,  66,  75  ff.,  86  f.,  108  ff.,  221  f.; 
development  of,  49,  54;  function  of,  156  f., 
211  f. 

Cerebral  ganglion  of  invertebrates,  24  ff. 

Cerebrin,  121 

Cerebro-spinal  fluid,  49 

Cerebrum,  66,  76  ff.,  156,  219  ff.;  develop- 
ment of,  49  ff.;  in  different  animals,  26, 
31  ff.;  influence  of,  on  reflexes,  158  f., 
171  f.,  241,  505,  532  ff.;  injury  of,  215, 
217  ff.,  252  ff.,  262,  274,  644,  f.,  663; 
localization  of  functions  of,  213  ff.,  235  ff.; 
as  related  to  consciousness,  213  ff.,  298, 
609  ff.,  618,  633  ff.;  removal  of,  158  f., 
217;  as  seat  of  mental  functions,  219, 281  ff. ; 
see  Brain 

Character,  262 

Chemical  changes  in  retina,  195,  325 

Chemical  elements  in  brain,  118,  279 

Chemical  integration  of  the  body,  151 

Chemical  senses,  28,  375,  479 

Chemistry  of  nervous  tissue,  117  ff.,  279  f. 

Chinese  music,  320 

Choice-time,  484,  488,  497 

Cholesterin,  120  f. 

Chorda  tympani,  179 

Choroid,  183  f.,  194 

Chromatolysis,  105,  240 

Chromosome,  37 

Ciliary  muscle,  183,  188  f.,  427 

Circulation,  in  brain,  68,  70,  125,  215,  291, 
526,  643  f.,  663;  during  emotion,  505  ff., 
523  ff.;  as  a  means  of  co-ordination,  151; 
in  nerves,  137 

Clang,  313  f.,  318  f.,  369,  395 

Classification  of  mental  functions,  664,  666; 
of  odors,  307;  of  sensations,  300  ff.;  of 
sounds,  312  f.;  of  tastes,  310 

Clearness,  597 

"  Climbing  fibres,"  109  f. 

Cochlea,  200  ff.,  280 

Cochlear  nerve,  82,  90,  203 

Coelenterate,  nervous  system  in,  17  ff. 

Coenaesthesia,  350,  518  f. 

Cold  spots,  181,  344  ff.,  366,  405 

Collaterals,  42,  73,  88,  111,  265,  268 

Collectors,  162,  240,  622  f. 

Color  blindness,  335  f.,  343 

Color  mixture,  330  ff.,  340,  343,  452 

Color-tone,  327,  333,  337 

Color  triangle,  344 

Color  vision,  196,  324,  327  ff.,  352,  475,  490; 
theories  of,  340  ff. 

Columns  of  the  cord,  40,  46  ff.,  71,  73  ff., 
79  ff.,  89  f.;  of  Clarke,  74,  90;  of  Goll,  75 

Commissures,  40,  43  f.,  71  f.,  89 

Comparison,  376,  440,  488,  601  ff.;  mediate 
and  immediate,  601;  process  of,  602  f. 

Compensatory  movements,  155,  209  ff. 

Competition,  between  stimuli,  173;  dyn- 
amogenic  effect  of,  533 

Complementary  colors,  332,  334,  339,  342  f. 

"Complexes,"  509,  586 

Concatenation,  64,  280  f. 


696 


INDEX  OF  SUBJECTS 


Concept,  466 

Conduction,  alterations  of,  133,  139;  in 
axon,  111,  133  ff.,  279  f.;  in  gray  matter, 
111,  286  ff.;  irreciprocal,  111,  290;  in 
nerves,  127  ff.;  in  nerve-net,  19  f.;  as 
primary  function  of  nervous  tissue,  16,  33, 
143,  279  f.;  rapid,  due  to  long  nerve- 
fibres,  20  f.;  reversible,  133  ff.;  at  syn- 
apse, 111,  286  f.,  290;  velocity  of,  in 
nerves,  131  ff.,  473;  in  other  tissues,  132 

Conductivity,  15  f.,  127,  133 

Conductor,  16,  65,  127  ff.;  artificial,  139  ff.; 
indifferent,  284  f. 

Cones  of  the  retina,  190  ff.,  325  f.,  352 

Confluxion,  441,  444,  447,  451 

Conjunctiva,  180  f.,  184 

Consciousness,  651,  666,  685;  centre  of,  483, 
564,  597;  cerebral  conditions  of,  609  ff., 
618,  635;  double,  679;  elements  of,  500  f., 
511;  field  of,  431,  483,  597,  618;  intensity 
of,  597, 609  f.,  650;  limits  of,  430  f.;  relation 
of,  to  learning  and  habit,  496,  563  f.;  to 
bodily  movements,  145  f.;  seat  of,  213  ff., 
633  ff.;  theories  of,  610  f.;  unity  of,  679, 
684  ff. 

Conservation  of  energy,  646  ff.;  inappli- 
cable to  relations  of  mind  and  body,  650  f., 
665;  within  the  nervous  system,  648  ff. 

Consonance,  321  ff.,  529 

Constant  errors,  409,  437  ff. 

Contours,  prevalence  and  rivalry  of,  452 

Contrast,  in  perception  of  space,  439,  441, 
443,  447,  451;  in  taste,  311;  in  tempera- 
ture, 347;  in  vision,  339  f.,  343,  435,  452 

Convergence,  of  the  eyes,  189,  427;  of  ner- 
vous impulses,  93,  161  f.,  171,  403,  622  ff. 

Convolution,  see  Gyre 

Co-ordination,  as  function  of  nerve-centres, 
63  ff.,  93,  281  ff.,  285  f.;  in  movements 
initiated  from  cortex,  241,  254,  261  f., 
562,  565,  622  ff.;  in  reflexes,  153,  155, 
157  f.,  169,  172,  241,  622;  in  sponge,  17; 
in  worm,  24 

Core  conductor,  139  f.,  284 

Core  model,  139  f. 

Cornea,  181  ff.,  186  ff.,  346 

Corpuscle,  Pacinian,  180  ff.;   tactile,  180 

Correlation  between  the  neural  and  the 
mental,  3,  6,  9  f.,  124  ff.,  213  ff.,  281,  292  f., 
298  f.,  375,  381,  458,  483,  485,  592,  607  ff., 
629  ff.;  conception  of,  629  ff.;  limits  of, 
656  ff.,  664  ff.,  677  f. 

Corresponding  points,  248  f.,  424  ff. 

Cortex  of  cerebellum,  54,  86,  103,  108  ff.; 
of  cerebrum,  31  ff.,  104,  222  ff.,  264  ff.; 
development  of,  52  ff.,  62;  as  seat  of 
mind,  634 

Covering  points,  424 

Crab,  nervous  system  of,  24  f.,  288;  learning 
by,  546 

Cross-education,  565 

Crossing  of  nerves,  243,  285  f. 

Cuneus,  222,  247 

Curare,  136 

Current  of  action,  134,  137  f.,  230 

Current  of  injury,  141  f. 

Current  of  rest,  141  f. 

Cutaneous  senses,  179  ff.,  244,  344  ff.,352, 
363  ff.,  473,  475  ff.,  517 


DEAF-MUTES,  211 

Deafness,  250,  316 

Decerebrate  animal,  152,  155,  158  f.,  168  f., 
525 

Decussations,  69,  79,  ff.,  91  f.,  247,  250 

Deep  sensibility,  181 

Degeneration  of  nerve-fibres,  73,  87  f.,  112  f., 
239,  288 

Dendrites,  42,  97,  101  ff.,  265  f.,  268;  func- 
tion of,  107  f.,  161,  287  ff.;  mobility  of, 
289,  613 

Departing  station,  251 

Depression,  501  f.,  531 

Depth,  perception  of,  413  f.,  416,  423,  426  ff., 
458,  465 

Determining  tendency,  495  ff. 

Development,  of  the  mind,  654  ff.;  of  the 
nervous  system,  in  the  individual,  36, 
275,  279,  298,  655,  672;  in  the  race,  13, 
298;  of  space-perception,  381,  384  ff., 
390  f.,  410,  431,  458,  463  ff. 

Diaschizis,  243 

Dichromatic  vision,  J536,  340,  343  f . 

Diencephalon,  48 

Difference  tone,  323 

Diffusion,  in  living  cells,  14,  290 

Discrimination,  376  ff.,  403  f.,  410,  665; 
auditory,  315  ff.,  369;  as  a  factor  in  learn- 
ing, 550,  554  f.,  571;  of  intensities,  353  ff., 
378  f.;  olfactory,  374;  tactile,  348,  396  ff.; 
time  of,  360,  488  ff.,  497,  571;  visual,  326, 
329,  331,  336 

Disposition,  543,  583 

Dissociation,  534  f. 

Distribution  of  nervous  impulses,  93,  159  f., 
168,  622  ff.;  of  training,  581,  616 

Divided  space,  illusion  of,  438  ff.,  448 

Division  of  labor  in  animal  economy,  15, 
64  ff.,  175,  351 

Dizziness,  211,  350 

Dorsal  column,  40,  48,  80  f. 

Dorsal  horn,  46 

Dorsal  root,  40,  43  f.,  46  ff. 

Dot  figure,  598,  618 

Double  contact,  411  f.,  637 

Double  images,  414,  424  ff.,  436,  465 

Drainage  theory,  611  ff.,  618  ff. 

Dreams,  587,  643 

Dualism,  630,  640,  652 

Ductus  cochlearis,  202  f.,  205 

Duplex  theory  of  vision,  196,  340 

Dura  mater,  70 

Dynamogenesis,  532  f. 

EAR,  196  ff.;   an  analytic   organ,  206,  319; 

development  of,  56;   inertia  of,  474,  476, 

479;  internal,  28,  155  f. 
Ear  drum,  197 

Eccentric  projection,  303,  384,  390,  411 
Economy  in  learning,  577  f.,  581  f. 
Ectoderm,  38  ff. 
Effector,  16,  65,  99 
Eigenlicht,  325,  357,  369,  433 
Einfuhlung,  450 

Einstellung,  602;   see  also  Adjustment 
Electrical  excitation  of  cortex,  228  ff.;    of 

nerve,  130  f.;    of  sense  organs,  305,  309, 

325 
Electrical  phenomena  in  cortex,  230;    in 


INDEX  OF  SUBJECTS 


697 


nerves,  137  ff.;  in  the  retina,  138,  194; 
during  emotion,  509  f. 

Electrical  theory  of  nerve  impulse,  138, 
140  ff.;  of  the  synapse,  289  f. 

Electrophysiology,  130 

Electrotonic  currents  in  nerve,  139  f. 

Electrotonus,  138  f. 

Elements  of  consciousness,  500  f.,  511 

Elimination  by  subtraction,  491,  497  f. 

Embryo,  37  ff.;  mental  life  of,  658 

Embryology,  36  ff. 

Eminent  men,  brain  weights  of,  60  f. 

Emotion,  509,  521  ff.,  661;  bodily  symp- 
toms of,  523  ff.;  central  factors  in,  522, 
644;  sensory  factors  in,  523,  644;  sup- 
pression of,  586;  theories  of,  522  f.,  525 

Empiristic  theory,  385,  418,  458  f. 

End-brain,  31,  48  ff. 

End-bulb,  180  f. 

End-organs,  175  ff.,  280,  299,  304,  351  f., 
375,  473,  479 

End-plate,  motor,  100,  136,  212 

Endoneurium,  99 

Enlargements  of  the  cord,  69  f.,  74 

Entoderm,  38 

Epicritic  sensibility,  349 

Epineurium,  99 

Equally  tempered  scale,  320 

Equilibrium,  157,  209,  282 

Erect  vision,  453  ff. 

Eustachian  tube,  197  f.,  200 

Excitability,  see  Irritability 

Excitation,  method  of,  228  ff.,  244,  249  ff. 

Excitement,  501  f.,  506,  508,  531 

Exercise,  nutritive  effects  of,  59,  62,  581,  616 

Expectancy,  480,  502,  507,  556,  602;  physi- 
ology of,  621  f. 

Experience,  380,  418,  458.  468,  542,  549  f., 
563,  593  f.,  646,  657  f.,  679,  685 

Extensity,  383,  418  f. 

Extent,  perception  of,  364,  371,  409,  431  f., 
440,  467  f.,  490 

External  ear,  196  f.,  394 

External  meatus,  196  f. 

Extirpation,  229  ff.,  238,  241,  262  f.,  663 

Eye,  axes  of,  420  ff.;  development  of,  49, 
56;  fixation  of,  420,  448  f.,  459  f.;  move- 
ments of,  185  f.,  189,  246  ff.,  269,  414  f. 
417,  420,  427,  447  ff.,  454,  457,  459  ff., 
530,  659;  muscles  of,  185  f.,  420  f.; 
nerves  of,  189,  191;  nerve-centres  of,  25, 
30,  56,  83,  92,  158,  189,  237,  246  ff.;  per- 
ceptions of,  413  ff.;  positions  of,  421  f., 
465;  rotation  of,  420  ff.;  structure  of, 
182  ff.;  torsion  of,  421  ff. 

FACILITATION,  170  ff.,  511,  584,  590,  599,  612 

Faculties,  255,  274,  391,  566,  571,  581,  655, 
666;  development  of,  657 

Fatigue,  causes  of,  538  ff.;  curve  of,  536, 
538;  feelings  of,  350,  536,  538  f.;  level  of, 
537;  mental,  401,  537  f.,  541;  of  muscle, 
536  ff.;  of  nerve-centres,  538  ff.,  611  ff.; 
of  nerve-fibres,  136;  products  of,  538  f.; 
of  sense  organs,  308,  337,  453,  537;  varie- 
ties of,  539  ff. 

Fechner's  law,  361,  377  ff.,  480 

Feeling,  302,  373,  496,  500  ff.,  564  f.,  567, 
658,  666;  aesthetic,  505,  513  f.,  528  ff., 


664;  classification  of,  512  ff.,  526;  com- 
mon, 518  f.;  complex,  518  f.,  521  ff.;  con- 
tent of,  514;  definition  of,  500;  dimen- 
sions of,  501  f.,  506  ff.,  526;  experiments 
on,  504;  expression  of,  505  ff.;  intel- 
lectual, 502  f.,  513  f.,  527  f.,  544,  584, 
590  f.;  intensity  of.  514;  moral,  513  f., 
664;  nature  of,  500,  503,  511;  neutral, 
515  f.;  number  of,  501;  physiology  of, 
504  ff.;  rhythm  of,  514  f.;  sensuous, 
513  f.,  518  ff.;  sthenic  and  asthenic,  524; 
teleology  of,  527;  theories  of,  504  f., 
510  ff.,  522  ff. 

Feeling  of  familiarity,  503,  544,  590  f.;  of 
strangeness,  544 

Feelings  of  innervation,  407  f. 

Feeling-tone  of  sensations,  502,  512,  515  ff.. 
519  ff.,  633,  635 

Fenestra  ovalis,  198,  205 

Fenestra  rotunda,  198,  205 

Fibrillar  theory,  114 

Field  of  touch,  396  ff.;  of  vision,  415  ff.,  436, 
438,  456  ff.,  461  ff. 

Fillet,  lateral,  82  f.;  mesial,  81,  83  f.,  90,  246 

Filum  terminale,  69  f.,  72 

Fishes,  brain  of,  27,  29  ff.;  intelligence  of,  217 

Fissure,  calcarine,  222,  247  ff.;  central, 
220  ff.,  236  ff.,  244  f.,  271;  of  cerebellum, 
86;  of  cerebrum,  33,  51  ff.,  220  ff.;  cing- 
ulate,  222;  of  the  cord,  45,  69  ff.,  79  f.; 
intraparietal,  221;  longitudinal,  52;  par- 
ieto-occipital,  220,  222,  247;  postcentral, 
221;  precentral,  221;  of  Rolando,  220  ff., 
236  ff.,  244  f.,  271;  of  Sylvius,  51  ff.,  75, 
86,  220  f.,  249  f. 

Fixation  of  the  eye.  420,  448  f.,  459  f. 

Flatworm,  20 

Fore-brain,  31 

Fore-dog,  153,  155 

Fore-period,  480,  482  ff.,  489 

Forgetting,  546,  573  ff. ;   curve  of,  575  ff. 

Fornix,  77  f.,  85  f.,  95,  219,  223 

Fovea  centralis,  193,  195,  247  f.,  334,  420, 
459  f. 

Fraunhofer's  lines,  327  ff. 

Free  nerve-ending,  180  f. 

Functions,  fundamental  of  animals,  14; 
intellectual,  219,  224,  251  ff.,  273  f. 

Fundamental  system,  26  ff.,  47,  56  f.,  94,  96 

Fundamental  tone,  319 

Fusion,  321  ff.,  403  f.,  410,  450  f.,  599 

GANGLION,  cerebral,  24;  In  Invertebrates, 
20  ff.;  spinal,  26,  42  ff.,  50,  56,  73  f.,  104, 
106;  spiral,  203 

Genetic  theory  of  space  perception,  386 

Geniculatum,  78  f.,  84  f.,  91  f.,  96, 246, 248, 250 

Geometrical  illusions,  437  ff.;  theories  of, 
447  ff. 

Geometrical  senses,  352,  383,  390,  410,418 

Germ  layers,  38 

Gestaltqualitaten,  601 

Golgi  net,  113;  Golgi  stain,  103,  112,  264  f., 
267,  269 

Gray  matter,  45,  53  f.,  73,  102,  106,  118,  222, 
226;  conduction  in,  110,  286  ff. 

Ground  bundle,  47,  94 

Growth  of  the  brain,  59  ff.;  of  the  ner- 
vous system,  57  ff. 


INDEX  OF  SUBJECTS 


Gyre,  221;  angular,  221,  246  f.;  dentate, 
219;  frontal,  221;  fusiform,  222;  lin- 
gual, 222,  247;  occipital,  222;  parietal, 
221  f.;  postcentral,  220  ff.,  235  ff.,  244  ff.; 
precentral,  220  ff.,  235  ff.,  245  f.,  261, 
268  ff.;  supramarginal,  221;  temporal, 
221;  sigmoid,  235 

HABENULA,  78,  95 

Habit,  263;  formation  of,  545  ff.;  orders  of, 
556  ff.;  law  of,  615  ff. 

Hair-receptors,    179  f.,  346 

Hallucinations,  306,  436,  644 

Hammer,  197  ff. 

Harmony,  321  ff.,  529 

Head,  25,  28;  receptors  of,  25,  27  f. 

Hearing,  acuity  of,  368  f.;  end-organ  of, 
196  ff.,  351  f.;  pathway  of,  82,  90  f.,  203, 
250;  sensations  of,  312  ff.,  367  ff.;  space- 
perceptions  of,  392  ff.;  theories  of,  205  ff. 

Heart,  beat  of,  149,  262,  505  ff.;  nerves  of, 
157,  163 

Heat,  sensation  of,  347  f. 

Hemianopsia,  247  f.f  253 

Hemiplegia,  263 

Hemispheres,  cerebral,  51  f.,  213  ff.;  pre- 
dominance of  the  left,  263,  565 

Heredity,  37,  520 

Higher  units,  498,  559  f.,  562,  569,  578  f., 
583,  597,  621;  neural  mechanism  of,  622  ff. 

Hind-brain,  48  ff. 

Hind-dog,  153,  155,  174 

Hippocampus,  95,  223 

Hood  or  tegmentum,  83  ff. 

Hormones,  148,  151 

Horn  of  Ammon,  220 

Horns  of  the  cord,  46,  72  f.,  79  f.,  89 

Horopter,  426 

Hypermetropia,  187 

Hypophysis,  76  ff. 

Hysteria,  535,  544 

IDEAS,  512,  593;  learning  by,  553  f.,  563; 
physical  basis  of,  638 

Identical  points,  424,  426 

Idiocy,  656 

Illusion,  339,  409,  430,  434  ff.,  519,  544; 
angle,  442  ff.;  of  area,  447  f.;  confluxion, 
441,  444,  447,  451;  of  divided  space, 
438  ff.,  448;  geometrical,  437  ff.;  of 
Hering,  445  f.;  of  motion,  453;  of  Muller- 
Lyer,  440  ff.,  448,  450  f.;  one-dimen- 
sional, 438  ff.;  of  Poggendorf,  440,  444, 
446,  451;  theories  of,  447  ff.,  456;  of 
time,  440;  of  touch,  440;  twisted-cord, 
446  f.;  vertical-horizontal,  437,  447  ff.;  of 
Zollner,  444,  446  f. 

Imagery,  583 

Imagination,  219,  243,  253,  274,  589,  661 

Incus,  197  ff. 

Index  of  refraction,  186  f. 

Indifference  point,  346  f.,  365  f.,  468,  514  ff. 

Indifferent  conductor,  284  f. 

Individual  differences,  60  ff.,  187,  221,232, 
306,  314  ff.,  326,  331  f.,  335  f.,  338,  368, 
398,  438,  456,  471  f.,  476,  491,  537,542, 
554  f.,  571,  583,  591,  600,  655 

Inertia  of  the  senses,  471  473  ff.,  479 

Infancy,  mental  life  in,  658  ff. 


Inhibition,  148,  162  ff.,  170  ff.,  262,  274, 
287,  507  f.,  511,  532,  539  f.,  551,  562,  572, 
579  f:,  584  ff.,  590,  599,  612  f. 

Injury  of  the  brain,  215,  217  ff.,  252  ff.,  262, 
274,  644  f.,  663;  of  the  cord,  218;  of  the 
nerves,  218,  306,  348  f.,  400  f. 

Instinct,  146,  542,  550,  600 

Insula,  52 

Intellect,  219,  224,  251,  259  f.,  262,  435,  527 

Intelligence  of  animals,  33,  216  f.,  545  ff. 

Intensity  of  sensations,  300,  302,  353  ff.; 
of  consciousness,  597,  609  f.,  650 

Interaction,  640  ff.,  654 

Interbrain,  48  ff.,  76,  78  f.,  83  ff.,  91  f.,  223 
248,  250;  in  different  vertebrates,  30  f. 

Interference,  496  f.,  534,  540,  551,  570, 
579  ff.,  584  ff.;  associative  and  reproduc- 
tive, 580,  585;  see,  also,  Inhibition. 

Internal  capsule,  52  f.,  85  f.,  93,  223 

Internal  ear,  200  ff.,  350 

Interval,  316  f.,  320  f.,  323 

Introspection,  5,  7  ff.,  307,  430,  463,  500, 
551,  559,  594,  608,  672,  681 

Intuition,  385 

Iris,  183  f.,  188  f. 

Irradiation  of  impulses,  169 

Irritability,  15  f.,  64  f.,  127,  129  ff.,  133,  175, 
228  f.,  539,  545,  585,  587,  612,  615,  619 

Island  of  Reil,  52,  85  f.,  221,  250,  262 

Itch,  347 

JAPANESE  music,  320 
Joint-sense,  182,  349 
Judgment,  593,  601  ff.,  665  f. 
Jump  of  the  eye,  459  ff. 
Just  noticeable  difference,  358,  361  ff.,  432; 
just  noticeable  interval,  473  ff. 

KEPHALIN,  122 

Kinesthetic  sensations,  317,  349  f.,  407  ff., 

414,  417,  423,  427,  431  ff.,  447  ff.,  454  f., 

486 
Knee  jerk,  167,  171  f.,  532  f. 

LABYRINTH  of  the  ear,  200  ff.,  350 

Language,  252  ff. 

Lapses,  585,  623 

Latent  time,  166,  287,  290,  473  ff.,  479 

Lateral  line  in  fishes,  27 

Layers,  of  cortex,  265  ff.;  of  embryo,  38; 
of  neural  tube,  40  ff.,  45,  53;  of  retina  190  f. 

Learning,  542  ff.;  as  a  function  of  the  ner- 
vous system,  35,  96,  113,  145,  147,  159, 
216  f.,  254,  262  ff.,  282,  286  f.,  615  ff.; 
curve  of,  548  ff.,  556  f.,  560  ff.,  568,  575; 
in  animals,  544  ff.;  in  man,  555  ff.;  re- 
lation of  consciousness  to,  563  f. 

Lecithin,  122 

Le  Long,  case  of,  256 

Lens  of  the  eye,  183  f.,  186  ff.,  427 

Life,  mechanics  of,  290,  292 

Limit,  physiological,  557;  of  the  scale  of 
intensities,  356  ff.,  362,  364  f.,  368;  of 
the  scale  of  tones,  314  f.;  of  the  visible 
spectrum,  328 

Line  of  regard,  420 

Lipoids  of  the  brain,  118  ff.;  function  of, 
119,  280 

Listing's  law,  421  f.,  459 


INDEX  OF  SUBJECTS 


699 


Lobe,  220  ff.;  frontal,  51,  85  f.,  220  ff.,  227, 
255,  257,  259  ff.,  262  ff.,  274;  limbic,  222, 
268;  occipital,  85,  92,  220  ff.,  246  ff., 
252  f.,  274.;  olfactory,  31  f.,  50,  158,  219, 
250;  optic,  30,  103;  parietal,  220  ff., 
245,  252  f.,  262;  pyriform,  95,  250,  268; 
temporal,  51,  85  f.,  220  ff.,  249  f.,  252  f., 
257  ff.,  262,  272,  274 

Local  sign,  384,  388  ff.,  394,  398  f.,  401,  405 
ff.,  410,  414,  422,  431,  433,  456  f.,  464,  614 

Localization  of  sensations,  384,  390,  467, 
511,  633,  635;  auditory,  392  ff.,  492  f.; 
cutaneous,  348,  396  ff.,  406  f.,  410;  gus- 
tatory, 392;  olfactory,  392;  visual,  414 

Localization  of  functions,  74,  152  ff.,  219  ff.; 
history  of,  227  f.,  234,  235  f.,  255,  257  f., 
264;  in  human  brain,  238  ff.,  298,  614, 
634;  methods  of,  87  f.,  223,  228  ff.,  273 

Locomotion,  155,  157  f.,  209,  242,  254 

Logic,  593,  603 

Lustre,  453 

MACULA  LTJTEA,  191,  193  f. 

Malleus,  197  ff. 

Mammillary  body  76  ff.,  86,  95   222 

Mammals,  brain  of,  29  ff.;  intelligence  of, 
547 

Man,  nervous  system  of,  33  ff. ;  intelligence 
of,  549,  553  ff. 

Map  of  the  cortex,  224  ff.,  233,  264 

Masson  disk,  370 

Materialism,  631,  640  f.,  669;  criticism  of, 
659,  663,  674  ff. 

Materiality,  concept  of,  681;  inapplicable 
to  mind,  681 

Maturity,  661  f. 

Maze-test,  547 

Mean  gradations,  359,  376 

Meaning,  as  aid  to  memory,  577,  579;  re- 
call of,  601 

Measurement  of  sensation,  356  ff.,  377  f. 

Mechanism,  concept  of,  as  applied  to  the 
nervous  system,  3,  63  ff.,  96,  275  ff.,  562; 
of  association,  617  ff.;  of  attention,  611  ff.; 
of  co-ordination,  159  ff.,  622  ff.;  of  dis- 
crimination, 403,  624  f.;  of  habit,  615  ff.; 
of  life,  290,  292;  of  memory,  615  ff.;  of 
nerve-centres,  159  ff.,  286  ff.,  660;  of  per- 
severation,  587;  of  thought,  555,  589  f., 
606,  623  ff.;  of  trial  and  error,  551  f.;  of 
varied  reaction,  611  ff. 

Median  longitudinal  bundle,  83  f.,  89,  94 

Medulla  oblongata,  48,  66,  69,  75,  157 

Medullary  sheath,  see  Myelin 

Membrana  tympani,  197  ff. 

Membrane,  basilar,  202  ff.,  206  ff.;  of  brain 
and  cord,  68,  70;  of  Reissner,  202  f.,  205; 
semi-permeable,  14,  119  f.;  tectorial,  205; 
tympanic,  197  ff. 

Memorizing,  542,  566  f.,  570,  577  ff.;  econ- 
omy in,  578,  581  f. 

Memory,  analysis  of,  542  ff. ;  associative,  in 
animals,  546;  centre  for,  274;  disturb- 
ances of,  544;  experimental  study  of, 
572  ff.;  feelings  of,  502  f.;  neural  me- 
chanics of,  286  f.,  615  ff.;  span,  574,  577; 
training  of,  566  ff.,  570 

Mental  work,  291,  507,  509,  537  f.,  541,  590; 
elements,  382;  products,  382,  458,  463, 


512,  641,  654;  processes,  time  of,  470  ff., 
497,  614;  life,  in  embryo,  658;  in  infancy, 
658  ff.;  in  adolescence,/^  1;  in  maturity, 
661;  in  old  age,  662 

Mesencephalic  system,  94  f. 

Mesencephalon,  48 

Mesoderm,  38 

Metabolism,  in  brain,  215,  290  f.,  649;  in 
nerve,  135  ff.;  in  retina,  342 

Metaphysics,  its  place  in  psychology,  6, 
629  f.,  669,  686  f. 

Metazoa,  13,  16,  545  ff. 

Metencephalon,  48 

Method,  comparative,  13,  88,  216,  219,  231  f.; 
of  expression,  504;  of  impression,  504; 
introspective,  see  Introspection;  of  lo- 
calization of  functions,  87  f.,  223,  228  ff., 
273;  of  physical  science,  278;  of  physio- 
logical psychology,  7,  13,  297  ff.;  of  psy- 
chophysics,  357  ff.;  of  studying  animal 
intelligence,  547  ff.;  of  studying  memory, 
572  ff. ;  of  tracing  tracts,  87  f . 

Mid-brain,  77  ff.,  90,  92,  103,  189;  in  differ- 
ent vertebrates,  30;  development  of,  48  ff. 

Middle  ear,  197  ff. 

Mid-dog,  153,  155,  174 

Mind,  2,  10,  468  f.,  483,  625;  action  of,  on 
brain,  642  ff.;  dependence  of,  on  brain, 
215  ff.,  643,  671,  685;  development  of, 
644  ff.;  immateriality  of,  681;  indepen- 
dence of,  652;  nature  of,  385,  632,  652; 
reality  of,  653  f.,  668  ff.;  relations  of,  to 
body,  629  ff.;  to  brain,  632  ff.;  seat  of, 
631  ff.;  spatia  relations  of,  632  ff.;  spiritu- 
ality of,  682  f . ;  unity  of,  672,  678  ff. 

Mixture,  of  colors,  330  ff.,  340,  343,  452; 
of  cutaneous  stimuli,  347  f.;  of  odors,  308; 
of  tastes,  311 

Modiflability,  35,  217,  286  f.,  542  ff.,  615  ff. 

Molecules,  275  ff. 

Mollusks,  nervous  system  of,  20 

Monkey,  brain  of,  33  f.,  235  ff.;  intelligence 
of,  547,  549 

Monocular  vision,  413  ff.,  423,  430 

Motifs  of  perception,  413  ff.,  433,  436,  464 

Motility,  14  ff.;   of  dendrites,  289 

Motor  area,  228,  232,  235  ff.,  254,  267  ff. 

Movement,  automatic,  485,  496,  533  f.,  559, 
564,  610  f.;  compensatory,  155,  209  ff.; 
expressive,  504  ff.,  523  ff.,  528  ff.,  534, 
637;  forced,  157,  210;  ideomotor,  535; 
involuntary,  534;  learned,  145,  147,  159, 
242,  254  f.,  262  ff.;  perception  of,  211, 
364,  387  f.,  404  f.,  433  f.,  453;  reactive, 
480  f.,  484;  reflex,  145  ff.;  rhythmical, 
149,  164  f.;  skilled,  242,  254  f.,  262  ff.; 
unconscious,  145;  voluntary,  147,  535, 
538,  645 

Muller-Lyer  illusion,  440  ff.,  448,  450  f. 

Muscle  sense,  317,  349  f.,  363  f.,  374,  407  ff., 
440;  centre  for,  244  f.,  253;  end-or- 
gans of,  181  ff.,  349;  pathway  of,  90; 
theories  of,  407  f.;  in  visual  perception, 
414,  417,  423,  427,  431  ff.,  447  ff.,  454  f., 
466 

Muscle  spindle,  181  ff. 

Music,  feelings  aroused  by,  521  f.,  529  f.; 
intervals  in,  316  f.,  320  f.,  323,  521;  scale 
of,  316,  320  f. 


700 


INDEX  OF  SUBJECTS 


Myelencephalon,  48 

Myelin,  26,  45,  58,  119  ff.;    myelin  sheath, 

26,  45,  58,  99  f.,  119,  142  f. 
Myelinization,  58  f.,  223  ff.,  246 
Myopia,  187 

NARCOTICS,  120,  287,  289  f. 

Native  powers,  146,  386,  419,  457  f.,  464,  600 

Nativistic  theory,  385,  418  f.,  457  f. 

Near-sightedness,  187 

Neopallial  system,  96 

Neopallium,  31  ff.,  86,  219,  266 

Nerve,  67,  98  ff.;  abducens,  55,  87;  acces- 
sory, 56,  87;  auditory,  55  f.,  82,  87,  90, 
202;  blood  supply  of,  137;  cochlear,  82, 
202  f.;  conduction  in,  127  ff.,  473;  cra- 
nial, 55  f.,  76,  79;  development  of,  39, 
42  ff.;  excitation  of,  129  ff.;  facial,  55,  87; 
function  of,  21;  glosso-pharyngeal,  55,  87, 
178;  hypoglossal,  56,  87;  intermediate, 
87,  179;  oculomotor,  55,  79,  189;  ol- 
factory, 55  f.,  79,  106  f.,  177,  306;  optic, 
30,  55  f.,  79,  191,  193;  physiology  of,  127 
ff.;  quickness  of,  130  ff.;  spinal,  42,  66; 
trigeminus,  27,  55,  79  ff.,  87,  90,  178,  306, 
518;  trochlear,  55,  79;  vagus,  55,  81,  87, 
157,  163,  178,  524  f.;  vestibular,  82,  95, 
155  f.,  202  f. 

Nerve-cell,  97  ff.;  basket,  108  ff.;  branches 
of,  24  f.,  41  f.,  97.  286;  central  23  f.,  27  f., 
33,  65,  67,  159  ff.,  165,  167,  241;  chem- 
istry of,  105,  121,  280;  in  coelenterates, 
18;  of  cortex,  265  ff.;  development  of, 
41  ff.,  57  ff.,  97  f.,  104;  function  of,  286  ff.; 
giant,  240,  268  f.,  271;  of  Golgi  type,  97; 
multipolar,  103;  pigment  in,  106:  Pur- 
kinje,  101,  103  f.,  109  f.;  pyramidal,  102, 
104,  240,  265  ff.,  269;  of  retina,  190  ff.; 
size  of,  103;  stellate,  269;  unipolar,  43  f.; 
in  worms,  21,  23 

Nerve-centre,  28,  67,  74,  226  ff.,  281;  aux- 
iliary, 242;  catabolism  in,  291;  fatigue 
of,  538  ff.;  intellectual,  251,  262;  of  in- 
vertebrates, 20;  mechanics  of,  286  ff.; 
motor,  235  ff.,  253  f.;  perceptual,  252  f.; 
respiratory,  28,  157;  sensory,  244  ff.; 
speech,  255  ff.;  of  vertebrates,  25  ff. 

Nerve-fibre,  98  ff.;  association,  58,  96,  223  f., 
233,  246,  251  f.,  269;  catabolism  in,  135  ff., 
283;  central,  24,  28,  33,  56;  a  concentra- 
tion cell,  142;  in  cord,  75;  a  core  con- 
ductor, 140  ff.;  in  cortex,  266  ff.;  elec- 
trical properties  of,  134,  137  ff.;  fatigue 
of,  136;  function  of,  16,  20,  22,  35,  65, 
127  ff.,  154,  223,  279  ff.,  284  ff.,  304, 
473;  long  and  short,  20  f.,  25,  27  f.,  33 
56  f.,  94;  motor,  42,  46,  56,  73,  100,  128, 
132  f.,  151,  154,  212,  279;  in  nerves,  98  ff. ; 
nodes  of,  99  f.;  number  of,  75,  101;  of 
olfactory  nerve,  26,  56,  107;  projection, 
223,  242,  251;  sensory,  22,  26,  73  f.,  110, 
128,  132  f.,  176,  178,  182,  191,  203,  285, 
348  f.,  398;  size  of,  75,  100;  structure  of, 
98  ff.,  141,  279;  unmedullated,  99  f.,  128, 
132,  136 

Nerve  impulse,  127  f.,  165,  277,  279,  283  ff.; 
blocking  of,  287,  289;  in  different  nerves, 
284  f.;  speed  of,  131  f.,  473;  theories  of, 
135  ff.,  143  f.,  283  f. 


Nerve-muscle  preparation,  128  f. 

Nerve-net,  in  coelenterates,  18  ff.;  in  higher 
animals,  20,  112 

Nervous  elements,  97  ff.,  279  f. 

Nervous  system,  of  annelids,  22;  of  coe- 
lenterates, 17  ff.;  of  crustaceans  and  in- 
sects, 24:  development  of,  38  ff.,  275,  279, 
298;  elements  of,  97  ff.;  evolution  of,  35, 
298;  of  flatworms,  20;  function  of ,  13,  16, 
33,  63  ff.,  282  f.,  287;  fundamental  and 
accessory,  26  ff.;  growth  of,  57  ff.;  a 
mechanism,  3  f.,  63,  96,  275  ff.;  of  mol- 
lusks,  20;  plan  of,  63  ff.,  96,  127,  175, 
280  f.;  simplest  form  of,  17;  structure  of, 
63  ff.;  types  of,  20,  22,  25;  of  vertebrates, 
25  ff. 

Neural  crest,  39  f.,  42  ff. 

Neural  groove,  38 

Neural  tube,  38  ff.,  49 

Neurofibrils,  18, 21  f.,  41, 101, 106,  113  f.,  265 

Neuroglia,  40  f.,  46,  52,  71,  97,  101,  110, 
114  ff. 

Neurokeratin,  119 

Neurone,  100,  112,  160 

Neurone  theory,  98,  112  ff.,  286  f. 

Nissl  granules,  105 

Nissl  stain,  103,  105,  264 

Noises,  312  ff. 

Nonsense  syllable,  572,  577,  579 

Nucleus,  of  the  cell,  14,  100,  105,  288;  cau- 
date, 52,  78,  85  f.,  223;  of  cranial  nerves, 
80  ff.;  of  Deiters,  83,  95;  of  the  dorsal 
columns,  79  ff..  89;  lenticular,  52,  85  f., 
223,  259  f.;  olivary,  80  ff.,  95;  pontine,  83, 
86,  95;  red,  84  f.,  95,  223;  terminal,  93  f. 

Nutrition  dependent  on  exercise,  59,  62,  581, 
616  f. 

OBJECT  of  sense,  310  ff.,  380  ff.,  654 

Object  blindness,  252,  263 

Occasionalism,  646 

Odors,  classification  of,  306  ff. 

Old  age,  brain- weight  in,  61  f.;   mental  life 

in,  544,  662 
Olfactometer,  374 

Olfactory  bulb,  31,  50,  56,  79,  107,  177 
Olfactory  lobe,  31,  50,  158 
Olive,  69,  76  f.,  79  ff.,  86 
Olive,  superior,  90 
Optic  chiasm,  56,  76,  91,  222,  247 
Optic  lobes,  30,  103 
Organ,  concept  of,  636  f. 
Organ  of  Corti,  203  ff. 
Orientation,  247,  253 
Ossicles  of  the  ear,  197  ff. 
Osmosis  in  living  cells,  14,  119  f. 
Oral  or  snout  sense,  27,  31 
Otoliths,  203,  211 

Overlapping,  498  f.,  556,  558  ff.,  562 
Over-learning,  572,  576 
Over-tone,  314,  319  f.,  322  f.,  369 
Ovum,  development  of,  36  ff.;   fertilization 

of,  36  f. 
Oxygen  consumption  of  brain,  125,  215,  291; 

of  nerves,  137 

PACINIAN  corpuscle,  180  ff. 
Pain,   179,  181,  245,  302,  344  ff.,  348,  477, 
516  f.,  532,  539  f. 


INDEX  OF  SUBJECTS 


701 


Pain  spots,  181,  344  ff.,  477,  517 

Paired  associates,  573,  578 

Pallium,  31,  86,  219;  development  of,  51  ff. 

Papilla,  of  skin,  180;  of  tongue,  178,  310, 372 

Paracentral  lobule,  222 

Paradoxical  sensation  of  cold,  346,  348 

Parallax,  427,  429,  458 

Parallelism,  607,  631,  682 

Paralysis,  238  f.,  241  f.,  245,  254,  263 

Paramecium,  behavior  of,  545,  612 

Paraphasia,  253,  260 

Paraxon,  98 

Parietal  organ,  30  f. 

Path,  auditory,  82,  90,  203,  250;  branching, 
159  ff.,  168  f.,  611  ff.;  cerebellar,  95; 
converging,  161  f.;  cutaneous,  89  f.;  gus- 
tatory, 90;  kinesthetic,  90;  motor,  239, 
251;  olfactory,  92,  107,  250;  optic,  91  f., 
246,  269,  479;  reflex,  22,  89,  92;  sensory, 
81,  89  f.,  246,  267;  of  voluntary  reactions, 
478,  485,  487 

Pathological  method,  231,  252  ff.,  594 

Peduncles,  of  cerebellum,  78,  82  ff.,  86;  of 
cerebrum,  76  ff.,  84  f.,  223 

Perception.  301  ff.,  347,  375  ff.,  403,  435, 
440,  442,  449  ff.,  458,  466,  483  f.,  593  ff., 
665,    673;     centres   for,    245   ff.,    249   f., 
252  ff.,  260;  of  differences,  358  ff.,  601  ff.; 
errors  of,  434  ff.;  in  learning,  551  f.;   me-  , 
diate  and  immediate,  593;  of  movement/ 
211,  364,  387  f.,  404  f/  433  f.,  467;  process 
of,  594  f.;  of  relations,  555,  664,  f.;  selec-/ 
tion  in,  596  ff.;   of  space,  380  ff.,  413  fT; 
time  of,  483  ff.,  497 

Perineurium,  99 

Perse verat ion,  586  f.,  .615 

Personal  identity,  678,  680  f. 

Personality,  262,  519,  673 

Perspective,  429  f.,  449 

Phenomenal  reality,  672,  680 

Phosphene,  325,  417 

Phosphorus  of  the  brain,  121 

Photo-chemical  substance,  190 

Photo-chromatic  interval,  333 

Phrenology,  227,  232 

Physiological  psychology,  1  ff.,  13,  297  ff., 
629;  assumptions  of,  653;  limitations  of, 
469,  625;  method  of,  7  f.,  13,  297  ff.,  356, 
380,  468 

Physiological  time,  472 

Physiology,  3  ff.;    method  of,  7 

Pia  mater,  70  f. 

Pineal  gland,  78,  634 

Pitch,  206  ff.,  313  ff.,  490,  492 

Pithecanthropus  erectus,  34  f. 

Plane  of  regard,  421 

Plasticity,  35,  217,  544,  581 

Plateau  in  the  curve  of  learning,  561  f. 

Pleasantness,  501  ff.,  508  ff.,  515  ff.,  526, 
532,  552.  564  f.,  567,  583 

Plexiform  layer,  266,  268,  273 

Plexus  in  cortex,  267  f.,  273 

Poggendorf  illusion,  440,  444,  446,  451 

Point  of  regard,  420,  422 

Pons,  66,  75,  76  ff.,  82  ff.,  222;  development 
of,  49,  57 

Posture,  157,  162,  173,  209 

Practice,  401  ff.,  451  f.,  455.  471,  480  f.,  488, 
496,  499,  534,  542,  556  ff.,  595 


Practice  curve,  548  ff.,  556  f . ,  660  ff. .  568, 575  f . 

Precuneus,    222 

Pre-established  harmony,  646 

Preparation  for  reaction,  480,  482  ff.,  489, 
491,  493  ff.,  590,  602 

Presentations  of  sense,  301  ff.,  380  ff.,  413  ff 
613  f.,  659 

Primary  position  of  the  eye,  421  f. 

Primates,  brain  of,  34  f . ;  intelligence  of  33, 
547,  549,  551  f. 

Product,  concept  of,  641 

Projection  fibres,  223,  242,  251 

Projection  of  sensations,  303,  384,  390,  393, 
411,  414,  633 

Proprioceptors,  154,  156 

Protagon,  123  f. 

Proteid  or  protein,  14,  118;  in  brain,  118  f. 

Protopathic  sensibility,  349 

Protoplasm,  composition,  14 

Protozoa,  13,  544  f. 

Psalterium,  85  f. 

Pseudoscopic  vision,  428,  430 

Psychic  blindness,  247  ff.,  252  f.;  deafness, 
250,  252  f. 

Psycho-analysis,  586 

Psycho-galvanic  reaction,  509 

Psychology,  1,  7,  264,  273;  comparative, 
14  ff.,  216  f.;  experimental,  4,  629, 
genetic,  381,  655;  introspective,  381;  phy- 
siological, see  Physiological  psychology 

Psycho-physics,  6,  9,  353  ff.,  649;  methods 
of,  357  ff. 

Pupil  of  the  eye,  183  f.,  188  f. 

Purkinje  cell,  101,  103  f.,  109  f. 

Purkinje  phenomenon,  196,  333 

Purposiveness  of  reflexes,  153  f.,  216,  527 

Pursuit  movement  of  the  eye,  460  ff. 

Puzzle  test,  555  f.,  605 

Puzzle-box  test,  548  f.,  551,  553 

Pyramids,  69,  76  f.,  79^. 

QUADRIGEMINA,  30,  49,  77  f.,  83  f.,  90,  92, 

95,  189 
Quality  of  sensation,  302  ff.,  324  ff.,  353  ff., 

408,  614 
Quantity  of  sensation,  300,  302,  324,  353  ff., 

614 

REACTION,  general  concept,  15;  anticipatory, 
217;  instinctive,  146;  learned,  145,  147, 
545  ff.,  569;  mental,  583,  594;  uncon- 
scious, 145;  varied,  545,  547  f.,  550,  555, 
563,  569,  595,  598,  600,  604  ff.;  voluntary, 
147 

Reaction  time,  166,  472  f.,  476  ff.;  analysis 
of,  473,  483  ff.;  associative,  493  ff.;  com- 
plex, 487;  discriminative,  488  ff.,  497; 
experiment  in,  481;  movement  in,  480  f., 
489;  muscular,  485  ff.,  489;  reduced,  473, 
483;  sensorial,  485  ff.,  489,  491;  simple, 
472,  476  ff.,  497;  to  stimuli  of  different 
senses,  476  ff.;  of  different  intensities, 
479  f. 

Readiness,  483,  485  ff.,  489,  493,  495  ff., 
503,  580,  584,  612 

Reading,  252,  257,  461,  498  f.,  560,  595 

Reality,  conception  of,  656,  669  f.,  677;  of 
atoms,  675  ff.;  of  the  mind,  653,  656,  662, 
667,  668  ff. 


•\ 


702 


INDEX  OF  SUBJECTS 


Reasoning,  603  ff.,  664  ff.;  deductive  and 
inductive,  605;  mediate  and  immediate, 
606 

Rebound  after  inhibition,  164  f.,  173,  599, 
612  f. 

Recall,  503,  543  f.,  550  f.,  563,  582  ff.;  diffi- 
culties of,  585  f.;  factors  determining, 
583,  600;  partial  or  selective,  582  f.,  600  f . ; 
threshold  of,  584 

Receiving  station,  246,  248,  250  f.,  253,  260, 
272 

Receptive  field,  161,  168 

Receptor,  16,  65,  99,  127,  175  ff.,  280,  304, 
341  ff.,  351  f.,  375,  477;  distance  receptor, 
25,  217;  simplest  type  of,  176 

Receptors,  of  annelids  and  arthropods,  25; 
of  jelly-fish,  18;  of  vertebrates,  27  f. 

Reciprocal  innervatipn,  162 

Recitation,  value  of  in  memorizing,  582 

Recognition,  543,  590  ff.,  665,  678;  false, 
544;  feelings  of,  590  f.;  mediate  and  im- 
mediate 592,  593 

Rectus  internus,  etc.,  185  f. 

Reflex,  128,  145  ff.;  clasp,  153;  compound, - 
169  ff.;  duration  of,  166;  extensor  thrust, 

154,  164,  168,  173;   flexion,  153,  161,  164, 

167,  169,  171,  173;  force  of ,  167  f.;  local, 
22  ff.,  94,  146,  154;   in  man,  155;   mental 
influences  on,  532  f.;  patellar,  167,  171  f., 
532 f.;  postural,  162,  173;  protective,  153, 

155,  173,  532;    pupillary,  92,  145  f.,  161, 

168,  188  f.,  248;    scratch,  155,  161,   164, 
167,  172  f.;  pinipjs.  146  f.,  170;  spread  of, 
168  f.;  stepping,  154,  164;   winking,  164, 
167;   wiping,  154 

Reflex  action,  145  ff.,  281,  538;  cerebral  in- 
fluence on,  158  f.,  171  f.,  241,  262;  char- 
acteristics of,  159  ff.;  fatality  of,  173;  in- 
terpretation of,  154;  in  man,  155;  varia- 
bility of,  174 

Reflex  arc  or  path,  22,  89,  92,  145  f.,  154, 
159  ff. 

Reflex  preparation,  151  ff. 

Reflex  time,  166  f.,  287,  472;   reduced,  167 

Refraction  in  the  eye,  183,  186  ff. 

Refractory  period,  131,  164  f.,  540,  615 

Reinforcement,  170  f.,  532  f. 

Relations,  perception  of,  555,  601 

Relativity,  law  of,  375  f. 

Relay  station,  92  f.,  151 

Relief,  501  f.,  506  ff.,  531 

Relearning,  572,  577 

Reproduction,  in  protozoa,  14,  36;  in  met- 
azoa,  16,  36  f.;  of  associations,  543,  582  ff. ; 
see  Recall 

Reproductive  tendencies,  493  ff.,  583  ff., 
605  f.;  selection  among,  590 

Reptiles,  brain  of,  29  ff. ;  intelligence  of,  547 

Residual  defects,  261 

Resistance  in  gray  matter,  287,  289  f. 

Respiration,  automatism  of,  148;  centre  for, 
81,  125,  148,  157,  262;  in  emotion,  505  ff., 
525 

Restitution  of  function,  241  ff.,  259  ff.,  663 

Restlessness,  501 

Retention,  542  ff.,  545  f.,  572  ff.;  curve  of, 
575;  loss  of,  546,  574  ff.,  585,  616  f.;  par- 
tial, 573;  physiology  of,  286  f.,  615  ff. 

Reticular  formation,  79  f. 


Retina,  183  f.,  186,  189  ff.,  280,  325  f.,  438; 

layers  of,  190  f.:   periphery  of,  326,  334; 

projection  of,  upon  cortex,  248  f. 
Retinal  field,  415  ff. 
Retinal  image,  186  f.,  189,  382,  415  f.,  419, 

453  ff. 

Retinal  light,  325,  357,  369,  433,  463 
Retraction  of  dendrites,  289,  613 
Reward  and  punishment,  547  f. 
Rhythm  as  a  source  of  feeling,  529;   as  an 

aid  in  memorizing,  578  f.,  584 
Rhythmical  movement,  149,  164  f. 
Right  and  wrong  cases,  359 
Right-handedness,  263  f.f  565 
Rivalry,  308,  311,  392  f.,  452  f.,  598  f. 
Rods  of  the  retina,  190  ff.,  325  f. 
Roots  of  the  nerves,  40,  42,  44,  47,  50,  56, 

73  f.,  134 
Rotation,  sensation  of,  210  ff.,  350;   of  the 

eye,  420  ff. 

SACCULE,  201,  203,  208,  211 

Salts  of  the  brain,  118 

Saturation  of  color,  324,  327,  337 

Saving  method,  573 

Scala  tympani,  201  ff. 

Scala  vestibuli,  201  ff. 

Scale,  of  colors,  328,  330  f.,  344;  of  ex- 
tensity,  383;  of  intensity,  356  f.,  362;  of 
tones,  314  ff.,  318.  320  f. 

Sclerotic,  182,  184,  194 

Segments  of  the  nervous  system,  in  an- 
nelids, 22  ff.;  in  vertebrates,  25,  74 

Selection,  neural  mechanism  of,  623  f.;  in 
the  process  of  learning,  547,  549  ff.,  564; 
in  perception,  596;  in  recall,  590;  in 
thought,  606 

Self,  511,  519,  571,  658  f.,  665,  668  ff. 

Semicircular  canals,  155, 200  f.,  203  f.,  208  ff., 
350 

Sensations,  16,  282,  285,  297  ff.,  381  f.,  639; 
classification  of,  300;  feeling-tone  of,  502, 
512,  515  ff. ;  measurement  of,  356  ff.,  377  f . ; 
pure,  391,  594,  596,  659;  quality  of, 
302  ff.,  324  ff.,  353  ff.,  405,  408;  quantity 
of,  300,  302,  324,  353  ff.;  simple,  302  f., 
327,  341,  351,  380,  391;  subjective,  306; 
unit  of,  361,  377  f.;  visceral  or  organic, 

350,  518  f.,  525  f. 
Sensation-circles,  397  ff.,  405 
Sense-organ,  67,  175  ff.,  299,  473  ff.,  479; 

development,  of,  38,  56;   see  Receptor. 
Sensitivity,   175,  345,  357,  364  ff.,  368  f., 

371  ff.;   see  also  Irritability;   differential, 

358  ff.,  392,  404,  431  f. 
Sensorium,  213  f.,  284 
Sensus  communis,  214,  350,  518  f. 
Series  of  sensations,  356  f.,  383,  386  ff. 
Sheath,  medullary  or  myelin,  26,  45,  58,  99  f.. 

142  f.;  primitive,  99 
Shock  of  difference,  403 
Sight,  see  Vision. 
Skill,  acquisition  of,  555  ff.,  565 
Skin,  senses  of,  179  ff.,  244,  344  ff. 
Sleep,  113,  291 
Smell,  27,  -158,  175  ff.,  219,  250  f.,  304  ff., 

351,  372  ff.,  392,  475,  478 
Snout  sense,  31,  219 
Solipsism,  668 


INDEX  OF  SUBJECTS 


703 


Somesthetic  area,  244  ff.,  403  f. 

Soul,  1  f.,  10,  215  f.,  625,  686;  see  Mind 

Sounds,  312  ff. 

Space-form,  383,  418  f.,  466 

Space-perception,  245,  364,  371,  380  ff., 
413  ff.;  theories  of,  385  f.,  418  ff. 

Span  of  attention  or  apprehension,  560, 
597;  of  immediate  memory,  574,  577 

Spatial  series,  383,  386  ff.,  406  f.,  419,  422, 
659 

Spatial  quality,  418  f. 

Specialization  of  organs,  15  f.,  18  f.,  22,  36, 
64  f.,  175 

Specific  energies  of  nerves,  284  ff.,  304,  350 
ff.,  389  f.,  418,  420;  of  parts  of  the  brain, 
284  f.,  614 

Spectrum,  327  ff.,  341 

Speech,  centres  for,  227,  233,  255  ff.;  dis- 
turbances of,  253,  255  ff. 

Spider,  learning  by,  545  f. 

Spider  cell,  114  f. 

Spinal  cord,  25  ff.,  66,  69  ff.,  226,  241;  con- 
sciousness in,  216  f.;  development  of,  39 
ff.,  59 

Spinal  dog,  153  ff. 

Spinal  frog,  152  ff. 

Spinal  ganglia,  26,  42  ff.,  73  f.,  103,  106 

Spinal  preparation,  151  ff. 

Spirituality,  concept  of,  682  f. 

Sponge,  behavior  of,  17 

Spreading  of  reflexes,  169 

Stable  colors,  334  f.,  337,  340,  344 

Stains  of  nervous  tissue,  101,  103,  124,  264; 
fibril,  103,  106,  265;  Golgi,  103,  264  f.: 
Nissl,  103,  105,  264;  osmic  acid,  87; 
Weigert,  72,  264 

Staircase  effect,  536,  538 

Staircase  figure,  594,  618 

Stapedius  muscle,  198,  200 

Stapes,  197  ff.,  205 

Stentor,  behavior  of,  544  f.,  612 

Stereognosis,  253,  410 

Stereoscope,  427  f.,  598 

Stereoscopic  vision,  414,  427  f.,  430 

Stimulation,  method  of,  229  f. 

Stimulus,  4,  6,  14  f.,  282,  303;  adequate, 
175,  304  f.,  307  ff.,  311,  324,  327  ff.,  332, 
341,  346  ff.,  367;  general,  17,  129  f.,  175, 
305,  309  ff.,  324  f.,  346;  mixed,  308,  311, 
330  ff.,  340,  343,  347  f.,  389,  452;  in  re- 
action time  experiments,  471,  476  ff.; 
and  sensation,  357  ff. 

Stirrup,  197  ff.,  205 

Striatum,  31,  51,  78  f.,  86,  219 

Stripe  of  Baillarger,  269 

Stripe  of  Gennari,  269,  271 

Strychnine,  147,  160,  619 

Subconsciousness,  2,  672  f. 

Subcutaneous  senses,  181,  348  f.,  363  ff. 

Sub-excitation,  485  ff.,  580 

Subjective  idealism,  668 

Suggestion,  402,  535 

Summation  of  stimuli,  170  f.,  612 

Surface  of  separation,  14,  65,  119  f.,  289  f. 

Surprise,  602 

Suspensory  ligament,  184,  188 

Sympathetic  system,  66  f.,  74,  99;  develop- 
ment of,  44  f.;  function  of,  150,  189 

Sympathetic  vibration,  206  f. 


Synapse,    107,   111    ff.,   266,   287    ff.,   479, 

609  ff.,  619 
Synthesis,  93,  302  f.,  383  f.,  391,  407,  410, 

419,  422,  435,  466,  468,  665,  683 

TALBOT'S  law,  474 

Taste,  28,  306,  309  ff.,  351  f.,  372  f.,  392, 
475,  477  f.;  centre  for,  251;  end-organs 
of,  178  f.,  307;  pathway  of,  00,  178  f. 

Taste-buds,  178  f.,  307,  309  ff.,  478 

Tectal  system,  94  f. 

Tegmentum,  83  ff. 

Telegraphy,  learning  of,  556  ff.,  561,  621 

Telencephalon,  48 

Temperature  sense,  179,  181,  245,  344  ff., 
365  f.,  406  f.,  477 

Tendon  spindle,  182 

Tension,  feeling  of,  501  f.,  506  ff. 

Tension  theory  of  consciousness,  610  f. 

Tensor  tympani,  197  f.,  200 

Tentorium  cerebelli,  68,  70 

Terms  of  the  judgment,  593 

Testimony,  reliability  of,  589 

Thalamus,  30,  49,  51  ff.,  77  f.,  84  ff.,  90, 
92  f.,  96,  246;  in  different  vertebrates,  30  f. 

Things,  299  f.,  303,  381  f.,  422,  512,  633, 
655,  658  ff.,  666,  673  f.,  676  f.,  679  ff. 

Third  dimension,  perception  of,  413  f.,  423, 
426  ff.,  449,  458,  465 

Thomson's  law,  453 

Thought,  555,  589  f.,  593  ff. 

Threshold,  of  consciousness,  2,  458,  463, 
671;  of  difference,  358  ff.;  of  excitation, 
357,  478;  of  recall,  584;  of  spatial  dis- 
crimination, 326,  396  ff.,  565 

Tickle,  347 

Timbre,  314,  318  f.,  369,  395 

Time,  illusions  of,  440;  of  mental  processes, 
470  ff.,  497,  614 

Time-form,  470,  514 

Tone-color,  314,  318 

Tones,  312  ff.;  difference,  323;  end-organs 
for,  206  ff.;  scale  of,  314  ff.,  318,  320  f.; 
simple,  313 

Tonus,  155,  209,  211,  325 

Touch,  analysis  of,  179,  306,  310,  344  ff.; 
centre  for,  244  ff.;  illusions  of,  440;  in- 
ertia of,  473  ff.,  476;  pathway  of,  81, 
89  f.;  perceptions  of,  391,  396  ff.,  466  ff., 
492 

Touch  compasses,  396,  401 

Touch-corpuscle,  180 

Touch-spots,  132,  179  f.,  344  ff.,  365,  398  ff. 

Tract,  81  ff.,  cortico-spinal  or  pyramidal,  47, 
73,  83  f.,  88  f.,  93,  232,  239  f.,  242,  269; 
development  of,  47;  direct  cerebellar,  95; 
list  of  tracts,  89;  methods  of  tracing, 
87  f.;  naming  of,  88  f.;  olfactory,  50,79, 
92;  optic,  91.  See  Path. 

Training,  401  ff.,  542,  547  f.,  565  ff. 

Transference  of  training,  402,  550,  563, 
565  ff.,  581,  588,  605 

Treffermethode,  573 

Trial  and  error,  learning  by,  550  ff.,  582, 
617,  620;  in  perception,  594  f. 

Twilight  vision,  196,  333,  335,  340 

Twisted  cord  illusion,  446  f. 

Two-point  threshold,  396  ff.,  565 

Tympanic  membrane,  197  ff.,  367 


704 


INDEX  OF  SUBJECTS 


Tympanum,  197  ff. 

Typewriting,  learning  of,  558  ff.,  576  f.,  621 

UNCONSCIOUS  movements,  145 

Unicellular  animal,  13,  14 

Unit,  of  perception,  442  ff.,  597,  623;  of 
reaction,  498,  558  f.t  562,  569,  578  f.,  584, 
621  ff.;  of  sensation,  361 

Unity,  conception  of,  670  f.,  676;  of  con- 
sciousness, 679,  684  ff.;  of  the  mind,  672, 
678  ff. 

Unpleasantness,  348,  501  ff.,  508  ff.,  515  ff., 
526,  532,  552,  564  f.,  567,  585  f. 

Utricle,  201,  203,  208,  211 

VARIABILITY,  of  brain- weight,  60;  of  per- 
ception, 359,  376;  of  reflexes,  174;  of 
reaction  time,  476,  481;  of  voluntary 
effort,  537 

Varied  reaction,  545,  547  f.,  550,  555,  563, 
569,  595,  598,  600,  604  ff.,  615;  physio- 
logical theory  of,  611  ff.,  620 

Ventral  horn,  46,  151,  154 

Ventral  root,  40  ff. 

Ventricles  of  the  brain,  39,  49,  51,  77  ff.,  81, 
86 

Verbal  amnesia,  253,  260 

Vermis,  86  f. 

Vertebrates,  nervous  system  of,  25  ff. 

Vertical-horizontal  illusion,  437,  447  ff. 

Vestibular  nerve,  82,  95,  155,  203 

Vestibule,  200  ff.,  208,  211,  350 

Vicarious  function,  242 

Visceral  sensation,  350 

Vision,  acuity  of,  326,  371  f.;  aesthetics  of, 


530  f.;  binocular,  414  f.,  423  ff.,  436,  458, 
465;  clear,  420,  457,  459  ff.;  erect,  453 
ff.;  indirect,- 326,  334,  422,  461;  inertia 
of,  474,  476,  479;  monocular,  413  ff.,  423, 
430,  466;  peripheral.  326,  334  ff.;  sen- 
sations of,  324  ff.;  stereoscopic,  414. 
See  Eye,  Color  vision. 

Visual  area,  246  ff.,  285 

Visual  perception,  413  ff. 

Visual  purple,  125,  195 

Visual  sensations,  285,  324  ff.,  369  ff. 

Visual  size,  430 

Vital  force,  279,  291 

Vitreous  humor,  183  ff.,  187 

WARMING-UP,  536,  538,  545,  615 
Warmth-spots,  181,  344  ff.,  366,  405 
Weber's  law,  361  ff.,  432;  interpretations  of, 

374  ff. 

Weigert  stain,  72,  264 
Weight,  perception  of,  363  ff.,  601  f. 
Wernicke's  region,  257,  259  f. 
White  matter,  45,  53,  74,  101,  118  ff.,  223 
Will,  243,  483  ff.,  496,  645,  664,  666;    will 

to  learn,  582 
Will-time,  484,  488,  497 
Word-blindness,  252,  257,  263 
Word-deafness,  250,  253,  257,  260,  263 
Writing  centre,  254  f.,  257,  261 

YELLOW  spot  of  retina,  191,  193  f.,  326,  335, 
371,  465 

ZOLLNER  illusion,  444,  446  f. 
Zones  of  the  retina,  334  ff.,  343 


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